Chimney flashings & weathering
In short
Where a stack passes through the roof it needs a proper set of lead weatherings — an apron at the front, stepped flashings and soakers at the sides, and a back gutter behind. It is the single commonest source of a roof leak, and the commonest place to find a bodge.
Also called: Chimney flashing leaking · Lead flashing chimney · Stepped flashing · Chimney soakers
Signs you need this
- Damp at ceiling level in a top-floor room near a chimney
- Staining on a chimney breast at roof level
- A mortar fillet where the stack meets the roof
- Black bituminous paint around the base of a stack
- Lead flashing lifted out of the joint, or flapping
- One long unbroken run of lead down a stack side
A chimney interrupts a roof slope, and every side of that interruption needs its own detail. At the front, where water runs down onto the stack, there is an apron flashing. Down the sides, individual soakers interleave with each course of slate or tile, covered by a stepped flashing that follows the mortar courses up the stack. Behind, where water is dammed by the stack, there is a back gutter. Four details, all in lead, all doing different jobs.
Done properly they are dressed to shape, tucked into a chase raked out of a mortar joint, wedged with lead wedges and pointed in. Done improperly — which is what you find far more often — the whole junction is filled with a mortar fillet, or painted over with black bituminous compound, or both. Mortar fillets crack because brick and lead move at very different rates; bituminous paint hides the crack until the timber behind it has gone.
Lead has rules and most failed leadwork failed because they were ignored. Lead expands and contracts substantially with temperature, so pieces have to be kept short and lapped — flashings in the order of 1.5 metres for code 4, not run as one continuous strip along the whole side of a stack. A single long piece looks neater on the day and splits within a few years, invariably in the middle. It is one of the most reliable diagnoses in roofing: a leak at a chimney with one long unbroken flashing is a fatigue split.
The back gutter is the detail that gets skipped most, and it is the one that causes the worst damage. Water running down the slope above a stack has to be collected and taken round it, and on a stack of any width that needs a proper lead-lined gutter with a tilt fillet, not just an apron dressed up the back. Where it has been left out, the water sits against the back of the stack and rots the timber behind it — which is out of sight until the repair is a structural one.
Code matters too. Lead sheet is specified by code number, which relates to its thickness, and using something too thin for the position means it fatigues faster and tears at the fixings. Code 4 is the usual minimum for flashings and soakers; back gutters and more exposed details often want code 5.
The good news is that this is a contained, well-understood job that lasts a very long time when done correctly. New leadwork on a chimney, properly sized, properly lapped, chased, wedged and pointed, should outlive the covering around it — and it takes one of the biggest single causes of roof leaks off the table permanently.
What the job covers
- Full set of weatherings — apron, soakers, stepped flashing, back gutter
- Lead of the correct code for each position
- Pieces sized and lapped for thermal movement, never one long run
- Chases raked into mortar joints, wedged and pointed properly
- Old mortar fillets and bituminous paint removed, not covered over
- Timber behind the stack checked for rot while it is open
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
Why does my chimney leak when it was flashed recently?
The commonest reason is lead run too long. Lead moves a great deal with temperature, and a flashing laid as one continuous strip down the side of a stack fatigues and splits, usually across the middle, within a few years. Correct practice is short lapped lengths — around 1.5 metres for code 4 — which is why a job that looked tidier on the day can fail sooner.
Is a mortar fillet acceptable instead of lead?
No. Brick and mortar and lead all move differently, and a mortar fillet at a roof-to-wall junction cracks — it is when, not if. Once cracked it holds water in the joint rather than shedding it, which is worse than nothing. The correct detail is lead chased into a mortar course, wedged and pointed.
What is a back gutter and do I need one?
It is a lead-lined channel behind the stack that collects the water running down the slope above and takes it round each side. On any stack of reasonable width, yes, you need one — and it is the detail most often left out. Where it is missing, water stands against the back of the stack and rots the timber, which is invisible until it is expensive.
Can flashings be replaced without touching the roof covering?
Partly. The stepped flashing on the stack face can be renewed on its own, but the soakers interleave with each course of slate or tile, so getting those right means lifting the covering either side. A flashing job that does not disturb any slates has usually only done half of it.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.