Wet Area Sheeting and the Shower Recess
You are lining a bathroom in a renovation. The shower is a walk-in recess 1200 mm wide by 900 mm deep, formed by a tiled hob and a half-height wall. The existing wall framing is sound but the studs are at 600 mm centres, and one stud is set back 8 mm from the others, leaving a hollow behind the sheet. The client wants large-format tiles (600 x 300 mm) on the shower walls. The floor is timber-framed and the shower floor will be tiled to a fall. The waterproofing membrane is to be applied by a separate applicator after sheeting.
1. What sheeting and framing issues must be resolved before the waterproofing applicator arrives, and why does the order matter?
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Waterproofing is only as good as the substrate under it. The applicator will not fix framing, so every substrate issue must be closed out first.
Issues to resolve:
- The 8 mm set-back stud creates a hollow behind the sheet. Large-format tiles are stiff and will bridge the hollow, but the sheet will deflect when loaded or when the tile adhesive shrinks, and the membrane over a moving joint can crack. Pack the stud out or plane the others so the framing is in one plane within tolerance before sheeting.
- Stud spacing at 600 mm may be too wide for the sheet type and tile weight. Check the sheet manufacturer's fixing and support requirements; large tiles on a flexible substrate need closer support, often 450 mm or a thicker sheet.
- The shower recess needs the correct sheet for a wet area, fixed with the right fasteners and with joints set out so they do not coincide with the tile joints where possible.
- The floor-to-wall junction and the hob need a rigid, sound substrate with the correct fall, because the membrane turns up the wall and any movement at the junction will show as a crack in the tile line.
The order matters because the membrane is a thin, flexible layer. Any deflection or movement below it transfers through it. Once the membrane is on, fixing the framing means cutting the membrane, and the warranty and the integrity are gone. So framing, packing, sheet fixing and joint treatment are all signed off before the applicator starts.
2. The applicator says the membrane can go straight over the sheeting with no primer. How would you check that, and what would you do if you disagreed?
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The membrane system is a proprietary product, so the manufacturer's instructions govern: which substrates need a primer, which need a sanding or a skim coat, and what the minimum and maximum recoat times are. Some sheet substrates are porous and need a primer to stop the membrane drying too fast and pin-holing; some are smooth and need a key. The applicator's blanket statement is not evidence either way.
Check by asking for the product data sheet and the manufacturer's substrate table, and compare it with what is actually on the wall: sheet type, surface texture, and whether the joints have been taped and set as the system requires. If the data sheet says primer is required and the applicator will not use it, that is a defect risk you should not accept.
If you disagree, raise it before work starts, in writing, with the data sheet reference. Offer to get the manufacturer's technical line to confirm. If the applicator still refuses, escalate to the builder or client, because the waterproofing warranty will sit with whoever specified the system. Document the sheeting condition with photos before the membrane goes on, so there is a record of the substrate you handed over. The judgement is that you are responsible for the substrate you provide, and the applicator is responsible for the membrane, but the join between the two is where failures happen.