# Forum Home Renovation Bathrooms  Bonding Screed to Membrane

## Watters

The green shed has a video on screeding. The screed they build in it is a bit thin but the interesting aspect of the video is that they used mixed Davco Powder Mastic to bond the screed to the waterproof membrane:  https://www.youtube.com/watch?v=QhLOhTlJ_Ro 
Is that acceptable or is there a better product to use for bonding the screed to the waterproof membrane below it (Crommeln shower waterproof membrane SBR rubber sitting on compressed fibre cement sheet in the shower recess, and sitting on tile underlay in the rest of the bathroom - substrates primed with diluted Crommelin)? 
Davco powder mastic is a flexible cement based tile adhesive, so as a "non-rubber modified flexible cement glue" might do the job...

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## Petervm

> Is that acceptable or is there a better product to use for bonding the screed to the waterproof membrane below it (Crommeln shower waterproof membrane SBR rubber sitting on compressed fibre cement sheet in the shower recess, and sitting on tile underlay in the rest of the bathroom - substrates primed with diluted Crommelin)? 
> Davco powder mastic is a flexible cement based tile adhesive, so as a "non-rubber modified flexible cement glue" might do the job...

  In my book absolutely not, going by your previous posts you*'re* putting a 50mm screed down right? if so the screed should just sit on top of the waterproofing, even better if you separate the screed from the waterproofing by putting down builders plastic first, so that the compressed fiber cement can move independently from the screed, thereby eliminating any possible stress on the waterproofing membrane.

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## Watters

> In my book absolutely not, going by your previous posts you*'re* putting a 50mm screed down right? if so the screed should just sit on top of the waterproofing, even better if you separate the screed from the waterproofing by putting down builders plastic first, so that the compressed fiber cement can move independently from the screed, thereby eliminating any possible stress on the waterproofing membrane.

  Interesting idea about the builders plastic sheet, thanks. RE builders plastic, that's going to move around a bit when putting down the screed. Did you use anything to stop it doing that while moving the dryish cement/sand around with the float and compacting it to create the screed? Or did you do a wetter screed and just trowel it? 
Crommelin said yesterday pretty much the same thing, i.e. to leave the screed just sitting on the waterproof membrane. 
[EDIT: Crommelin said not to use a powder mastic but to use a flexible cement based glue. I asked if Monoflex was okay for that then. They said yes.

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## phild01

Don't like the idea of a floating screed and the swamp that would take up residence there.

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## Watters

> Don't like the idea of a floating screed and the swamp that would take up residence there.

  Sounds like Monoflex is the go then...

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## Watters

Just got more feedback from Crommelin. They said to use a slurry coat of cement mixed with either Crommelin high performance bonding agent or Crommelin Powerflex.
Nuff said.

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## Optimus

> Just got more feedback from Crommelin. They said to use a slurry coat of cement mixed with either Crommelin high performance bonding agent or Crommelin Powerflex.
> Nuff said.

  But how can you trust them, watters?

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## Watters

> But how can you trust them, watters?

  Ahh, the public online forum pain. Yes I do, I spoke to their tech guru this time. 
I'm putting the magic sauce of the cement slurry in this post in case anyone else wanders down this path: 
1. Crommelin high performance bonding agent or Powerflex.
2. Mix the additive with enough cement to discolour the additive to grey but to remain extremely liquid enough to broom around on top of the Crommelin shower waterproofing membrane/wettite. 
I also then checked with Efflock about Efflock as well because the cement slurry will almost certainly be partly tainted by Efflock which I want to use in the screed. They said it "should" be okay. 
So you know what I'm going to do? I'm going to get out a bit of waste CFC and test the whole thing out before I commit... 
Mama Mia...

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## Petervm

Watters, it's very likely that any movement in the sub floor will, if you glue the screed to the waterproofing, result in that movement transmitting through the screed and possibly resulting in the tiles cracking and also putting excessive stress on the waterproofing, which could lead to failure of the membrane. 
A few years ago I actually did glue my screeds in shower bases for a while using Monoflex to glue the tiles down and some of the tiles did end up cracking due to sub floor movement.

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## Watters

> Watters, it's very likely that any movement in the sub floor will, if you glue the screed to the waterproofing, result in that movement transmitting through the screed and possibly resulting in the tiles cracking and also putting excessive stress on the waterproofing, which could lead to failure of the membrane. 
> A few years ago I actually did glue my screeds in shower bases for a while using Monoflex to glue the tiles down and some of the tiles did end up cracking due to sub floor movement.

  Swamp versus cracked tiles, what a choice... Maybe that's why they weren't that keen on the screed being bonded... 
I used to think the WA building code of double brick with cavity on a concrete slab was overkill. I'm not so sure anymore. 
How about a compromise then, unbonded but no builders plastic sheet? At least that means not having to worry about other stuff interacting with the Efflock in the screed.

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## Petervm

A few more thoughts - 
Weren't you going to waterproof under the screed and on top? if so that should eliminate any possibility of a "swamp" forming under the screed, only thing you then have to do is make sure the tiles are adhered 100% so there are no voids between the tile and waterproofing membrane for water to sit. 
Also I assume you're going to use a glazed tile which would be waterproof and if you use eflock in the grout that should waterproof the grout, which only leaves the edges between the tile and wall which you could fill with silicone/polyurethane/MS polymer, before you install the wall tiles. You could just about call that three lots of waterproofing!   
Finally if your shower base consists of one piece of compressed cement sheet, no joints then sticking the screed to the waterproofing should be ok, although you still might need to account for thermal movement of the cement sheet and screed contracting & expanding at different rates, I assume the waterproofing membrane should be able to accommodate that movement.

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## Watters

> Weren't you going to waterproof under the screed and on top? if so that should eliminate any possibility of a "swamp" forming under the screed, only thing you then have to do is make sure the tiles are adhered 100% so there are no voids between the tile and waterproofing membrane for water to sit.

  Correct, but with an open neck around the puddle flange between the lower and upper layers of waterproofing membrane, plus Efflock water repeller in the screed. In theory no water should get through but I wouldn't put a bet on it all the same. The open neck means any water that does get in won't get trapped in the screed. The bottom layer of waterproofing with fabric will lap into the neck of the puddle flange.   

> Also I assume you're going to use a glazed tile which would be waterproof and if you use efflock in the grout that should waterproof the grout, which only leaves the edges between the tile and wall which you could fill with silicone/polyurethane/MS polymer, before you install the wall tiles. You could just about call that three lots of waterproofing!

  JH also suggest using a wet area sealant in that area after the wall tiles are up but because of mould risk I am not so sure, bed it with the flexible grout? Both the floor and wall tiles are porcelain not ceramic. My understanding is that porcelain tiles in general tend to have much lower porosity than ceramic tiles regardless of glossy or matt finish. The wall tiles are Italian Flaviker 300mm x 300mm x 8mm and are somewhat matt. I gather that Flaviker tiles are more usually used in commercial applications than residential. My wife chose them... The floor tiles are 97mm x 97mm on a lattice and have a somewhat glazed surface with rough bits embedded in the surface which will help prevent slipping on the floor in the shower. Indeed, Efflock will be in the grout, in the tile adhesive and in the screed, it will be everywhere!   

> Finally if your shower base consists of one piece of compressed cement sheet, no joints then sticking the screed to the waterproofing should be ok, although you still might need to account for thermal movement of the cement sheet and screed contracting & expanding at different rates, I assume the waterproofing membrane should be able to accommodate that movement.

  I'm still not sure whether to bond the screed or not. At least I have a solution now on how to do that if that is the way it is going to be. Need to think some more on this. Yes, the membrane will easily be able to cope with such movement but if it ever tears it is game over. It is indeed one piece of 15mm CFC for the shower recess and solid with no joints, just a hole for the puddle flange and fixing holes. However, it is surrounded by perimeter YT under shower recess wall plates. The following picture shows the arrangement that is in the making:

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## Petervm

I hope you're a young bloke Watters so that you get to enjoy the bathroom because it should last for a generation or two or three...

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## Watters

> ....it should last for a generation or two or three...

  Thanks, hope so.... Here's how the other side will look, decided hob would look too outdated...
[EDIT] Polurethane has bond breaker over it, not polymer.

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## Petervm

Who ever makes your shower screen should, if they are up with current standards, have a bottom channel that fits over the top of the shower angle so that the shower screen sits on top of the angle instead of how your drawing shows it on the shower side of the angle, I think you will achieve a neater finish that way. At least that's how I see most showers that I do finished.

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## Watters

> Who ever makes your shower screen should, if they are up with current standards, have a bottom channel that fits over the top of the shower angle so that the shower screen sits on top of the angle instead of how your drawing shows it on the shower side of the angle, I think you will achieve a neater finish that way. At least that's how I see most showers that I do finished.

  Great, you looked at the drawing, thank you. 
Yes, of course you are correct. However, I found an old thread on this forum where OldSaltoz suggested doing it this way as a better way to stop water breaching. Evidently there can be some pump action under the shower screen. That interested me given that YT will still remain under tile underlay in the main part of the bathroom. 
It will be a good day when I get to the point where I'm talking to shower screen folks.

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## Petervm

Bradnams is the company that my main builder uses they seem make reasonable screens - Showerscreens | Signature | Bradnams Windows & Doors

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## Watters

> Bradnams is the company that my main builder uses they seem make reasonable screens

  Thanks, I'll take a look at them when the tiling is done in due course.

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## intertd6

> Thanks, hope so.... Here's how the other side will look, decided hob would look too outdated...
> [EDIT] Polurethane has bond breaker over it, not polymer.

   with the change of substrates I'm thinking it's not a situation that would be the best & could be a problem area with the expansion or contraction at this junction.
inter

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## Watters

> with the change of substrates I'm thinking it's not a situation that would be the best & could be a problem area with the expansion or contraction at this junction.
> inter

  Agree that having the change of substrates is not ideal. My neighbour's place has the same change of substrates by the way. It's no justification I know but his bathroom is still okay. I'm open to suggestions regarding the drawing but the two substrates will remain. For any future bathroom reno I would use Scyon for the whole floor, but that won't happen for this bathroom reno.

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## intertd6

> Agree that having the change of substrates is not ideal. My neighbour's place has the same change of substrates by the way. It's no justification I know but his bathroom is still okay. I'm open to suggestions regarding the drawing but the two substrates will remain. For any future bathroom reno I would use Scyon for the whole floor, but that won't happen for this bathroom reno.

   I'd say it should be treated as an expansion joint then, from substrate to the finished surface.
inter

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## Watters

> I'd say it should be treated as an expansion joint then, from substrate to the finished surface.
> inter

  Thanks. Updated diagram below. Presumably bond breakers are needed on bottom flat tails of angles?

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## METRIX

This is where DIY always falls over, a job is done half the way it should be, then you have to start coming up with un-conventional solutions to try and make something work, will the solution work ?, maybe, maybe not who knows ? 
As I said long ago for your bathroom, rip all that yellow tongue out, and replace with something suitable, not some of it, and certainly not just the shower recess. 
IMO you chose the worst position to put a join of two completely opposite substrates, the shower recess perimeter is the most critical part of the entire bathroom, it experiences the most expansion / contraction due to the hot showers, then cooling down, only to start again for the next shower, next day, next week, next year.

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## Watters

> ...rip all that yellow tongue out, and replace with something suitable...

  Please, not the "professionals" versus "DIY" debate again. It was the original "builder" who put the yellow tongue chipboard in the wet area in the first place, not me. 
It is no defence I know but my neighbour's property has been renovated the same way at least two years ago, i.e. CFC in shower recess, YT left everywhere else. However, I don't know if that renovator bothered to do tile underlay with stud adhesive or not for the main bathroom floor. If he had then he would have had to make the shower screed and the bathroom floor screed differing maximum heights to restore the floor levels to same, which is what I will be doing. I also don't know if he did an expansion joint between the two tiled surfaces (shower and main floor) but I will be creating an expansion joint using two angles filled between with sealant because of the two separate tiled areas. It is hard to know from looking on top what the neighbour's renovator did on top because it is hidden below the bottom of the shower screen. For all I know all he did was put a single angle there. Where his shower does differ is that it is a two wall shower whereas mine is a three wall one. 
Yes I am DIY and yes the approach I am taking is not the best I acknowledge that. I do consult and listen to a lot of input on this forum even so.
Would I do things differently another time? Yes I would. I have learned a lot along this journey with a lot of help from many people along the way, and a special mention there goes to you METRIX. 
So, yes, this reno is a dog's breakfast hybrid, but it will remain so which does mean unorthodox steps. Sorry about that.

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## Optimus

Wasnt the the original idea that you would do it yourself because you wanted it done properly & for your wife to save money on her investment?

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## METRIX

> Please, not the "professionals" versus "DIY" debate again. It was the original "builder" who put the yellow tongue chipboard in the wet area in the first place, not me.

  Times have moved on since your place was built and so have the materials used in these situations, even though compressed has been around forever, and yes some builders still use YT as a wet area substrate, as this is an acceptable minimum method of construction, doesn't mean it's recommended. 
And yes I will compare DIY to Professional, because your example shows exactly what happens when you don't have the experience to understand what you are doing is right or wrong, or has knock on effects.
If your example can save one DIY'R not attempting what you're doing, and getting in a Licensed Pro who will offer warranty then it's worth comparing. 
Bathrooms and Waterproofing are difficult tasks to do correctly to achieve a long term solution, some think it's easy and do it themselves. only to end up in a pickle with an inferior result, then usually pass the problem onto the next person who buys the house.   

> Yes I am DIY and yes the approach I am taking is not the best I acknowledge that. I do consult and listen to a lot of input on this forum even so.
> Would I do things differently another time? Yes I would. I have learned a lot along this journey with a lot of help from many people along the way, and a special mention there goes to you METRIX.

  Herein lies the problem with seeking detailed information from a forum, everyone has their own advice for what's right or wrong, as you know there are 100 different ways to do a job.
How do you decipher what's correct and what's rubbish, the more questions you ask the more answers you receive confusing the situation even more.

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## Skinah

Some info to help as I did a DIY job on mine with zero experience. Its my house that I wont sell so I get to fix any mistakes I make. 
1. Make sure you research your screed correctly. Your not on a concrete slab like I was and most screeds I saw mention a minimum thickness if you go unbonded. Unmodified Traditional screed needs to be a minimum of 40mm if compacted very well and some say 50mm. Your drawings show your going much thinner so this will be critical to select a screed that can handle floating and also your thin sections. 
2. Get some polypropelene fibres 12mm long and throw in your screed and don't mix it too long as it can break the fibres up. You soak the fibres first to separate them to get an even mix. Ebay has them shipped from the UK in a small amount. I throw the excess screed out in my dirt driveway and the batches I used the PPF in have stood up very well to my car driving over them compared to other mixes. 
3. You*'*r*e* correct about not putting the top membrane all the way up to the puddle flange but you have not considered the effect of putting an additive in the screed to make it water resistant and then packing it against the flange riser. If water does not soak into the screed due to additives it cant reach the puddle flange by fixing one issue you create another. One idea is to put some foam around the flange, pack the screed and then when it drys pull out the foam so a 2mm gap is left at the puddle flange. Your top membrane creates a funnel to pore it down this gap. 
4. Not a fan of your top alu angle, just use a single continual alu angle. Putting another one on top will look ugly and wont be needed. Go thick with the alu angle if you have an expansion joint as the glass will push on this and create movement.

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## Watters

Thank you Skinah for your input. You have restored my faith in people genuinely wanting to help other people. Likewise, the villa is ours and we have absolutely no intention of selling it. How many grams of PPF per 20Kg bag of cement did you use for the screed mix?    

> 1. Make sure you research your screed correctly. Your not on a concrete slab like I was and most screeds I saw mention a minimum thickness if you go unbonded. Unmodified Traditional screed needs to be a minimum of 40mm if compacted very well and some say 50mm. Your drawings show your going much thinner so this will be critical to select a screed that can handle floating and also your thin sections.

  
Noted but I really wish there was some sort of gunky acrylic or similar alternative to sand/cement that tile adhesive would stick to. Maybe I should talk to Sika again about this. From memory if I recall correctly AS 3958.1-2007 says something along the lines of: 
Bonded screeds - Thickness can range from 15mm to 40mm unreinforced (don't think I'd ever want to go down to 15mm unreinforced.).
Unbonded/Floating - Minimum thickness of 40mm high if unreinforced (can't remember if you can go thinner with unbonded/floating if you provide reinforcement). 
Also, I recall a post where OldSaltOz mentioned that screeds less than 30mm thick are prone to cracking. 
Anyway, I guess that's why I was looking to bond the screed so I could thin down the screed height a little bit if I wanted to. Interestingly the original screed was only 25mm thick and had no reinforcement at all. It seems the subject of bonded vs unbonded vs floating screeds is a passionate subject for many people. Some say bond because you don't want a swamp (don't think I would get a swamp with Efflock at 1:50 in the screed but anyway), others say don't bond even on lightweight flooring such as Scyon, CFC or tile underlay because if you bond you may get a point load somewhere that can create uneven stress on the screed/membrane and bad things can happen. Bonding, they say, can also aggravate curling and shrinkage. Then again Crommelin's products for bonding (either High Performance Bonding Agent - which is really their Acrylic modifier product in different packaging - or Powerflex) both provide flex to a bonded screed afterwards anyway. So damned if you bond and damned if you don't bond.   

> 2. Get some polypropelene fibres 12mm long and throw in your screed and don't mix it too long as it can break the fibres up. You soak the fibres first to separate them to get an even mix. Ebay has them shipped from the UK in a small amount. I throw the excess screed out in my dirt driveway and the batches I used the PPF in have stood up very well to my car driving over them compared to other mixes.

  
I guess you mean this stuff:   Concrete & Screed fibres - 900G bag of 6mm polypropylene (PP) fibres | eBay  
The blurb says it reduces cracking and shrinkage, sounds good. It also evenly disperses in the mix. 
I was going to use 50mm square galv mesh but PPF sounds like much more fun and can't corrode either. However, with Christmas fast approaching and trying to get it in from the UK I would be lucky to have it this side of Christmas. Sika also make PPF as well (18mm long) in 600g bags. I'll go to the green shed and see how much it would cost to get it in:   Fibres | Sika Australia Pty. Ltd.  
[EDIT] Just ordered some bags of Sika PPF....   

> 3. You're correct about not putting the top membrane all the way up to the puddle flange but you have not considered the effect of putting an additive in the screed to make it water resistant and then packing it against the flange riser. If water does not soak into the screed due to additives it cant reach the puddle flange by fixing one issue you create another. One idea is to put some foam around the flange, pack the screed and then when it drys pull out the foam so a 2mm gap is left at the puddle flange. Your top membrane creates a funnel to pore it down this gap.

  
Yes, good point will do. I will also need to consider what to do regarding the grout as well. I will be using Mapei Ultracolor Plus which repels water and, ordinarily, you use the grout to lock the grate in place above the puddle flange cap. Hmmm.    

> 4. Not a fan of your top alu angle, just use a single continual alu angle. Putting another one on top will look ugly and wont be needed. Go thick with the alu angle if you have an expansion joint as the glass will push on this and create movement.

  
Agree with your point but too late. I did the two 3mm thick aluminium angles (filled between with Sikaflex 11fc) because of the substrate change and also to keep the two tiled areas either side totally separate. A (made to order) C section will go over the top of the two angles and then a rubber tread on top of that down the track so it will be just like a slim hob with a shower screen fitted directly behind it A LA OldSaltOz's suggestion a while back. It will only protrude above the finished tile surface by 5mm or 10mm depending on height of the final screeds. Photo is after primed coat has gone on:

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## Skinah

> Thank you Skinah for your input. You have restored my faith in people genuinely wanting to help other people. Likewise, the villa is ours and we have absolutely no intention of selling it.

   Likewise, it is good to see another person like myself that believes in giving it a go but only after first LEARNING how to do the job. I get annoyed seeing a weekend warrior grab materials and start without bothering to learn. Not that I know how to do it in your case, but I'll throw some info your way on what I picked up on my learning curve. 
 Yes those are 6mm fibres you linked to, but I used 12mm long ones. Reinforcement was not needed in my situation so I threw them in as over engineering and they did work as the mix I used them in first go was too wet and they helped for sure with shrinkage cracks compared to previous attempts. Its a pain trying to get a good consistant mix for a large room when you are doing the packing and the person mixing has a different idea that screed should look like concrete  :Smilie:  If it is your first time screeding, then mix all the dry stuff in bulk, then if your mix is too wet you already have dry sand and cement to throw in to fix it. To test the water amount, grab a trowel and pat the mix many times over, if water forms when you pat it is too wet, if you can get a nice compacted finish without water then it is right, if you pat the screed and it wont compact nicely and shape good then it needs more water.  
 18mm PPF is probably fine but I'm not an expert, you need to determine if they are suitable for your job or if steel mesh is needed. The dose is around a hand full per wheel barrow load and they don't weigh much. I have some left over and have finished my screeding now (done probably a tonne of dry material in my house now) so if your stuck I can help if you read up on them and wish to use them. They can be a pain to get an even mix through the screed, plus if mixed for long times you can cut the fibres from the sand rubbing on them and when they get too short they are no longer effective. That is probably more for concrete trucks that mix for ages during transit and then the screed is pumped into place. 
 Re thickness and materials you should speak with Tilers as they know their trade, Sika know their own products and they wont tell you about a competitors product. Maybe call parex group as they make screed, additives, grout and adhesives. I used lanko latex instead of your Efflock and their support line was very helpful. 
 RE grout. You want water proof grout as a pinch of prevention is better than a pound of cure as my grand dad would say. Best to stop the water getting under the tiles in the first place. The grout is also how water escapes from the layers of screed underneath if your in a hurry and tile on green screed. All cement based products NEED WATER to gain strength and if you dry them out they never gain their full strength. This needs to be understood if your going to do things differently.  
 Link worth reading  http://www.laticrete.com.au/LinkClic...8%3d&tabid=688 
 Tilers place screed down and then tile on top in X days when the screed contains water still and then the grout allows the water to escape slowly. If you use a water proof grout then the water escaping from green screed can cause issues like the picture 2 in that link. I used epoxy grout which is water proof but any movement in your house can cause a crack and then let water in. Water proof grout will not allow the water back out like normal grout and a wicking action. By fixing one issue you can cause another so it is important to allow a path that works for your puddle flange to allow water to exit. 
 3mm alu angle is good, some of it is 1.6mm I think and that is too thin to have supported by a flexible sealant on one side and glass pushing on the other. No problems your all good  :Smilie:  Do you have a link to OldSaltOz post you refer to? He has good advise that I have read in the past so interested in reading it. My guess is it is based around hot water on ALU frames making the air expand and then when the shower is turned off the air cools and sucks water into the alu channel as the air shrinks. Water trapped = mould. 
 Your membrane looks very neat and tidy for one not yet fully done, well done. don't scratch at the crommelin membrane for at least 4 days or it will peel. 
 Your 5 to 10mm above the tiles is good as it will help stop water getting out under the door of the shower. We used a D shaped alu strip that is 5mm? high and it is needed. When you splash and kick water if will go under the door in a shower of your size with nothing at the door. With our 5mm mushroom D shape we get 1 mouth full of water under the door for a 30 minute shower. 
 If interested you can see my work in progress here which I am about to update with more pictures... http://www.renovateforum.com/f205/sh...nsuite-119419/

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## METRIX

Have you considered Lanko 189, have used this for a few screed situations, it goes off QUICK, and is so strong it takes a sledgie to break it after 24 hours

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## Watters

*@Skinah:*
Thanks for that. And yes I'll take a look at your project thread.
Sorry couldn't find the OldSaltoz link again, although I did find this one again:  http://www.renovateforum.com/f247/sc...oofing-112091/ 
This one interested me because there will also be waterproofing above the screed as well. There are plenty of sobering stories of waterproofing above the screed going wrong when the upper membrane was done and the screed hadn't dried properly.  At least it's not winter anymore... 
OldSaltOz contributed a lot of valuable gems for sure. I'd really like to know what his views were on the "bonding screed or not over lightweight substrates that might flex" conundrum. 
The lower membrane is done now and nobody is allowed to walk in the bathroom for the next 5 days. Then it will be screeding time. So, I've got 5 days to work through a screed strategy before D-day arrives...  *@METRIX:*
Thanks for the tip, I'll look into it. Strong sounds good, regular sand/cement screeds don't have a lot of strength. I guess I'm hoping to have a screed mix which gives plenty of time to play sand castles during the day and then sets within a day or two and gets bone dry within a week. Lanko 189 plus Efflock plus PPF? ... options... Need to do more research and reading and calling people.... I'll get myself some Lanko 189 and have a play with a loose piece of waterproofed villaboard and see what happens...

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## METRIX

> *@METRIX:*
> Thanks for the tip, I'll look into it. I guess I'm hoping to have a screed mix which gives plenty of time to play sand castles during the day and then sets within a day or two and gets bone dry within a week.

  You wont be playng sandcastles with this stuff, it goes off quick, and sets rock hard soon after  :Smilie:

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## Skinah

From what I can dig up it looks like you will need to float the screed and go 50mm thick and also use 3 to 1 sand to cement and make sure it is very well compacted and no excess water used. Do not use fans or heaters in an attempt to get the screed dry enough to put a membrane on top. 
If you're in a hurry to get the bathroom working then use the efflock or any of the many SBR additives on the market in the screed and skip the top membrane as you have one under the screed. Petervm has given you good advice above and has posted you need to use builders black plastic to stop it bonding. Good luck and make sure you know how the full system is going to work before starting the screed.

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## autogenous

CFC the whole bathroom, especially upstairs. Less movement, no substrate change for different movement rates.

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## Watters

> From what I can dig up it looks like you will need to float the screed and go 50mm thick and also use 3 to 1 sand to cement and make sure it is very well compacted and no excess water used. Do not use fans or heaters in an attempt to get the screed dry enough to put a membrane on top. 
> If you're in a hurry to get the bathroom working then use the efflock or any of the many SBR additives on the market in the screed and skip the top membrane as you have one under the screed. Petervm has given you good advice above and has posted you need to use builders black plastic to stop it bonding. Good luck and make sure you know how the full system is going to work before starting the screed.

  Thanks, appreciate the input. Btw, I am in no hurry... 
Not too keen on the plastic sheet, might cause a problem for two layer waterproofing. Some people recommend if wanting to do that to just leave the screed sitting on the WP membrane with nothing in between. Also makes it easier to work with and compact the screed (no sheet to move around as you work). By the way if you do bond and use Powerflex for bonding there would still be a flex capability afterwards. I also don't know if a hard tough screed A LA Lanko 189 is what one wants on timber based substrates. A strong screed which can flex sounds more appropriate, perhaps the PPF will help there. 
What I know I will do so far: 
1. 3:1 sand/cement. Sand will be washed sand from Bunnies in bags. Would have preferred kiln dried sand, [thinks out aloud] maybe I should tip the bags onto some fibro in the garage to aerate.
2. 18mm PPFs for reinforcement.
3. Leave the screed to cure for "two" weeks (AS3958.1-2007 and the Laticrete article Skinah provided refer) before doing any waterproofing on top. I can build the bath side storage cupboard doors while waiting...
4. Keep the neck of the two puddle flanges open regarding the two layers of waterproofing.
5. Use Efflock to help waterproof the screed. Many tile shops sell Efflock and it seems to be a good product so I will use it (already bought it anyway).
6. Keep path down into flange open because of using Efflock (thanks Skinah).
7. Accurately measure all portions e.g. absolute max of 10 litres of water for 20kg of cement is 0.5 : 1 ratio which is the maximum water to cement ratio (less than this would be better). 
That's all I know right now for sure, but it's a beginning. Today I have a lot of reading to do, calls to make (including to Lanko to see if they have something like Lanko 189 which gives you a lot more time to work with the screed but similar strength and permits some degree of flex - if not, I'll just use the regular cement) and do more research.

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## METRIX

Geez, by the time you actually get around to finally putting down your screed in the shower recess, I will have fully ripped out and replaced two entire bathrooms.  :Cool:

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## Watters

> Geez, by the time you actually get around to finally putting down your screed in the shower recess, I will have fully ripped out and replaced two entire bathrooms.

   :Yipee:

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## Watters

> Do you have a link to OldSaltOz post you refer to? He has good advise that I have read in the past so interested in reading it.

  *@Skinah*
Just came across the OldSaltoOz post again regarding screeds thinner than 30mm being at risk of cracking. Here it is:  http://www.renovateforum.com/f209/an...estion-115131/ 
It is also interesting what he had to say about most homes today being tiled with little or no fall. Makes one wonder why bother with a bathroom floor or toilet room screed (shower understandably needs falls) if that's the case?

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## inferno6688

Watters, 
have you done your screed? keen to hear what you ended up doing 
i am about to waterproof and screed, so i am going through the same research as you! 
i want to WP above and below the screed too. Are you going debond the floor/wall corners above the the screed? 
thanks

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## Watters

> Watters, have you done your screed? keen to hear what you ended up doing
> i am about to waterproof and screed, so i am going through the same research as you!
> i want to WP above and below the screed too. Are you going debond the floor/wall corners above the the screed?
> thanks

  Okay, as you have asked, I'll explain what I did and am doing but I am definitely not going to say that what I'm doing is the best way to do things whatsoever. I saw in your separate thread someone suggested using Lanko epoxy primer over scyon, then Lanko self leveller reinforced with water substitute acrylic, then the waterproof membrane. This is very interesting as it avoids the headaches of cement/sand screeds and probably would have been a better solution for the substrate environment I am working with. That said, I do like the fact that my screeds will add weight onto the joists, bearers and brick piers which I feel will be a good thing given recent history of the bathroom. The previous incarnation of the bathroom also had a cement/sand screed incidentally. 
Yes the screeds are done, three in total (shower recess, bathroom main floor, and the toilet). I did one screed each day, the main bathroom floor one being the most intense especially as I was doing the mixing and screeding as well. I mixed dry ingredients first (thank you Skinah!). The villa has a built-in garage with the interior entry door close to the bathroom so I mixed the screed mix on the concrete floor of the garage, which made life a lot easier, brilliant (thank you Petervm!). 
I used Lankolatex 751 and Efflock in the screed and also added 18mm Sika polypropylene  fibres. ParexDavco told me that they had tested Efflock with their Lanko products and had not found any issues. Lankolatex 751 helps give the screed additional strength and flex  capability. Efflock makes the screed pretty much waterproof although Lankolatex 751 helps with that as well. Efflock also protects against efflorescence. Adding the polypropylene fibres should help to minimise cracking in the screed. Lankolatex 751 and Efflock in the screed makes the screed mix feel a bit like plasticine to work with. The polypropylene fibres were a pain to work with though. I separated the polypropylene fibres in a clean bucket of water first before adding them to the screed mix but l still sometimes found clumps of them in the screed mix as I was mixing which I then either removed or separated further by hand. 
I bonded the screeds using Lankolatex 751 mixed 1:1 with cement plus a little bit of water. The standard doesn't say you must use "unbonded" for my environment but uses wording like "if unbonding" and the like. A lot of people said I should go unbonded but I chose not to because of the high risk of the screed lifting and curling if unbonded. I am well aware that by going bonded that movement in the subfloor is more likely to transmit through the  screed (crack the screed) and possibly some of the floor tiles. That said, Lankolatex 751 is SBR based and, coupled with the two layers of  SBR waterproofing membrane, and also the flexible tile adhesive (Laticrete 335) should all help to absorb movement. 
The screeds have been covered by builders plastic sheet and will remain so for another couple of days (7 days max). It is not possible to wet the top of the screeds, the screeds are already fully repelling the water and will not absorb water. After the full 7 days the screeds will continue air drying for another two or three weeks before the upper waterproofing gets done (there are plenty other things that need doing in the meantime, the kitchen needs a makeover as well, need to make some cabinets etc etc). Waterproofing above the screed requires that the screed be pretty much bone dry which is why I need to wait. 
Yes, there will definitely be bond breakers for the floor/wall corners above the screed. I used 3:1 washed sand/cement (stronger) in my screed so some shrinkage is a given and there's always the possibility of some wall movement (tiles on waterproof membrane on villaboard on wood frame) one day. The above waterproofing will first use a small bead of Soudal Multibond SMX35 (polymer sealant with fungicide), plus, after SMX35 has cured, reinforcing fabric with integrated bond breaker. Why? Well, even though there will gaps between floor/wall tiles and edges when tiling (as per AS3958.1) it will just make tiling easier than if putting down a 12mm silicone bead for bond breaker above the screed. Note, as per James Hardie's instructions, after tiling, there will be a silicone bead between floor and wall tiles and also at corner wall tiles which will be colour matched to the grout used everywhere else. 
I didn't use any mesh for reinforcement. As with the subject of bonding or not there are also many (often passionate) arguments for and against using mesh (e.g. mesh itself can compromise the screed etc). 
The waterproof membrane being used is Crommelin's Wetite/Shower waterproofing membrane although K10 Plus seems to be a better product from the many posts about membranes on this forum. Crommelin's membrane doesn't require a primed surface although that was done for the lower waterproofing. For the upper waterproofing, the screed may not be primed given that priming for Crommelin's SBR membrane involves diluting the liquid membrane with water and the fact that the screeds are now repelling water! That said, priming for Crommelin's membrane can, optionally, also be done using Powerflex or the like. I need to make a couple more phone calls....

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## inferno6688

Thanks for the detailed response Watters. 
funny you say that K10 is better than Crommelin, as i am not leaning towards using Crommelin over K10 or Ardex! Looking at the coverage, the Crommelin product seems to go further each bucket to achieve the results.  
I have decided to slightly thicken my screed to a minimum of 20mm at the waste so i can use a straigth sand/cement mix 
This is what i am planning
1) silicon bond breaker. Might use the bandage tape with bond breaker too in the shower area, but just silicon for the rest of the bathroom. Very costly using the bond breaker bandage! Will also use the Gripset bandages around tap penetrations
2) waterproof - leaning towards Crommelin
3) screed- sand cement.- looking at additives like Powerflex or Gripset 11Y
4) under tile heating elements
 5) then another layer of waterproofing on top of the screed
6) tile adhesive - still looking. will need to check if a powder mastic can be used with Crommelin WP 
To help with timing, while i am waiting for the screed to fully cure before the next layer of WP, i will tile the walls and leave off the bottom row of tiles. This will also help protect the top layer WP, and with the bottom row of tiles left off i can still water 200mm up the wall on the top layer.  
Can you guys think of anything i have missed?

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## Petervm

"6) tile adhesive - still looking. will need to check if a powder mastic can be used with Crommelin WP"  For the shower floor I recommend you use a glue that's suitable for swimming pools such as Laticrete 335 or Davco SMP Evo and if you want to avoid having multiple types of glue left over, use either one for the whole bathroom. I prefer 335 because it doesn't require priming on bare Villaboard. On walls I usually use Construction Chemicals - Drymastic or Gripflex for large format porcelain tiles. - http://www.constructionchemicals.com.au/products/ceramic-tile-adhesives/  Most glues should be compatible with Crommelin WP including the ones I mentioned above.

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## Gund

We were also waterproofing below and on top of the screed. Two coats each. A tiler recommended to use a water/cement slurry to bond the dryish screed to the membrane. Been using the shower since 7 months without any issues.

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## Watters

> I prefer 335 because it doesn't require priming on bare Villaboard.

  Regarding bare villaboard, my waterproofing goes all the way up the villaboard i.e. wherever wall tiles are going to go. So all of the tile adhesive will be going onto fully cured waterproof membrane. Yes, I'll be using Laticrete 335 everywhere (floor and wall).

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