# Forum Home Renovation Bathrooms  Waterproofing under or over screed?

## bosun

I'm going to screed a fall for a floor waste for the bathroom floor. The whole floor will be waterproofed before tiling of course.
Could someone help with some info please? (sorry if the info is already there somewhere in the forum - I had a quick look but no luck). 
The floor is 15mm compressed cement sheet on timber joists. 
1. Is there any preference between using a mortar bed vs a levelling compound (or some other product?)
2. Should I waterproof above or below the screed (with appropriate priming)?
3. If waterproofing above the screed, should I still prime the cement sheet before putting down the screed? 
Any helpful answers much appreciated 
cheers 
Bosun

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## Oldsaltoz

> I'm going to screed a fall for a floor waste for the bathroom floor. The whole floor will be waterproofed before tiling of course.
> Could someone help with some info please? (sorry if the info is already there somewhere in the forum - I had a quick look but no luck). 
> The floor is 15mm compressed cement sheet on timber joists. 
> 1. Is there any preference between using a mortar bed vs a levelling compound (or some other product?)
> 2. Should I waterproof above or below the screed (with appropriate priming)?
> 3. If waterproofing above the screed, should I still prime the cement sheet before putting down the screed? 
> Any helpful answers much appreciated 
> cheers 
> Bosun

  1. A mortar screed must be at least 25mm at the thinnest point or it may crack and fail. Levelling compound and other screed types available will go down to a few mm with no problems, and dry much much faster than screed. 
2. Above or below depends on how long you want the job to take, waterfroof first and you put your screed down and start tiling the walls.
To waterproof above the screed means waiting till it's completely dry, and can take days.
90% of new homes have the screed added after waterproofing and no problems. 
3. Will depend on what product you use to do the waterproofing. 
Good luck.   :Smilie:

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## bosun

Many thanks Olsaltoz. Just what I needed to know. 
I reckon you're the lighthouse among the shoals of DIY - thank you again for all your positive and extremely helpful input to the forum. 
cheers 
Bosun

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## an3_bolt

:Wat they said:  
Just to share - I have just done our parents holiday house. Waterproofing product I used to be compatible with the tilers adhesive was one that needed to be placed above the screed. Then the fun started! 
Winter time - the sand cement screed took a very very long time dry out properly......and even then the technical advice from the manufacturer was to apply an epoxy moisture barrier prior to applying the waterproofing membrane. Again - the membrane took an extraordinary amount of time to dry with near 100% humidity overnight..... 
If I had my time again:
1. Do it when it is not so cold - ie moderate temperatures. If unable to comply with this I would use a different system.
2. Another way of avoiding the drying screed problems is to use a modified screed - I have noticed that both Bostik and Davco/Lanko have these available which means you can get onto it very quickly.
3. The product for waterproofing you use will specify if you need to prime or not. Some are suitable for below screed and some are only suitable for above screed with correct falls to waste.
4. Screeding onto compressed cement - I have seen a cement slurry onto the compressed cement then followed by the screed and also the bondcrete seal/application (on their website) methods. I have also seen no prep at all with the screed just laid onto the cement sheet and also a pvc slip sheet on the sheet then the screed on top......which is right/best - I have no idea!
5. If in doubt about your waterproofing - get a pro in - it is an expensive exercise to demolish a bathroom and redo it (which is what it can end up being with a failed waterproofing membrane) - as i found out with an inherited bathroom from when we bought our house. 
I second the motion - "Free Beer for Life" for Oldsaltoz!! I certainly appreciate your help and sage advice too!

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## shauck

I'm curious. What is the preferred method and compatible products? I know, I'm generalising but is there sort of an answer to this?

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