Repairing render where an extension meets the house

Why render cracks where an extension meets the house

An extension and the house it joins are two structures. The extension sits on newer foundations, its blockwork shrinks as it dries and it may settle slightly in its first years; the old house has long since finished moving. Render carried continuously across the line between them is asked to stretch across that difference, and it usually answers with a straight vertical crack at or near the junction.

The right repair method depends on whether the movement has stopped. If the crack is old, fine and not widening, the junction may be stable, and the render can be cut out along it and rebuilt with reinforcing mesh bedded across the line. If the crack opens and closes with the seasons or keeps widening, the render needs a deliberate joint there instead: stop beads on each side so the movement happens at a straight line rather than tearing the render. A crack that tapers, steps through the masonry or is accompanied by sticking doors needs a surveyor first.

Render repair ↗

Reading the crack

A few observations go a long way. Note whether the crack is exactly on the junction or a little to one side, whether it is the same width top to bottom, and whether it has reappeared after an earlier repair. Tap the render on both sides: if it rings hollow, water has probably got in and loosened it, and the repair area will be larger than the crack.

Look at what is above. A leaking flashing where the extension roof meets the house wall, or a gutter that overflows at the junction, will keep wetting the crack. Water getting in at that point is a reason for the render to fail even where the structure is stable.

When contractors disagree, compare what each has seen. One may have tapped and inspected the junction closely, another priced from photographs. An explanation based on an unseen wall should stay provisional however confidently it is put.

Is the junction still moving?

The simplest test is time. Mark the ends of the crack with a fine pencil line, note its width and the date, then look again after a few months and through a change of season. A crack that has not grown is probably old movement that has stopped; one that has lengthened or widened is still moving.

The age of the extension matters. Blockwork shrinks as it dries in its first years, and a new extension often settles slightly, so a crack in a recent extension may be settling-in movement. A crack that appears in an older extension, or after nearby drainage or ground works, needs more investigation.

Signs beyond the render, such as cracks continuing through the masonry or the inside plaster, or doors and windows near the junction starting to stick, point to movement in the structure and need a surveyor before the render is touched.

Repair options at the junction

A typical repair cuts the render back on both sides of the crack to sound edges, clears loose material, repairs the masonry and any open joint, and rebuilds the render in the same number of coats. Where the junction is stable, mesh is bedded across it. Where it is not, stop beads are fixed on each side and the render on each wall finishes to them, leaving a clean joint. Either way, the flashing or gutter feeding water to that point is repaired at the same time.

If the extension is block and the house is stone, the render belongs on the extension only. Bradford Council’s stone guide advises against rendering traditionally bare stone, so the extension’s render should stop on a bead at the stone corner, with the stone left clean.

The quote should show the expected repair area and how a larger area found when opening up will be agreed.

The commonest false fix is a rigid filler pushed into the crack and painted over. If the junction is still moving, the line comes back within a year or so, often with hollow render on either side where water has got in behind the filler.

Finishing the repair

A patch at a junction is usually visible at first because the new render is cleaner and its texture fresh. On a painted wall, repainting the whole elevation once the patch has cured hides it. On through-coloured render the patch weathers in over time but rarely matches exactly.

A beaded joint is meant to show. At the inside corner between extension and house, it reads as the natural line between two parts of the building. In the middle of a flat wall, it needs to be straight, plumb and the same width all the way up.

If the whole extension is being recoated at the same time, the coating has to suit both the old render and the new patch, and the stopping point is best taken at a corner.

When two quotes describe different repairs

One proposal may cut out a narrow strip and fill it; another may form a movement joint with beads; a third may re-render the whole extension wall. Before comparing totals, ask why. The difference usually comes down to whether each contractor thinks the junction is still moving, how much hollow render they found, and whether they have allowed for the water source.

A narrow filled strip is the cheapest and the most likely to crack again if the junction moves. A beaded joint accepts the movement. A full re-render of the extension wall may be justified if much of its render is hollow, but not simply to hide the crack. Choosing the smaller patch only because it looks simpler can leave the cause in place.

Keeping an eye on it

Photograph the repair when it is finished and again a year later. If a fine line reappears exactly on a beaded joint, the joint is doing its job. If cracks appear beside it, the movement is larger than expected or water is getting in, and the flashing and gutter above are the first things to check.

The Bradford rendering overview compares local repairs with re-rendering a whole wall, for cases where much of the extension render has come away.