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    Hello everyone, hope you are doing well!

    I wonder if you could help. I am renovating a bathroom in a Victorian house, and my builder has proposed an unorthodox detail. Having sought advice from a few suppliers, I got confused as they are all saying different things, yet I'm sure that this sort of situation must arise often in practice and can be easily resolved...

    The attached drawing shows the detail, with tiling over an existing ground floor which is part suspended timber joists and part suspended concrete slab on brick sleepers.

    Our builder has proposed the construction shown on DETAIL 1, with new plywood substrate laid over existing floor joists (item 5), and a layer of cement board on top bonded to the plywood with the tile adhesive.

    To deal with the joint between concrete and timber substrates, he proposes to oversail the cement board over the concrete (item 6a) and to pour self levelling screed (e.g. Mapei Ultraplan Renovation) over the concrete to bring it level with the cement board (item 7). A second layer of cement board will then be laid on top, again bonded with tile adhesive and fixed down with extra screws (item 8). A liquid waterproof membrane (e.g. Mapei Aquadefence) then goes on top (item 9), then the tile adhesive and tiling. The theory is that this avoids having to have a movement joint between the two substrates which could be problematic for water penetration.

    Is this detail acceptable, from your point of view? If so, what thickness of cement board is OK to use? Can I get away with 6mm, or should it be 12mm?

    Also, do you think the plywood base is adequate (18mm marine grade ply)? Some suppliers are adamant that I need 2 layers of plywood which seems excessive. The existing timber joists are 45x95mm @400cc. The tiles are 9.5mm porcelain.

    Would be most grateful for any tips and advice!
    • CommentAuthortony
    • CommentTimeSep 3rd 2017
    It does not meet building regulations part L1b, I reckon the grout and tiles will crack above the change of structural substrate.

    Have you considered wet room floor, new over the whole of the mew room? Or anew. Floor dispensing with existing.

    There is a danger of insulation getting wet and causing the wood to rot.
    Thank you!

    By wet room floor do you mean screeded over whole bathroom? or a decoupling type membrane?
    Having a brand new floor structure in place of existing is unfortunately not possible. :(

    Re insulation getting wet, assume you mean from bathroom water? Would the Mapei liquid membrane not prevent this?
    • CommentAuthorowlman
    • CommentTimeSep 4th 2017
    There are lots of tanking solutions for wet room applications. Here is just one of them:-


    The floor wall junctions are particularly important, there are products mostly cloth type meshes to seal the junctions. The walls and corners too can be a problem, so best to sort them. As grout lines are vulnerable, so large format tiles tend to be better.
    As a first step I'd make sure the suspended portion of the floor is strong, firm and tight. Decoupling products can provide some movement flexibility. Cement board is just one solution to providing and impervious barrier, you could also look at construction boards, for the walls too, as they come in a greater variety of thicknesses are easier to cut exactly and provide some insulation, important if you put in electric UFH;-- BTW I would.
    Use correct shower drain products too all sealed prior to tiling.
    Decoupling membrane - something like this?
    Owlman, thank you for your advice too!
    • CommentTimeSep 4th 2017
    The builder's proposal is exceptionally thoroughly detailed, which is very unusual in my experience. If he's prepared to guarantee it then I'd be tempted to let him get on with it to his design. If you start changing the design, then you assume the risk if it doesn't work.
    • CommentAuthorCWatters
    • CommentTimeSep 4th 2017 edited
    Our builder has proposed the construction shown on DETAIL 1, with new plywood substrate laid over existing floor joists (item 5), and a layer of cement board on top bonded to the plywood with the tile adhesive.

    I'd want the plywood (and possibly the cement board) screwed down at say 200mm intervals.

    There might even be a case for adding additional floor joists or noggins to stiffen the floor first before putting down the plywood.

    There are also different grades of WBP plywood. One company local to me stocks three.
    • CommentAuthorowlman
    • CommentTimeSep 5th 2017 edited
    Having a structurally sound floor is obviously a very good plan, as is consideration for any potential movement considering the two differing substrates.
    It's only a small step from there to create a waterproof floor i.e. wet room as tony suggests. Your builder appears to have thought out the structural detail well enough but if you go down the wet floor route, and it's what I would do, then you need to eyeball the detail and products.
    Belt and braces is my rule of thumb, and the extra few quid now, in tanking solutions plus membranes will pay dividends later,-- don't skimp, water will find ANY weak spots, drain outlets are a good example, especially showers, which by nature can be difficult to get to.
    Thank you all for your advice!
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