Water Control Layer and Drainage Plane

An outer surface is not asked to stop all water. Cladding, render, brickwork and tiling all admit some, through joints, hairline cracks, capillary paths and pressure differences created by wind. The water control layer is the plane behind them that accepts what gets through, keeps it away from the rest of the build-up and returns it to the outside face.

What makes this a system rather than a sheet is direction. Every piece of the plane laps over the piece below it, every flashing tucks behind the plane above and discharges in front of the surface below, and every interruption, an opening, a projection, a change of material, is treated as a place where flow is diverted rather than merely covered over.

The comparison that clarifies it is with the air barrier system. A drainage plane works by overlap and gravity and can be complete with none of its laps sealed; an air barrier is not complete unless they are. Standing in front of an unfinished wall, a loose-lapped shingled sheet is a finished drainage plane and an unfinished air barrier, which is a distinction anyone can make by looking.

Where a single membrane is asked to serve both purposes it has to satisfy the stricter definition of complete, and that decision belongs to the designer and should be recorded. Where the drawing does not say which convention applies, the lap treatment is settled on site by whichever trade installs the sheet, and the looser convention is the one their own craft conventionally uses.

Lap direction and flashing are one mechanism rather than two components. A flashing performs only if the plane above laps over its upstand and it discharges in front of the surface below; get either relationship the wrong way round and the flashing collects water and carries it inward with more certainty than if it were absent. Nothing about the material tells anyone which way round it should sit.

The plane and the drained gap depend on each other. A plane that sheds perfectly delivers water into the gap, and if the gap is bridged at a batten, a lintel, a floor edge or a run of squeezed adhesive, water stands against the construction at that point instead. Equally, an open gap with no exit at its base simply holds what the plane delivered to it.

Fixings turn the plane into a perforated surface by design. The plane has to be able to shed around each penetration, which depends on the fixing being placed where water is already moving downward past it rather than where it is being held. This is why a penetration that would be trivial in the field can be consequential immediately below a sill or a tray.

The backup and the plane together decide whether laps stay laps. Where the background is uneven, unsupported across a stud bay, or moves relative to the sheet, laps lift and open under wind pressure, and the sequence that was correct on the drawing stops being correct on the wall. Tapes and sealants used to hold laps then have to bond to a surface that is moving, which is a different demand from the one made of them in the field.