Corbels and Oversails on Victorian Chimney Stacks

What the projecting courses are doing

Look at the top of a Victorian stack and you will usually see the brickwork step outwards in two or three stages before the pots. Those stepped courses are the corbels, and the wider band of projecting brick directly under the flaunching is the oversail. They look ornamental, and they are, but their main job is practical: they push rain off the face of the shaft so that it falls clear instead of running down the brickwork below.

The shaft beneath is thinner than the head, so the projection works like a very small eave. Water that lands on top of the stack runs to the edge of the oversail and drips away from the wall. Without that edge the same water hugs the face, soaks the upper courses and carries salts down into the joints. A drip line of this sort is one of the oldest weather details in masonry, and it was built into the head of almost every stack on a terrace.

The head is also the most exposed part of the whole chimney. It stands above the ridge, takes the wind from every side and has nothing to shelter it. How much of its condition you can judge without a ladder is covered in What to Look For on a Chimney From the Ground, and it is worth reading before you decide what to describe when you ring.

How corbels and oversails wear

Projecting courses fail from the top surface downwards. The upper face of each step is a small ledge, and ledges hold water. Frost gets into the brick, the face flakes off, and the arris, the sharp outer edge that does the dripping, rounds over. Once that edge has gone, water no longer leaves cleanly. It creeps back under the brick and runs down the shaft, which is how a head that looks fairly sound from the street can be the source of damp lower down.

Mortar is the second weak point. The joints between corbel courses are exposed on three sides, and where the pointing has washed out, rain gets behind the projecting bricks instead of simply landing on them. A corbel brick that has lost its bedding can be resting on its neighbours with very little holding it in place. Loose projecting bricks are the first to come down in a gale, and they land on the path, the yard or the roof below.

Earlier repairs matter as well. A hard cement fillet or a smooth rendered slope laid over the steps traps moisture and hides movement behind it. A crack that follows a single course, or a section of oversail that seems to lean outwards, tells us whether the trouble is at the surface or in the bed, so mention either when you call. Why the mortar choice matters as much as the workmanship is explained in Repointing a Chimney: Why the Mortar Mix Matters More Than the Finish.

Repair, partial rebuild or full rebuild

The right answer depends on how deep the damage goes. Where the brick faces are sound and only the pointing has failed, raking out and repointing the corbel joints is enough. Where individual bricks have lost their faces or their arrises, those bricks can be cut out and replaced with matching ones, provided their neighbours are sound enough to take the new bedding.

Where several courses have loosened, or the oversail leans, the sensible course is to take the head down to a sound level and rebuild it. That means the projecting courses are relaid to their original profile, bedded fully and tied back into the shaft, and then the pots and flaunching are reset on top. It sounds more drastic than it is. The head is a small volume of brickwork, and doing it once properly is better than patching it each time a brick drops. Repoint, Rebuild or Remove? How the Decision Gets Made sets out how that decision is usually made.

On a terrace the head is shared, so work here nearly always involves the neighbour’s flues and pots as well, and Shared and Party Chimney Stacks: What You Can and Cannot Do Alone is worth reading first. Anything inside the flue, such as lining, testing or sweeping, is not roofing work and belongs to a HETAS or Gas Safe registered installer or a sweep. Our work is on the outside of the stack, and the head is where much of it starts.

Why the projection should not be flattened off

Now and then a head is “simplified”. The projecting courses are chipped back flush with the shaft and a smooth cement coat is run over the top, because it looks tidier and seems to get rid of the broken bits. It does get rid of them. It also removes the drip. With the oversail gone, rain runs down the face every time, and a flat cement top cracks along its edge within a few winters because it has nothing to shed the water over.

There is also the look of the thing. A row of Victorian stacks with matching profiles is part of how a terrace reads from the street, and one head cut flat stands out for the wrong reasons. Restoring the original profile is no harder than tidying it away, and it keeps the stack doing the job it was built for instead of asking a coat of cement to do it.

If a quotation proposes taking the projection off, ask why, and ask what will throw the water clear instead. What Should Be on a Chimney Repair Quotation? lists what a written quotation should say about the head. Please do not climb up to look at any of this yourself. Describe what you can see from the ground, or send a photograph, and we will tell you what we think before anyone goes near the roof. The page for chimney repairs in Birkenhead explains how we handle the access.