How to properly waterproof a shower tray area before tiling

Most leaks in a renovated bathroom don't come from the tiling itself, but from what is underneath, in the shower area, applied before the first tile was laid. It's the part that can't be seen when the job is finished, and the most expensive to put right afterwards.

Membrane or resin: two systems for the same function

• Waterproofing membrane for shower areas: a specific system that waterproofs the whole shower area (floor and base of the walls) before tiling. It is bought with a 10% margin for overlaps.
• Waterproofing resin/paint for bathrooms: especially advisable in renovations where the existing shower tray isn't lifted. It covers not only the shower but also the rest of the walls that may get regular splashing, next to the washbasin for example — capillary damp in a bathroom isn't limited strictly to the shower tray. Coverage of 5 m² per litre per coat.

Worked example with resin: for 6 m² with 2 coats: (6 × 2) ÷ 5 = 2.4 litres. This is the same calculation the PresupuestoYa calculator applies automatically for the Waterproofer trade.

The drain: the point that fails most

The trap or drain with a waterproofing collar integrates a flange that connects directly to the membrane or resin, sealing that critical point. It's one of the points with the most leaks in a badly waterproofed bathroom: using a generic drain with no collar and relying only on silicone around it doesn't give the same guarantee as a system designed to integrate with the waterproofing.

Always overlap the membrane onto the drain with the manufacturer's specific collar, not with generic silicone: it is the point of the shower with the most direct water pressure and the first to fail if the seal isn't that of the system itself.

Corners and junctions: don't replace them with more resin

The reinforcing tape and corners are prefabricated pieces (tape and internal/external corners) to reinforce the floor–wall junctions and corners, the points most prone to cracking. Don't replace them with more coats of resin at those points: the prefabricated tape and corners have a flexibility that resin alone doesn't offer, and it is precisely that flexibility that prevents cracking at the junction.

Frequently asked questions

Is waterproofing with membrane the same as with resin? They do the same job, but resin is especially useful when the existing tray isn't lifted and it also covers splash areas outside the shower, such as next to the washbasin.

Why isn't it enough to seal the drain with silicone? Because generic silicone doesn't give the same guarantee as a specific collar integrated with the waterproofing system, at the point of highest direct water pressure in the whole bathroom.

Can I reinforce a corner just with more coats of resin instead of the prefabricated piece? It isn't recommended: the prefabricated tape and corner gives flexibility that resin alone doesn't have, and that flexibility is what stops the corner cracking with movement.

Conclusion

Waterproofing a shower area well means dealing with three critical points at once: the general surface (membrane or resin), the drain (with a specific collar) and the corners (with a prefabricated piece, not just resin). Skipping any of the three is what makes a newly finished bathroom renovation start giving damp problems months later.