Cracked or stained render: causes and checks

Reading cracks in render

Cracks fall into a few recognisable patterns. Fine crazing across the surface is usually shrinkage as the render dried; Cornish Lime’s curing guide notes that plastic shrinkage cracks show within the first 48 hours and that wind, heat or strong sun drying the work too fast causes more. Diagonal cracks running from the corners of windows and doors follow stress at the opening, which is why K Rend specifies reinforcing mesh at those stress points. Cracks that join into a network of plates, with hollow sound when tapped, mean the render has come away from the wall.

A crack that is widening, steps through the brick or stone joints, or reopens after repair may reflect movement in the wall itself. That needs a surveyor or structural engineer before any render work. A local repair can deal with debonded or cracked render in one place; it cannot fix a moving wall.

Painting over a symptom changes what is visible. A coating can bridge hairline crazing, but it does not reattach hollow render or explain why a crack keeps returning.

Reading stains

Stains usually point to where water is coming from. Dark streaks below sills, copings and window heads mean the drip on that detail is not throwing water clear. A vertical stripe under a gutter joint or downpipe points to the rainwater fitting; SPAB’s advice is to keep gutters clear and in good repair so water is not concentrated on one part of a wall. A dark band along the base of the wall can come from rain splashing back off paving, or from render taken down past the damp-proof course so it draws moisture up.

Green growth is algae, which thrives on walls that stay damp and shaded. It is cleaned with a biocidal wash, such as the products render manufacturers make for it, rather than a pressure washer that can damage the surface. If it returns quickly, the wall is staying wet, and the reason is usually one of the details above.

White, powdery deposits are salts carried out of the wall as it dries. They are common on new render and on walls that have been soaked, and they usually brush off once the wall dries out. If they keep returning, water is still moving through that area and the source is worth finding before any coating goes on, because coatings applied over salts tend to blister and flake.

Render repair ↗Render colour change ↗

What the position of a mark tells you

Where a crack or stain sits on the wall often tells you more than how it looks. Marks directly below a sill, coping or window head point to a failed drip. A vertical stripe below a gutter joint points to the rainwater fitting. Cracks at the corners of openings point to stress there. A band along the bottom of the wall points to splash-back, or to render carried down past the damp-proof course.

Change over time tells you more. A crack that was there last winter and has not altered is less urgent than one that is new or widening. A stain that appears after rain and fades in dry spells is water running down or soaking in at that point; one that never dries suggests the wall behind is staying wet.

Take one photograph of the whole elevation and one close enough to show the mark, and date both. Comparing dated photographs over a few months shows whether a crack is growing far more reliably than memory does.

The options for Harrogate rendering and repairs include local repairs, coatings and replacement; the position and history of the mark usually show which one is needed.

Matching the fix to the fault

Hollow render is cut back to sound edges and patched in coats; Tendring District Council’s guide says the extent is found by tapping and only the minimum should be removed. Surface crazing can be filled with a flexible exterior filler and coated. A strip blown off along the base by frost is cut back and re-rendered to finish on a bell-cast bead just above the damp-proof course, so the bottom edge drips clear instead of sitting in splash-back.

Where an older stone wall is cement-rendered and the render is cracked and holding water, SPAB’s advice is that the cement may need careful removal by hand and replacement in lime, after a test patch. For stains, fix the cause first, such as the failed drip or the leaking gutter, then clean the wall, then recoat if the colour is still uneven. Recolouring without fixing the cause brings the stain back in the same place.

If several faults appear on one wall, they are not necessarily connected. Treat each according to what it is, and be wary of one large treatment chosen mainly to hide all of them.

For example, on a hypothetical rendered Edwardian semi with a crack below one bay sill and a green streak beside the downpipe, the crack might be cut out and patched after tapping, the downpipe joint resealed, the streak cleaned with a biocide, and the elevation repainted once the patch has cured. Each fault gets its own fix, and the repaint follows the repairs.

Maintenance that stops both coming back

Most cracking and staining starts at the same few places, so a short check each autumn catches it early. Clear the gutters and look at the downpipe joints. Look under every sill and coping for streaks that show a drip has stopped working. Look along the base for a dark band or render that has crept down over the damp-proof course. Tap anything that looks bulged.

At completion of any repair, photograph the repaired area from the same position used before the work, and keep the materials, repair limits and date together. A labelled folder is enough. It helps distinguish a later recurrence in the same place from a new defect elsewhere.

Where a cause remained uncertain, say so in the record. A conditional repair should not be remembered later as a fully diagnosed cure. Keep any recommendation for a follow-up inspection or another specialist with it.

Judge the appearance against the repair that was agreed. A local patch may stay visible because the surrounding wall has aged or its texture is hard to copy. If a wider colour treatment is wanted for looks, treat it as a separate job from the repair.