Chimney Repair Guide: Repointing, Flashing and Repairs

The classic sign is a damp patch on the chimney breast in a bedroom or the loft, appearing or worsening after wind driven rain rather than after every shower. Rain that only causes damp when it comes from one direction is nearly always getting in at the stack.

In the loft, look at the underside of the roof where the chimney passes through. Staining, tide marks on the brickwork, or damp timber next to the stack point straight at failed flashings.

From the ground with binoculars you can often see the cause: mortar missing between bricks, a cracked or spalled brick face, a cement fillet instead of proper lead, or a leaning stack.

Repointing means raking out the old failed mortar joints to a proper depth and refilling them with fresh mortar mixed for the brick. It is the most common chimney repair and, done well, buys another twenty to thirty years.

The depth matters. Smearing new mortar over old joints looks fine for a season and falls out within two winters. Joints should be raked to at least twice their width before refilling.

The mix matters too. Soft Victorian brick wants a softer, more permeable mortar. A hard cement mix on old brick traps moisture and causes the brick face to blow off in frost, which is exactly the spalling you see on so many older North East stacks.

Where the chimney passes through the roof, the junction should be sealed with lead: a front apron, stepped side flashings chased into the mortar joints, and a back gutter behind the stack. Done properly it is a fifty year detail.

What we usually find instead is a sand and cement fillet. Cement is rigid, the chimney and roof move at different rates, and it cracks. Once it cracks, water runs straight down inside the stack. Repointing a chimney with a cracked cement fillet still in place solves nothing.

The back gutter is the piece most often skipped. On the uphill side of a chimney, water collects against the brickwork and needs a formed lead gutter to take it round the sides. If yours does not have one, that is very likely where the damp is coming from.

An open unused chimney is a hole in your roof. Rain falls straight down the flue, soaks the brickwork from inside, and shows up as damp on the breast in a room where nobody has lit a fire in thirty years.

The fix is a capping pot or cowl that keeps the rain out while still allowing the flue to breathe. Sealing a chimney completely without ventilation is a mistake and causes condensation inside the stack.

Where a chimney is genuinely redundant and in poor condition, taking it down to below the roof line and tiling over is often more sensible than repeatedly repairing it. It removes the maintenance permanently and the roof reads perfectly well without it.

If more than a few bricks have spalled, the stack leans, or the mortar has gone soft right through, repointing is putting good money after bad. A rebuild above the roof line takes the stack down course by course, salvages what is sound, and rebuilds with matching brick and correct new flashings.

It is a one to two day job on most houses once scaffolding is up, and it resets the clock completely. New lead, new pointing, new cap, new pots.

We will always tell you if a repair will do. We will also tell you if it will not, because patching a failing stack twice costs more than doing it once properly.

Repointing a typical domestic stack runs from around £450 to £1,200 depending on size, access and scaffolding. New lead flashings all round usually land between £350 and £900. A full rebuild above the roof line is generally £1,200 to £2,500.

Scaffolding is often the deciding factor. If you are having any other roof work done, do the chimney at the same time and share the access cost. It can save several hundred pounds.

Most chimney jobs are one to two days. We leave the site clean and the stack watertight before we take the scaffold down.