# Forum Home Renovation Tiling  How thin can the screed be on a shower base?

## ErrolFlynn

I've removed the old screed.  And I'm down to the slab (such as it is).  The slab looks like it used to be a dump for any old cement that might have been left over from somewhere else.  It undulates and is very hard. The waste pipe protrudes about 30mm from the slab.  On one side of the waste pipe the slab is relatively flat but the other side runs up and down.  So, on one side of the waste pipe the screed will be 30mm (the low end) rising to maybe 35mm (at the wall).  However, on the other side of the pipe the screed will also be about 30mm next to the waste pipe (low end) but when placing a slope on the screed the cement will only be about 10 or 20mm thick at the 'high'.  Is 10 to 20mm sufficient?

----------


## Oldsaltoz

> I've removed the old screed.  And I'm down to the slab (such as it is).  The slab looks like it used to be a dump for any old cement that might have been left over from somewhere else.  It undulates and is very hard. The waste pipe protrudes about 30mm from the slab.  On one side of the waste pipe the slab is relatively flat but the other side runs up and down.  So, on one side of the waste pipe the screed will be 30mm (the low end) rising to maybe 35mm (at the wall).  However, on the other side of the pipe the screed will also be about 30mm next to the waste pipe (low end) but when placing a slope on the screed the cement will only be about 10 or 20mm thick at the 'high'.  Is 10 to 20mm sufficient?

  A sand and cement screed less than 25mm will want to crack.
There are materials available that will perform much better and dry or set a lot faster then screed. even a levelling compound with some sand added works. 
PS. Cut the waste pipe level with the slab and run the waterproofing membrane down into it. 
Good luck.   :Smilie:

----------


## ErrolFlynn

I don't think I can cut the waste pipe; cutting it would take it off vertical.  The waste pipe is the end of an elbow. So, cutting it will place it an an angle. I'll try and dig out more concree from the slab so that I have at least 25mm of screed.

----------


## Oldsaltoz

> I don't think I can cut the waste pipe; cutting it would take it off vertical.  The waste pipe is the end of an elbow. So, cutting it will place it an an angle. I'll try and dig out more concree from the slab so that I have at least 25mm of screed.

  The reason for cutting the pipe level with the base / slab is so water that passes through the shower base will drain away. 
If the pipe is left above the slab base and the water proofing has been done how will water get out?

----------


## ErrolFlynn

I suspect my setup is unusual.  The screed will go down first.  The waterproofing is then going on top of the screed and itll extend down the waste pipe a bit.  I would happily have cut off the waste pipe but if I did I doubt Id be able to fit the drain-grate, because the waste pipe is an elbow.  There doesnt appear to be much straight section left on the elbow.  If fact Ive had to file the elbow opening in order for the drain-grate to fit.  I gather that the drain grate needs about 5mm movement in the waste pipe.  That has been a lot of filing!

----------


## ringtail

The seemless epoxy flooring guys do shower screeds using a epoxy/sand mix and it can be feathered down to nothing without cracking, and its waterproof. ( still needs to be waterproofed)

----------


## ErrolFlynn

Concrete tends to be brittle, I know.  And the thinner it gets the weaker it becomes.  I'll dig out a bit more to be sure.  But the fact that the screed will be resting on a slab of concrete has to give it a good deal of protection from cracking.

----------


## ringtail

> Concrete tends to be brittle, I know.  And the thinner it gets the weaker it becomes.  I'll dig out a bit more to be sure.  But the fact that the screed will be resting on a slab of concrete has to give it a good deal of protection from cracking.

  
Not really. The substrate has no real bearing of the screeds ability to resist cracking. If anything it might make it worse but sucking any moisture out of the screed. If you do go with a traditional screed you would want to bondcrete the original slab so it doesnt suck the moisture out of the screed.

----------


## ErrolFlynn

I was advised to get some Bondcrete.  I bought a small bottle of it.  Reading the label gives me the impression that my purpose of it is a minor use for the product.  I was going to splash it around and spread it with a brush. (Actually, with the uses cited I thought I was had aquadhere glue but with a different label.) I was going to put my screed on with the Bondcrete still wet.  But now that I think of that, I'll put the question: should the screed go on top of wet or dried Bondcrete?

----------


## Oldsaltoz

Wet

----------


## heavytrevy

Never use boncrete in a wet area.  :Rolleyes:

----------


## heavytrevy

A cement slurry is the norm, works best for bonding a screed 
How about the OP using some ardex a 45  which is a quick no slump mortar.
tile on in 90mins.
mix with fine sand an it will still be strong right down to a feather edge.   

> Not really. The substrate has no real bearing of the screeds ability to resist cracking. If anything it might make it worse but sucking any moisture out of the screed. If you do go with a traditional screed you would want to bondcrete the original slab so it doesnt suck the moisture out of the screed.

----------


## ErrolFlynn

Never heard of ardex a 45.  Looked it up on the web and it sounds quite good. Perhpas next time.  Unfortunatley, I've already bought my premixed cement and the bondcrete.  Actually, it was the wetseal guy who gave my job a once over and recommended the bondcrete.  Before I got started on all this I got some coaching from a tiler.  It was the tiler who demostrated installing a screed, and his advice was prior to putting the screed down to throw a bit of cement powder around the place on the floor, then splash some water on top of it, and then straight away put the (very dry) screed down.

----------

