What Is Chimney Flashing?

Why the joint needs sealing

A chimney stack rises through the roof, and where brick meets tile there is a gap. Tiles and slates are laid to shed water in overlapping layers, but they cannot run up the side of a brick wall. Something has to bridge the join, accept the movement between two different materials, and send water down onto the roof covering. That something is the flashing. Without it, rain would run down the face of the stack and straight into the roof space.

The same job is done wherever a roof meets a wall, at a dormer cheek, a gable, a wall abutment or a skylight, but chimneys have more sides and more complicated junctions than most, because the stack stands free on the roof slope. The water coming down the slope meets the stack head-on, and has to be diverted around it.

The parts of a chimney flashing

A well-made flashing is a set of linked pieces, each with a particular job.

• Apron: the lead sheet across the lower face of the stack, which takes water running down the slope and lets it flow onto the tiles below.
• Step flashings, or soakers with a cover flashing: pieces cut in a stepped pattern up each side, which follow the brick courses. Each step is tucked into a mortar joint and laid over the tile.
• Back gutter: a channel behind the stack on the upper side, which catches water coming down the slope and carries it around both sides of the stack.
• Cover flashing: the upper edge of the lead, turned into a raked-out mortar joint (a groove cut in the bricks) and sealed, so water cannot run behind it.
• Tray or damp-proof course: in some stacks, a sheet of lead built in at the base which sends water that enters the brickwork back out.

The terms soakers and step flashing are explained in our page on soakers and step flashing. If you want the material side, what roofing leadwork is covers it. We use lead because it is long lasting, easy to dress into awkward shapes and holds its form.

How flashing fails

Flashings work for a long time when they are fitted properly, and then they stop working. These are the usual ways in.

• The edge of the lead lifts, because the mortar it was tucked into has crumbled, or the wedges have fallen out.
• The lead cracks from repeated expansion and contraction, especially where long sheets were used without allowance for movement.
• Sealant or mortar fillets, used as a cheap shortcut in place of proper lead dressing, shrink and crack.
• Theft or damage leaves bare gaps, and a section is replaced with a cheaper material that does not last.
• The stack moves slightly and pulls the flashing away, which is a sign of a bigger problem in the masonry.

The first sign is usually inside the house: a damp patch on the ceiling near the chimney, a stain on the chimney breast, or drips in the loft during heavy rain. Our guide to why a chimney leaks helps work out whether flashing is the cause or something else is, such as the flaunching or the stack itself.

Repair or replace

Small failures can be repaired. A section of lifted lead can be re-dressed and re-wedged, and a loose edge repointed. When the lead is cracked in many places, or old repairs have been made with sealant, replacing the flashing is more sensible than patching. A proper replacement strips off the old pieces, checks the tiles and battens beneath, cuts fresh chases in the mortar and fits new lead throughout. Beacon's approach is to show you photographs of what has failed first, so you can see whether it is a repair or a replacement. See our page on lead flashing repair or replace.

Why the details matter on older roofs

On older houses, the flashing may have been made from lead that has been in place a long time, and it is worth asking before anything is stripped whether it can be saved. Old lead often has plenty of life left once the edges are re-dressed and the mortar renewed. On listed buildings and in conservation areas, like-for-like lead is usually the expected material, and a change of metal may need consent from the council's conservation officer.

What flashing costs

One UK cost guide gives chimney leadwork as an average near £570, within a spread of £350 to £920, and another quotes flashing on all four sides at around £525 without scaffold and about £1,000 once access is included. Those are national figures to give you a sense of scale, not prices from us. Access, stack size and the state of the tiles shift the figure, and the price for your chimney follows the survey and is fixed in writing. For the whole picture of chimney services, return to chimney repairs in Basingstoke.