Rendering and External Wall Finish Insurance

Most render failures are not failures of the render. They are failures of the thing it was applied to, and the renderer gets the call.

Render needs a substrate it can bond to and that will not move differentially behind it. The problems are familiar: dense engineering brick or smooth concrete with no key, a painted or previously rendered wall where the old coating is failing beneath, blockwork with mortar joints raked inconsistently, mixed substrates meeting in the middle of an elevation, a wall that is damp from a different cause, and an EWI system fixed with the wrong fixings or density of fixings. Apply a good render to any of those and it debonds, cracks or blows within a few seasons.

So the survey before quoting is where the protection is. Record the substrate on every elevation, test adhesion on an existing painted or rendered wall rather than assuming, note mixed substrates and the need for movement joints or reinforcement at the junction, identify existing damp and its cause, and raise anything in writing before pricing. Where a client wants render over a substrate you have flagged, get the instruction. A renderer who flagged a painted wall and was told to proceed is in a different position from one who simply rendered it.

Render rarely lets water through the middle of a panel. It lets it in at the edges, and most of those edges are somebody else's detail.

The recognised entry points are a sill with no throating or a render that runs tight to a frame with no bead, a verge or coping with no drip, a reveal where the render meets a window without a seal, a render carried below the damp proof course so water bridges it, a missing bellcast at a change of material, an unprotected top edge, and a service penetration rendered around rather than sealed. Each puts water behind the render, where it moves laterally and appears somewhere unexpected inside.

So the detailing is where a contractor earns the job and protects itself. Beads and stop ends used as the system requires rather than where convenient, bellcast at the DPC with the render stopped correctly, drips and throatings maintained rather than filled, sealant to frames specified and applied by somebody, and penetrations sealed rather than rendered around. Where a drawing shows a detail that will not shed water, raise it. And photograph the details before any scaffold comes down, because afterwards nobody can see them.

External wall insulation changed what a render failure costs, and it is worth being clear about why.

On a solid wall, a render failure lets water into masonry that can dry out. On an EWI system, the render is the only weather protection for an insulation layer that cannot dry easily and loses its performance when wet. Water behind the render soaks the boards, the thermal performance falls, the boards can delaminate from the wall, and the remedy is stripping and replacing the system across the affected area rather than repairing a crack. On a block of flats or a housing scheme that is a very large figure across many properties.

There are two further consequences. EWI is a tested system: the adhesive, boards, fixings, basecoat, mesh and finish are approved as a combination, so substituting a finish or a basecoat steps outside the approval even if each product is good. And fire performance on insulated facades is a separate question entirely, in a market that has restricted fire related cover since the building safety failures, so a contractor rendering insulated facades should know what their policy says about fire rather than assume.

Render has to be applied and cured within conditions, and the commercial pressure to ignore that is built into the job.

Applying render in frost, when frost is forecast overnight, in rain, or in strong direct sun on a hot day produces early failure: frost damage, washed out surfaces, crazing, differential colour on monocouche, and poor curing. The pressure is obvious: scaffold is on hire, a gang is on site, a programme is running, and a decision to stop costs money today while the failure costs money in two years. Monocouche is particularly unforgiving because colour and finish are the product rather than a coating applied afterwards.

So the stop decision needs to sit with somebody and be recorded. Defined temperature and weather limits for the products used rather than judgement, forecasts checked for overnight conditions and not just the working day, protection and sheeting where a scaffold allows work in marginal conditions, and a record of the conditions when each elevation was applied. Where a main contractor instructs work in conditions you have flagged, that instruction belongs in writing, because the elevation will be looked at long after the conversation.

Water behind the render is the claim. We are an FCA regulated broker and will not tell you we are the best choice. These are the questions that decide it.