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I'm renovating a bathroom in my apartment and would like to fit 12mm Travertine tiles in a bath/shower area. For most of the walls in this area I have used cement board (Hardibacker 500) attached directly to suitably spaced studding. However, I have one wall which is a boundary between apartments and I'm not allowed to strip it back to the stud to re-clad it.

By drilling a couple of holes I've found that this wall seems to be constructed from double thickness (~30mm) of Gyproc Soundbloc and is painted. I had planned to remove the paint, screw and glue some 6mm cement board (Hardibacker 250) to the wall, and then seal all the edges with silicone. However, I'm worried that moisture could still permeate through the backer board and gradually weaken the plasterboard.

How I can prepare this wall for reasonable heavy tiles, taking into consideration that it'll be a wet area?

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You should be concerned; cement board is not a moisture membrane. It is used as a tile backer because even if it does get wet it stays structurally stable (which is good to keep your tiles from coming off) but it is not waterproof. –  auujay Mar 18 '13 at 15:13
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A membrane (whether thin sheet or liquid) applied over the backerboard, is a vastly superior means of waterproofing, This is because it sits immediately under the tile, as opposed to behind the backer.

Red Guard and HydroBan are some liquid membranes and Kerdi and NobleFlex are thin sheet membranes.

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Thanks - I've checked what's available in my area and am going to try out this Mapei shower waterproofing kit: mapei.com/public/GB/products/shower%20waterproofing%20kit.pdf –  teabot Mar 25 '13 at 10:24
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You can attach the cement board like you said (minus silicone) and them use RedGuard waterproofing membrane or similar membrane. Roll or trowel on two coats and water will not get through. You must use a mesh reinforcement at corners. Just be sure to plan it out well, as this stuff is not forgiving to work with and will set quicker than you think.

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