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I have a windowsill in a shower which had regular 6" ceramic wall tiles over drywall.

Some water had obviously pooled on this, got under the joint at the lip of the tiles and rotted the drywall. So I removed the drywall, left it to dry for a month and now I have to repair it.

I have some waterproof, for bathrooms, grout some special bathroom grade drywall and the same tiles.
Is there anything else I can do to prevent it happening again?

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Thanks for the better title! Specifically the problem is that water will pool on the flat sill and on the grouting. Am I better off using silicone around the tiles rather than grout? Or having the tiles overhang to make a drip ledge? –  mgb Jul 23 '12 at 20:33
    
I'd like a picture. But, you might be able to retile the windowsill, and by carefully buttering the back of each tile with the tile cement, build up a slope. This should stop the water from pooling. Also. Caulk every time you create an inside corner. –  Chris Cudmore Jul 24 '12 at 20:32
    
@ChrisCudmore - doing that. I was more concerned about when it ran to the edge of the sill and then onto the wall - how to stop it coming back under the horizontal tiles? –  mgb Jul 24 '12 at 21:26
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2 Answers

I would never use tile tor the window sill. The tile itself is waterproof, but the grout seams will always crack, the thinset is permeable, and the water will find a way to get behind the wall. Get a piece of stone custom fit, make sure it does not puddle water near the wall, and then you only have to worry about the edges which can be sealed with a silicone caulk

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+1. Preventing the water from pooling is key here. –  Tester101 Jul 24 '12 at 11:43
    
I can put a slope on the sill so the water (mostly) flows off. But regular tile doesn't give me a drip ledge - so as the water flows off it will go into the join between the edge of the sill and the wall below it. –  mgb Jul 26 '12 at 19:26
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Tiles and the thinset adhering them are water permeable. This is by design so that water can escape them.

If you have a tile surround in a shower, the backing of that tile should be a suitable vapor-proof membrane such as Kerdi. Drywall is not a suitable backing for tile in a wet area.

If you don't want to use a membrane then the backing of the tile should be a permeable material like concrete board. It can be backed with a waterproof membrane to keep your wall cavity dry.

If this is a tub-shower then the backerboard should extend all the way to the tub lip with the tile extending slightly over the lip... modern tubs have a 1/4" mounting lip for specifically this reason.

If it is a walk-in shower then you should have a suitable membrane continuous (or at least "shingled") to the floor pan.

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This is great information for anybody planning to build a shower, but what could be done to prevent the OP's situation given that they're likely not going to rebuild the entire tub surround? –  Tester101 Jul 24 '12 at 11:47
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