The short answer
A chimney apron flashing is the piece of lead across the lower, downhill face of a chimney stack. It turns up the brickwork, is tucked into a cut mortar joint, and then laps down onto the roof covering so rain leaves the stack and runs onto the tiles or slates. It is one part of a set of chimney flashings, not the whole job.
The apron is the first piece of lead rain meets on its way down a chimney. If it is thin, short or badly fixed, the ceiling below shows it.
Where the apron sits on the stack
A chimney stack rises through the slope, so water running down the roof meets a brick wall. Roofers deal with that wall in four places. The apron takes the front, the lowest face. Step flashings, cut as stepped pieces, take each side. A back gutter takes the high side, the one facing the ridge, the horizontal peak of the roof.
What does the apron actually do?
It bridges the gap between brick and roof covering. The top edge of the lead runs up the face of the stack and is turned into a mortar joint. The lower edge lies over the tiles or slates and sends the rain off the front of the stack in a clean sheet. Nothing is stuck on; the lead is shaped so water always runs over a lap, never behind one.
Because the apron lies across a plain horizontal line, it is the easiest chimney piece to see from the garden. A homeowner can often spot lifted or slumped lead from the ground. If a stack has a sloping bead of cement, called a fillet, instead of lead, it has no true apron at all.
Lead codes and what suits an apron
Every sheet of lead carries a code, a number that stands for thickness and weight. Code 4 is 1.80 mm thick and code 5 is 2.24 mm. Code 6 is 2.65 mm. The Royston Lead brochure lists codes 4, 5 and 6 among the weights for flashings, and keeps code 3 for soakers, the small folded lead pieces that sit under each slate or tile along a stack or wall.
Which code should a chimney apron be?
That is the roofer's call after seeing the stack. A wide apron on an exposed stack, or one that lies over a deep tile profile such as a Double Roman, needs more body than a short strip on a sheltered slope. Thicker lead keeps its shape better when it is dressed, the trade word for shaping lead by hand with a bossing stick.
The code also affects how long the job lasts. Lead can last for over 80 years, but only where it is thick enough for its position and free to move. We choose the code, tell you which it is, and it is named in the one written price after the survey. If a quote says only "lead flashings", ask which code and which pieces are meant.
How the apron laps over tiles
On flat slates the lead lies close to the surface and simply overlaps. On a profiled tile such as a pantile or Double Roman, the apron has to follow rolls and hollows, and that is where careless work lets water in.
Does the lead sit on top of the tiles?
Yes. The lower edge of the apron comes down over the covering, and the tiles under it must be cut or packed so the lead lies flat without forming a dam. On deep profiles the gaps beneath are closed with a mortar bed or purpose-made filler pieces, not left open. Lead pressed into a hollow without support sags and tears within a few winters.
The apron should also reach far enough down the slope to cope with wind-driven rain, which is common on exposed Somerset slopes such as the Quantock and Blackdown Hills villages. We look at the pitch and the tile type before deciding how far the lead drops.
Where a roof has been recovered around an old stack without renewing the lead, the apron often ends up too short, because the new covering sits higher. Our survey photos show the lap, so you can see it before any work starts.
Signs the apron needs attention
Look from the garden or the street. A bright, buckled or lifted strip of lead across the front of the stack is a warning. So is a dark mark on the wall of the chimney breast inside the house, which is often damp working through from a failed flashing rather than from the brickwork itself.
Can an apron be repaired without replacing it?
Sometimes. A small split can be patched, and a loose top edge can be re-fixed and re-pointed, meaning the mortar joint is refilled. But a patched apron on a stack with cracked or sagging lead elsewhere is a short-term answer. Our answer on cracked lead flashing explains when a patch is worth doing.
Check the sides and back at the same time. The apron is rarely the only piece to fail, and water often enters at the chimney back gutter behind the stack instead. Our lead work page covers the full range of flashings. Send a photo of the stack and we will do the survey, share pictures of your own chimney, and quote one written price naming the pieces to be renewed.
More on apron flashings
Is the apron the same as step flashing?
No. The apron runs straight across the front face of the stack, while step flashings are cut in stepped pieces and run up the sloping sides. Each piece laps over the next. A good chimney set has the apron, the steps and a back gutter working as one system.
Can I fit a chimney apron flashing myself?
It is a roof-edge job at height, and the lead must be shaped to the tile profile and chased into the brickwork correctly. Mistakes stay hidden until the first wet winter. For these reasons we advise against doing it yourself and recommend a roofer.
The full picture: Lead work in Taunton ยท More short answers