How Do Roofers Repair Chimney Leaks?

Before Anything: The Diagnosis

A chimney repair that starts with somebody deciding it is the flashing has a reasonable chance of being the wrong repair. There are five candidates - the flaunching, the pointing, the brickwork, the four pieces of lead, and the flue - and the job should start with establishing which.

That means the loft as well as the roof, and it means asking what the damp does over time, because damp that tracks humidity rather than rainfall is not a leak at all.

On the inter-war semis common around Bebington there is a local addition to that list: the small flat or shallow roof over a bay window, which has its own upstand against the wall and is frequently the actual source of damp that gets blamed on the chimney above it.

Access

Getting safely to a chimney is the part that surprises people on the quotation, and it deserves explaining rather than hiding.

A stack sits at the highest point of the building, usually in the middle of a slope or on a ridge, and it needs somebody to be able to work there with both hands for several hours. Depending on the house that means a scaffold, a tower, or a properly-secured roof ladder arrangement.

The important thing about it is that the cost does not scale with the size of the repair. Renewing one piece of lead and renewing all four need the same access. That is why a small chimney job can cost more than a larger job at ground level, and it is also the strongest argument for doing everything that needs doing while the access is there.

On a semi, it is worth asking whether the neighbour needs anything doing at the same time, particularly if the stack is shared.

The Work Itself

The old lead comes off. All of it, in the area being renewed. New lead is not dressed over old lead and it is not laid over a cement fillet.

The chase is raked out. A groove is cut into a horizontal mortar joint to take the top edge of the flashing. On a stepped flashing that is a series of steps following the brick courses. This is the part that is skipped in a bad job - lead bedded in mastic against the face of the brickwork instead of into a joint looks similar from the ground and lasts a fraction as long.

Soakers first, then the cover flashing. Where the stack passes through a slope, small individual soakers go between each course of tiles or slates, turned up against the brickwork. The stepped cover flashing then dresses down over them. Both pieces have a job and a flashing without soakers is half a detail.

The apron and the back gutter. The apron below the stack, dressed over the covering. The back gutter above it, formed with enough depth and fall to take everything coming down the slope and discharge it both sides.

Wedged and pointed. Lead wedges driven into the chase at intervals to hold the top edge, then pointed over. Clips at the free edge where exposure requires them.

Correctly sized pieces. Lead moves with temperature, so pieces are kept to sensible lengths - around a metre and a half for cover flashings - and lapped, rather than run in one continuous piece that will split along a fold.

If the Masonry Is the Problem

A different job, and often done at the same visit because the access is already there.

Repointing - raking out eroded joints to a proper depth and refilling them. Depth matters: scraping the surface and smearing new mortar over it fails quickly.

Flaunching - the mortar bed on top holding the pot, cut out and reformed with a proper fall so it sheds rather than collects.

Spalled brickwork - individual faces cut out and replaced where the face has blown, or the stack rebuilt above a sound course where it is widespread.

A vented cap if the flue is disused, which removes rain entry and still lets the flue breathe. A completely sealed cap causes condensation instead.

The Finish, and What to Check

Patination oil on the new lead. Without it, lead carbonate washes off the fresh surface and leaves white streaks down the tiles for the first year or two. Cosmetic, but its absence is a reliable sign of a job done in a hurry.

Debris cleared. Old mortar and offcuts in a gutter or a valley is the next leak.

Photographs. Ask for them. You cannot see any of this from the ground, and a set of before-and-after pictures is the only way you will ever know what was done.

And a written description naming what was found and what was renewed - not "chimney repairs". That is what lets you tell, if it leaks again, whether the next person is looking at something different.