Render And External Wall Repairs

Render is a coating, not a structure. When it fails it is usually reporting on something behind it.

Reading the crack

Fine, random, map-like cracking. Shrinkage in the render itself as it cured. Cosmetic. It lets water in over time, so it gets sealed and coated, but the wall is fine.

A straight vertical or horizontal crack in one line. Often at a control joint that was rendered over. The building moves, the joint moves, and the render has nowhere to go.

Stepped cracking following the mortar joints. This is the masonry behind moving, not the render. Worth understanding before anyone patches it, for the same reasons set out in [ceiling and wall cracks and what they mean](/blog/ceiling-and-wall-cracks-what-they-mean).

Cracks radiating from window and door corners. Stress concentration at openings. Very common, usually not serious, but they open up again if only filled.

Drummy and bulging render

Tap along the wall. Sound render is solid and dull. Failed render sounds hollow.

Drummy render has lost its bond to the wall behind. It is being held on by the surrounding render and by the paint, and it will eventually let go.

On an upper level or above an entry, that is a safety matter rather than a presentation one. Loose render falling from height is the reason this gets taken seriously in strata buildings.

Bulging is the stage after drummy. The sheet has separated and is moving away from the wall.

Why render lets go

Water behind it. The most common cause by a distance. Water gets in at a crack, a failed sill, a leaking downpipe or a missing flashing, sits behind the render and breaks the bond.

A weak or contaminated substrate. Render applied over paint, over dust, or over a surface that was never prepared.

No control joints. A large unbroken area of render on a long wall has nowhere to accommodate movement, so it cracks where it likes.

Salt attack. Rising damp brings salts up into the wall. They crystallise behind the render and push it off. This shows as a band of failure near the base of the wall.

The repair sequence

Find and fix the water first. Patching render over an active leak buys a season.

Cut out the failed render back to sound, bonded material, with a clean square edge rather than a feathered one.

Prepare the substrate properly, which usually means cleaning, and a bonding agent where the surface calls for it.

Rebuild in coats rather than one thick application, and match the existing texture. Getting the texture right is what decides whether the repair disappears or announces itself.

Allow it to cure before coating. Render that is painted too early traps moisture and fails again.

Then the coating

A flexible, breathable exterior coating over the whole elevation, not just the patch.

Breathable matters, because a wall needs to let moisture out. A sealed impermeable coating over a damp wall pushes the problem sideways and pops off later.

Coating the whole elevation also solves the colour match, which is impossible to get right on a patch of new render beside twenty years of weathered paint. That is the same reasoning behind doing [external painting](/blog/external-painting-and-when-to-repaint) by elevation rather than by patch.

What this costs if you wait

Render failure is progressive and it accelerates once water is behind it.

A cracked wall repaired early is a small job. The same wall two winters later is a scaffold, a large cut-out and a full recoat.