Waterproofing a New Building: What to Do While You Still Can

Almost everything on this blog is about repair — finding a leak, opening it up, putting it right at three times what it would have cost to do properly the first time. If you are building now, you are in the one position where none of that is necessary. Waterproofing during construction is not an add-on at the end; it is a set of decisions distributed through the whole build, most of which become unreachable once the next trade arrives.

The rule that governs all of it

Waterproofing is a system, and the weak point sets the performance. A terrace treated impeccably and a parapet junction left square will leak at the junction. Great material laid on green concrete will fail. There is no averaging — you do not get a partly waterproof building.

Stage by stage

Foundations and plinth

Only opportunity, ever, to treat the outside face of a basement or retaining wall. Once the excavation is backfilled, external treatment is gone as an option and every future problem becomes a negative-side problem — harder, more expensive, less reliable. This is the single highest-value stage.

Also: the damp-proof course at plinth level, continuous and unbridged, so you never meet rising damp. And drainage — get water away from the structure rather than asking a coating to hold it back.

Concrete work

The best waterproofing is dense, well-compacted, properly cured concrete. Honeycombing, cold joints, inadequate cover and rushed curing create the paths that everything downstream has to compensate for. Curing in particular: it is unglamorous, it costs almost nothing, it is skipped constantly, and it changes how the structure behaves for its whole life.

Plan construction joints deliberately and detail them with water stops rather than discovering their location later as a leak.

Wet areas — before the tiles

Bathroom and kitchen floors, and the walls of shower zones, get waterproofed before finishes. Not after. Membrane on the floor, turned up the walls, coved at the junction, sealed around every penetration. Flood-test each wet area before tiling — block the trap, hold water, inspect below. An hour's work that catches the fault while it costs nothing to fix. See bathroom waterproofing.

Roof and terrace

The falls are set now, in the screed, and they are permanent. Get the slope right toward outlets set at the correct level, cove every parapet junction, detail every penetration, then treat — and flood-test before tiling. The layer order matters and is very hard to correct later.

External walls, before plaster and paint

Now is when you get the junctions right: around window and door frames, at the sill and its drip, at balcony junctions, at every service penetration. These are the details that let driven rain into an otherwise sound wall, and they are cheap to do and awkward to retrofit.

Finishes

The last layer, and the least important structurally. If the stages above were done, the finish is decoration and protection. If they were not, no finish rescues it. This is also the point at which marble texture paint goes on the elevation — over a wall that is dry and sound, having been built to be dry and sound.

The five things to insist on

Cure the concrete. Properly, for the full period, in writing in the contract.
Cove every internal junction. Slab to parapet, floor to wall, base to wall in every tank.
Flood-test every wet area and the terrace before covering. Non-negotiable, cheap, and it is the only real verification you will ever get.
Detail penetrations individually. Every pipe, conduit, drain and fixing. Most leaks in new buildings are at penetrations.
Photograph everything before it is covered. A folder of dated photos of the membrane, the coving and the outlets is worth more in year eight than any warranty document.

On sequencing and other trades

The most avoidable failure in new construction is a perfectly good membrane punctured by the trade that came next. Protect it as soon as it cures, keep traffic off it, and do not let anyone drill, cut or fix through a completed waterproofing layer without the detail being made good. Say this on site, more than once.