Sand-Cement, Roughcast and Pebbledash on Chimneys

Three finishes that look alike from the garden

Most rendered stacks fall into one of three families, and telling them apart is the first step in deciding what a repair involves. Smooth sand-cement render is a flat, even coat, trowelled or floated and often painted. Roughcast, sometimes called harling, is a wet mix of sand, cement and small stones thrown onto the wall so the stones are buried in the coat and the whole surface is coarse and uniform. Pebbledash is different again: a plain coat goes on first, and loose chippings or pebbles are thrown onto it while it is still wet, so the stones sit on the surface like seeds on a bun.

From the garden you can usually separate them with binoculars and a little patience. Look along the face of the stack with the light coming across it rather than straight on. Pebbledash looks speckled and shows bald patches where stones have dropped out, roughcast looks knobbly all the way through, and smooth render shows only its tooling marks and any paint. Where the walls of the house carry one of these finishes, compare the stack with it, but do not assume they match, because chimneys were sometimes coated separately and at a different time.

The post-war houses described on our chimney repairs in Deeside page often have a rendered chimney whether or not the rest of the house is, so it is worth knowing which of the three you are looking at before anybody talks about repairs.

How each one fails

Smooth render fails by cracking and by losing its grip. Fine craze cracking is common and often harmless, but a coat that has gone hollow sounds dull when tapped, sags slightly, and eventually lets go in sheets. The edges of those sheets are usually sharp, and the underside is often damp or stained. On a stack, the weak points are the top edge under the pot, the corners, and the bottom where it meets the lead.

Pebbledash tends to announce its age sooner. The stones are only held by the surface of the base coat, so frost, wind and ordinary weathering gradually release them, and the grey base coat shows through in patches. The chippings end up in the gutter, on the drive and in the borders. That is untidy rather than urgent, but a bald patch has lost its first line of protection and the base coat underneath will absorb more water than it should.

Roughcast is the most forgiving of the three, because the stones are mixed through the thickness of the coat and a worn face still looks even. That is also its weakness: it can be failing quietly behind a surface that looks sound. Loose areas show up as a change of sound under a gentle tap, as cracks that run straight along a bed joint, or as a bulge. All three types can be blown off from behind by corroding metal beads or by water trapped between the coat and the brick.

How each one is repaired

The repair follows the failure. Sound render with a few local defects can be cut back to a clean, firm edge and patched. Hollow areas are tapped out until the edge of good material is found, which is often further than the visible damage, and the bare brick is brushed clean and dampened so it does not drag the water out of the new coat. The patch is then built up in layers rather than in one thick lump, and each layer is left to take up before the next goes on.

Pebbledash patches need an extra step. After the base coat is built up and has started to stiffen, the new stones are thrown on to it, and then pressed in lightly so they bed. The match depends on picking a stone of the same size, colour and shape and throwing it with a similar density, which is more skill than it looks. Roughcast patches are applied wet in the same way and blended into the surrounding texture, and smooth render patches are floated flat and finished to the existing tooling.

Where large areas have failed, a patch is the wrong answer. A face that is hollow over most of its area, or that sits over damaged brickwork, is better stripped and redone, and that decision usually can’t be made until the surface has been opened up. Our page on Repoint, Rebuild or Remove? How the Decision Gets Made explains how that choice is usually weighed up for the whole stack.

Why matching matters, and what to ask

A mismatched patch looks poor, but the appearance is the lesser problem. A new coat that is much harder or denser than the old one can sit proud, crack at its edges and channel water behind itself. A coat that is much softer wears away faster than the render beside it and leaves a step that collects water. The aim is a patch that behaves the same way as its neighbours: similar strength, similar thickness and similar texture.

Paint complicates the picture. Several coats of masonry paint can make roughcast look smooth, seal a hollow patch so it seems sound, and hide hairline cracks that would otherwise tell you something. If the stack has been painted, say so when you describe it, and expect any repair to include a plan for repainting the stack so the whole face ends up uniform. A patch on its own will always show against a freshly painted face.

When you get a written quotation, check that it says which finish is being matched, whether the work is a patch or a full recoat, and what is being done at the head and at the lead junction, because that is where most of the water gets in. The hub guide on What Should Be on a Chimney Repair Quotation? lists what to look for. Please do not climb up to examine a stack yourself; a photograph from the ground, taken with the zoom, is plenty to start a conversation.