Getting my wet room project back on track - is this a good plan?

[ojtiler]

Hi,I would appreciate any help confirming the final steps for my wet room project, please (see photos below)The room is about 2m x 2m. I have already virtually rebuilt the room - as the floor joists had bent under the weight of an old cast iron bath (now removed) and internal walls were built directly onto the floor boards!! I doubled up the joists and have built 2 new stud walls. The photos below show some of the stages of the build, so far. In order to accommodate the new layout, I have installed new supply & waste pipework.I'm using a Linear shower tray from Impey in the corner where the stud walls meet - in order to simplify the plumbing!:smilewinkgrin: The tiling I have sourced is 600x300mm porcelain tiles with a weight of approx. 24kg/sq.m.Unfortunately, although the build has gone really well so far, I've made a couple of errors😳...I left my research on wall material a bit late - and have already fixed 12mm WBP ply to the stud walls. Although I have not received any reply to a related post, I've pretty much made up my mind this is going to have to come off (as it may lead to tile movement/cracking later on) and I will fix backer boards instead. I've also screwed the new floor down (22mm WBP ply to be level with the Impey shower deck) before reading that this should be primed on the underside and edges joins. The floor ply is good quality WBP from Latham Timber, so I'm not sure if I really need to take it up to prime it? The 22m ply is in 2 panels, the join being directly over a joist line.So, here is my plan for the remainder of the build - I would be grateful for any replies suggesting any changes, or additional measures - as I really want to do the job right.

1. take off wall ply and replace with Marmox board (does this need to be minimum 15mm thick?, are Marmox boards ok health-wise?... I've read a couple of posts ref dust/silicosis on some backer boards:yikes:. Do the Marmox boards need treating e.g. with primer before I tile?)
2. unscrew floor ply, prime the back and edges and re-screw to the joists with screws ~150mm apart(do I need to over-board the floor with Marmox board, too?, this might reduce the risk of tile movement above the join in the panels?)
3. fix tiles to walls with Larsen Flexible Standard set powdered adhesive above battens on walls
4. prime floor & fix electrical underfloor heating (e.g. Impey), then screed off with levelling compound (is this right to put the ufh below the membrane?)(I want to fit the UFH over as much of the shower tray as possible, but how do I best deal with the change in level where there is no cable??.... just by applying more adhesive? - the shower tray is obviously on a gradient at this point)
5. apply Impey supplied primer & tanking membrane to floor, bringing the membrane ~75mm up the walls at the edge
6. finish tiling wall at bottom & fix floor tiles
7. grout with easypoxy from BAL

So - how does the plan sound? Have I missed any big steps?! Very grateful if anyone could take the time to share their experience and adviceMany thanks in advance....

 

[PHG Dave]

Hi, in response to your questions

1. 12.5mm Marmox boards would be fine, no issues of dust/silicosis due to the fact that they are polystyrene core with glassfibre reinforced polymer-cement skin which means they can be cut with a knife, are fully waterproof and do not need any priming prior to tiling.

2. 10mm marmox boards on the floor would give you a stable, waterproof and heat insulating floor on which to lay your heatmat.

 

[Gazebo]

I might be too late on this one but you could leave the ply on th walls in place and screw 10mm marmox directly to it.

 

[ojtiler]

Hi,


Many thanks for taking the time to reply to my post - and for the advice...


PHG - would I be ok with Marmox panels thinner than 10mm for over-boarding the floor? Reason I ask is that the wet-room flooor is already going to be a bit higher than the level of the adjoining landing (although I've just thought that I could fit the plywood I remove from the walls onto the floor to bring it up a bit!!:smilewinkgrin🙂.... I think Marmox comes as thin as 4mm? - would this be thick enough (there is no deflection in the floor).

Gazebo - thanks for the idea of over-boarding the walls, but the room is already really tight & I will have problems around the doorway area if my wall covering gets any thicker.

 

[bugs183]

I can't see a problem using the thinner boards, if the wood underneath is rigid and flat i'd set them in to flexible floor tile adhesive use a 6-10mm notched trowel, and bed the boards in. Get a straight edge on them to check for flatness, then screw them down as well.

 

[ojtiler]

I can't see a problem using the thinner boards, if the wood underneath is rigid and flat i'd set them in to flexible floor tile adhesive use a 6-10mm notched trowel, and bed the boards in. Get a straight edge on them to check for flatness, then screw them down as well.

bugs183.

Thanks for your reply. Yes - the floor is really solid (no deflection); the joists are fairly close together, doubled (to bring the floor level) and I have noggins across the centre of the room, too. Ref. the fitting technique you recommend for the Marmox boards, should I screw them immediately after bedding them on the adhesive?, or after they have set. I'm thinking the latter is probably better, to avoid distorting them as I tighten the screws?


Cheers.

 

[bugs183]

Wait for them to set, as you say the boards will distort otherwise.
Have fun!!

 

[ojtiler]

Cheers - thanks for clarification....