The short answer
A mortar fillet is a sloped bed of mortar that seals the joint where a stone roof meets a wall or chimney. Lead flashing does the same job as a folded metal strip. Fillets suit traditional stone work and are easy to match, but crack and need repointing; lead lasts longer but changes the look. The right choice depends on the wall, the building and the budget.
Fillets and lead at a wall or chimney
An abutment is any point where a roof slope finishes against something taller: a gable wall of a neighbouring building, a chimney stack, a parapet or a dormer cheek. Water running down the wall or the stack arrives at that joint, and it has to be turned back onto the slates.
There are two main ways to do it. A fillet (a triangular wedge of mortar, bedded into the joint and run along the line of the wall) fills the angle. Lead flashing is a strip of sheet lead tucked into a groove in the masonry and dressed down over the slates. Where slates meet a wall, a soaker (a little folded piece of lead) goes beneath every course, and the flashing laps over those. Soakers are what let the slope shed water without gaps.
On older stone buildings the fillet has a long history, and a lot of roofs around Stroud have both. Which one is right depends on what is there now, how exposed the wall is and what the building's owner or conservation officer will accept.
How a mortar fillet wears
A fillet is cheap to form and looks at home beside stone. Its weakness is movement. A wall and a roof do not shift at the same rate in sun and frost, and mortar has little give, so a hairline crack opens along the line where the fillet meets the wall. Rain gets into that crack, and a freeze-thaw cycle widens it.
The signs are cracks, a fillet that sounds hollow when tapped, pieces on the slates below, and damp stains on the ceiling beside a chimney breast. Those are what we look for first on a survey, inside the loft as well as out on the slope. A cracked fillet is not always urgent, but a missing one is, because the wall base is then open to the weather.
Our answer on lime mortar, torching and pointing on stone roofs covers the mortar itself. For a fillet the choice of mix matters because a mortar that is too hard can crack at the wall instead of moving with it.
How lead flashing compares
Lead is the better answer where movement is expected, because it bends rather than cracks. MyJobQuote's 2026 leadwork guide says flashing can last for over 80 years when correctly installed, and over 100 years for the higher codes. A lead code is simply a grade of thickness and weight. Leadworx calls Code 4 (1.80 mm) the usual choice for general flashing, Code 5 is thicker at 2.24 mm, and thin Code 3 (1.32 mm) is kept for soakers.
Lead has its own drawbacks. It needs to be dressed properly, with a chase cut into the stone or a pointed joint at the top, and the stone beside it can stain when rain runs over lead for years. On a listed building the conservation officer may prefer the fillet already there. Check with the planning team before replacing one with the other.
A lead flashing and a fillet can work together. A fillet below a flashing is a common repair, and in practice many roofs end up with a mixture of both.
What it costs to put right
None of these are our prices; they are national ranges from published guides. MyJobQuote lists £150 to £250 for patching a chimney flashing and an average of £570 to replace one in full. Checkatrade's repair guide quotes £50 to £70 per metre for lead flashing repairs and £80 to £120 per metre for stepped flashing in Code 5 lead, before VAT.
MyBuilder's 2026 chimney guide puts flashing repair at £250 to £1,200 and repointing a chimney at around £750 on average. Scaffold sits on top of all of these, and on a stack it is often the largest single cost.
| Detail | Typical UK range | Source |
|---|---|---|
| Chimney flashing patch | £150 to £250 | MyJobQuote, 2026 |
| Lead flashing repair, per metre | £50 to £70 | Checkatrade, 2026 |
| Stepped flashing, per metre | £80 to £120 | Checkatrade, 2026 |
| Chimney flashing repair | £250 to £1,200 | MyBuilder, 2026 |
After we survey, the written quote covers scaffold, lead or mortar, and labour.
Choosing a fillet or lead on your stone roof
The decision comes down to four questions. What is there now, and is it failing? How much does the wall move? What does the building's status allow? And how long do you want the repair to last? A fillet cracked at one corner on a sheltered wall may need nothing more than re-pointing. If water is coming in at a chimney that sits in the exposed middle of a slope, lead is the safer detail.
Either way, the slates around the abutment should be checked. The 2000 Cotswold District Council guidance observes that roughly half the slates have usually perished when a roof comes to be stripped, and slates beside a wall carry a lot of run-off. Pegs and battens in that zone deserve the same scrutiny as the joint itself. See how long a Cotswold stone roof lasts for the wider picture, and stone slate against stone tile if you are unsure what covering you have.
Our Cotswold stone roofing page covers the wider service. We photograph the joint at survey, quote for the repair, and give our 10-year workmanship guarantee in writing.