More roof leaks start at the chimney than anywhere else. A stack is a masonry column standing proud of the roof, exposed on four sides, with a joint all the way round it that has to move and stay watertight at the same time. When that joint is a cracked cement fillet rather than dressed lead, it is only a matter of time.
Repointing, flaunching, lead flashings, pots and stack rebuilds on period and modern chimneys.
The four things that go wrong
Chimney faults are predictable, which is good news, because it means a proper inspection can usually tell you the cause without opening anything up.
Failed flashing: old cement fillets crack and fall away, or lead has been patched with mastic rather than reworked
Perished pointing: mortar joints weather out, so the brickwork itself starts holding water
Cracked flaunching: the mortar cap that beds the pot breaks up, letting water straight into the stack head
Structural movement: sulphate attack or a leaning stack, which is a rebuild rather than a repair
Why cement fillets keep failing
A cement fillet is a triangle of mortar run along the join between the stack and the roof. It is cheap and quick, and it fails because masonry and roof coverings move at different rates while cement does not move at all.
The correct detail is lead: step flashings dressed into the mortar courses, soakers interleaved with the tiles or slates, an apron at the front and a back gutter behind. It costs more and it lasts decades rather than seasons.
On modern brickwork, a cement based mortar is fine. On soft Cotswold stone or old handmade brick, hard cement is actively damaging: it is stronger than the masonry around it, so moisture is forced out through the stone face instead of the joint and the stone spalls.
For period stacks in Burford, Woodstock, Charlbury and similar, we point in lime. It is softer, breathable and sacrificial, which is exactly what an old stack needs.
If a chimney is no longer used, capping it is usually cheaper than maintaining it, but it must be vented. A fully sealed stack traps moisture and you get damp appearing in the breast inside the house.
We fit a capping pot or vented cowl that keeps rain out and lets air move. Removing a stack entirely is an option, but it changes the roof and often the look of the building, so we treat it as a last resort.
How do I know the leak is the chimney and not the roof?
Staining that appears on the ceiling near the breast, or on the chimney breast itself, usually points at the stack. We confirm it by inspecting the flashings and pointing rather than assuming.
Can you repair rather than rebuild my chimney?
In most cases yes. Rebuilds are only necessary where the stack has structural movement, sulphate attack or brickwork that has broken down beyond repointing.
It is the sloping mortar cap on top of the stack that holds the pot in place and sheds water. When it cracks, water goes straight down inside the stack.
Should my unused chimney be capped?
Usually yes, with a vented cap. Sealing it completely traps moisture and causes damp on the chimney breast inside.
Do you use lime mortar on old stone stacks?
Yes, on soft Cotswold stone lime is the correct choice. Hard cement traps moisture and damages the stone face.
We get up to the stack, photograph every face and the flashing detail, and show you what we found.
Flashing, pointing, flaunching or movement. Treating the wrong one is the most common reason a chimney leak returns.
Lead code, mortar type and pot detail all stated in the quote, not left to chance on the day.
Lead dressed properly into the courses, mortar matched, waste cleared and everything left tidy.
We test the repair before we take the access down, because coming back for a scaffold twice helps nobody.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.