wet room on insulated solid floor

1 May 2018
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United Kingdom
I am converting one end of a garage to be a guest bathroom with level access shower. The existing concrete floor will be built up around 200mm using PIR insulation and screed.

My question is how do i fix the wetroom former? Can i lay the former directly onto plywood over the cellotex or should I omit the cellotex altogetether in the shower area and fill the void with more concrete or self leveling screed upto the right height to lay the tray?
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That's one I haven't seen before, however you may wish to consider using a shower tray riser kit (legs) to raise the tray, filling the void with insulation and securing/supporting the drain trap with expanding foam. This will avoid cold tootsies when first stepping into the shower.
Cant say I've ever seen a former being raised on a riser kit, formers usually always need to be sat flat to a solid floor and set level or a whole sub floor is built up if waste etc is to be run.

You said the floor is going to be celotex and screed? If so then the former can be set directly onto that. How it is set in would be down to the manufacturer, usually it uses a flexible floor tile adhesive. Other use their own bespoke methods, follow the MI for that. I wouldn't fit a former directly to the PIR as there could be too much point flexing IMO.
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Yes Cellotex or kingspan (120mm) and self levelling screed.

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