Cracked or Hollow Render: What Tap-Testing Tells You

What hollow render means

Render is meant to be stuck to the wall across its whole back face. Hollow render is render that has lost that bond. Sometimes the coats have separated from one another; sometimes the full thickness has come away from the brick, block or stone behind it. The surface can still look whole while a thin air gap sits behind it, with the loose section held up only by the bonded render around its edges.

The Building Conservation Directory's article on repairing lime renders explains that hollow and detached areas often show themselves through bulges and cracks, and that gently tapping with a chisel or the back of the hand confirms them. Bonded render gives a dull, solid knock; detached render sounds hollow or drummy. Tapping across the area traces the perimeter of the detachment and can show whether the separation is between coats or between the render and the wall.

Hollow areas matter because of water. Rain that gets into a crack can run down behind a detached section and stay there. SPAB explains that dense cement render on an old wall restricts evaporation, cracks as the wall moves and then traps the water that gets in, and that cement renders often fail in patches and come away. The same conservation article names water penetration as a cause of old render separating and bulging.

A hollow note does not always mean the render must come off straight away. Where most of a wall is well bonded, SPAB suggests leaving it and removing only the hollow-sounding patches, and the conservation guidance treats monitoring as a reasonable first step for render that has been detached for a long time without getting worse. When repair is needed, a patch repair cuts out the hollow area to a sound edge and rebuilds it.

Render repair ↗

Reading the cracks

K Rend's maintenance guidance sorts render cracks into four kinds: structural, settlement and shrinkage, stress cracks that are often found at window and door openings, and hairline cracks that affect the surface only. The kind matters more than the width, because it points to where the movement is coming from and whether the render is the problem or only the messenger.

Hairline crazing, a fine network of cracks over the face, is usually shrinkage in the top coat. In new lime render, Cornish Lime notes that plastic shrinkage cracks show within 48 hours if a coat dries too quickly, which is why fresh lime work is kept covered and damp. Diagonal cracks running from the corners of windows and doors are classic stress cracks, and they are the reason manufacturers such as K Rend specify glass-fibre mesh at reveals.

Stepped cracks that follow the brick or block joints, and cracks that also appear on the inside of the same wall, suggest the wall itself is moving. Those belong with a surveyor or structural engineer before anyone talks about render, because a new finish over a moving wall will crack along the same line.

For cracks that are not structural, K Rend's advice is to work out the likely cause with the applicator and, in general, to leave a crack as long as possible to stabilise before repairing it. Small cracks and holes in otherwise sound render can be filled with a flexible exterior filler; K Rend's Colour Fill is one made in colours to match its renders. A crack that keeps reopening after filling is a sign the movement has not stopped.

What you can safely check from the ground

Everything useful at this stage can be done with your feet on the ground. Walk along the wall in ordinary daylight and again when low sun rakes across it, because bulges and lifted edges throw shadows then. Look for lines where coats have parted, crack edges that stand proud, rust or dirt stains running from cracks, and any fragments on the path, the sill or in the gutter of a lower roof.

Photograph each crack with something for scale, such as a coin taped beside it at a height you can reach, and write the date on the record. Take the same photographs from the same spots a few months later. A crack that has not changed is a different problem from one that has lengthened or widened, and that comparison is valuable to whoever inspects the wall.

Leave the tapping to the person inspecting. Knocking on a bulge can bring it down, and render at first-floor height should never be tested from a ladder. If a section above a door, a path or a parking space is bulging or has started to shed pieces, keep people from standing underneath it and ask for it to be made safe before anything else is discussed.

Note what sits around each crack too: a dripping overflow, a leaking gutter joint, a sill with no drip, a bracket. Water from one of those can explain why one area failed while the rest of the wall held, and the render repair guide shows how such details are dealt with during the repair itself.

How the repair is set out

Once the hollow area has been tapped out and marked, the render is cut back to a sound edge, usually along straight lines so the patch can be finished neatly. Loose material is removed and the exposed background is cleaned and checked. Lime Green's preparation guidance, for example, asks for a clean, sound background and points out that raked joints alone do not give a render enough key.

The patch is built up in the same number of coats as the surrounding render and to the same overall thickness, so it finishes flush. On an older solid wall with lime render, the repair should also be lime; SPAB warns that cement repairs on old walls hold moisture in. On a modern cement or polymer render, the original system's base coat and finish are used, with mesh where the patch takes in a reveal or another stress point.

The edge of a patch is where a repair most often shows, so the new finish is textured to match before it sets. A through-coloured patch will look fresher than weathered render for some time; if that matters, the wall can be coated once the repair is ready. The Sheffield wall rendering homepage links to the other repair and finish options.