Practical guide
Wind-driven rain: details to check on a façade
When render fails on a wet wall, the cause is usually at a detail rather than in the middle of the face. Sills, frames, pipes, copings and the base of the wall are where driven rain collects and finds a way in. Checking those details before a wall is re-rendered saves doing the work twice.
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How driven rain gets behind render
Rain blown against a wall wets the whole face and then runs down it. Sound render sheds most of it, but water gathers on any ledge and at every junction, and it gets behind the render through cracks, open joints and gaps around frames and pipes. Once behind, it has trouble getting out, particularly through a hard cement render, which SPAB says restricts evaporation from older walls.
The signs are local: staining below a sill, a hollow patch under a downpipe joint, cracks radiating from a window corner, a damp band at the base of the wall. Each points to a detail worth checking before any new render is specified.
The top of the wall
Start at the top, where water first arrives. Gutters should be clear and joints sound; SPAB's advice is to clear them after the autumn leaf fall and check for leaks in heavy rain, because gutters crack where their bolt fixings rust and downpipes leak at the joints. Where render meets the soffit or eaves, K Rend's detailing sheet recommends a stop bead for a neat edge and, if needed, a sealant joint.
Parapets and garden walls need copings that oversail the face with a drip, so water falls clear instead of running down the render. Where a lower roof meets a rendered wall, ask how the flashing and the render edge will meet, because an open joint there lets water track down behind the render.
Windows and sills
Sills are the most common source of local damage. A sill should project clear of the render with a drip underneath, so that water falls off rather than running back along the underside and onto the wall; K Rend's detailing sheet gives a minimum drip of 40mm. Its notes also call for a stop bead at the reveal, with sealant to the frame, warning that rendering up to the frame without a sealed joint can let moisture in.
At the head of each opening, mesh bedded in the base coat and lapped at least 300mm onto the wall resists the cracks that start at the corners. When new render is planned, check that the existing sills project far enough for the new thickness. When insulation is planned, the wall face moves outward by the board thickness, so the Energy Saving Trust's warning that windows, pipes and roof edges are affected applies: sills will need extending or replacing.
Pipes, brackets and fittings
Every bracket and pipe through the wall is a potential leak. Downpipe clips, cable fixings, satellite brackets, vents and outside taps should be taken off before rendering where possible and refixed afterwards with the holes sealed. K Rend recommends stainless steel brackets for anything fixed to finished render, because rust from cheap brackets stains the surface.
Downpipes need checking joint by joint. A pipe that leaks at a joint wets the same strip of wall every time it rains, and the render below will fail in a line. Fix the pipe before the render, not afterwards.
The base of the wall
Render must stop above the damp-proof course. If it runs down to the ground, it bridges the DPC and draws ground moisture up into the wall. K Rend's detailing sheet is clear that render should not bridge the DPC, that the DPC should project past the face of the render, and that a bellcast bead forms the drip for roughcast or dash finishes, with a drip bead under textured finishes.
Check the ground outside too. Paths, patios or soil built up against the wall above DPC level defeat the detail, however well the render is finished. If the ground has crept up over the years, lowering it or adding a drainage channel belongs in the job.
When the new render is thicker than the old
Details that worked with the old render may not work with the new. K Rend's K Mono, for example, must be at least 15mm thick after scraping, and a base coat with a thin finish adds its own depth. If the wall is stripped first, the new face may finish close to where the old one was; if a new system goes over sound existing render, the face moves outward.
Either way, measure at survey stage. Check that each sill still projects with its drip clear of the finished face, that downpipe brackets are long enough to hold the pipe off the new surface, and that the render will stop at a bead rather than running over the edge of a stone surround. Sorting those points out on paper is far easier than improvising on the scaffold.
Reading the marks before choosing a fix
For example, imagine three marks on one hypothetical wall: a vertical stripe below a downpipe joint, a stain spreading from under a sill, and a dark band along the bottom 300mm. The first points to the pipe, the second to the sill and its drip, the third to the base detail or the ground level. Each needs a different repair, and none is solved by a new coat of paint on its own.
The location of a mark is a clue, not a diagnosis, and two causes can overlap on the same patch. That is why a survey checks the details around a mark as well as the render itself.
When the work is finished
Before the scaffold comes down, look at the details rather than the big flat areas: the sealant at each frame, the drip under each sill, the bead at the base and the refixed pipes. Ask for photographs of high-level details you cannot see from the ground. Keep the product names and colour references with them.
The Scarborough rendering homepage links the guides to repairs, single walls and whole-house work if the survey finds more than a detail at fault.
Common questions
Should render go down to the ground?
No. It should stop above the damp-proof course on a bellcast or drip bead. Render carried down to the ground bridges the DPC and keeps the base of the wall wet.
Can sealant fix a leaking window junction?
Sealant at the frame is part of the correct detail, usually against a stop bead, but it will not make up for a sill with no drip or a cracked reveal. Check the whole opening.
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