Chimney flashings do one job: they shed water away from the junction where the roof meets the stack. When they fail, the evidence usually turns up somewhere else entirely — a damp patch on a bedroom ceiling, a stain that darkens after every south-westerly, crumbs of mortar in the gutter below. The repair itself looks simple enough. In practice, it is one of the easiest details on a roof to get subtly wrong, and a flashing that leaks is nearly always the result of the same handful of errors repeated. Here are the ones worth knowing about before you start.

Mistake 1: Treating Sealant as a Repair

The most common shortcut is a tube of mastic. A smear of silicone or a strip of self-adhesive flashing tape over a tired joint will hold for a season or two, and it looks tidy from the ground. It is not a repair. Sealant traps water against the lead, hides the real defect, and makes the eventual proper fix harder because the new metal can't sit flat on a bed of cured goo.

Sealant does have a legitimate role — as a bedding compound beneath a lap, or a small dab where a fixing passes through a sheet. It is not a substitute for overlap. Water is kept out by geometry: laps that shed, upstands that rise above the waterline, and a chase that holds the top edge. If the lead has split, corroded through, or pulled out of the stack, replace that section.

Mistake 2: Skipping or Botching the Chase

Cover flashing has to be fixed into the chimney, not laid against it. On brick and stone stacks, the top edge of the lead is folded into a horizontal chase — a raglet — cut into the mortar joint or the masonry itself, then wedged and pointed. Get this wrong and nothing above it will save you.

• Cutting the chase too shallow. The lead then sits proud of the wall, catches the wind, and works loose as it expands and contracts in the sun.
• One deep pass with a grinder. Rake the joint and cut in two or three shallow passes instead. A single aggressive cut blows the arris off the brickwork and leaves a ragged edge that no amount of pointing will tidy.
• Leaving dust in the chase. Brush and blow it out before wedging. Mortar will not bond to grit.
• Pointing flush with a hard cement mix. A stiff, cement-rich mortar grips the lead and tears it as it moves. A softer lime-based mix, or lead wedges with a flexible pointing, lets the metal breathe.

Mistake 3: Getting the Laps and Falls Wrong

Every lap must run with the flow of water, never against it, and each one needs to be generous. On a typical pitched roof, a hundred millimetres of horizontal overlap is a sensible minimum; go wider on shallower pitches, and check the guidance for the thickness and length of lead you're using, because longer pieces move more.

On the sides of a stack, stepped flashing needs soakers behind each step, dressed over the tile or slate and hooked over the batten or underlay — not stapled on top of the covering where the wind can lift them. Cut them too short and the water runs past the lap.

The back of the chimney is where most leaks begin. It needs a back gutter with a raised edge on the upstand and a proper fall into the roof covering, plus a saddle or cricket behind a wide stack to push water around it. A shallow, narrow back gutter silts up with grit and moss within a couple of winters, and then it simply overflows into the roof.

Mistake 4: Blaming the Flashing When the Chimney Is at Fault

Plenty of "flashing leaks" are nothing of the sort. Water is getting in at the top of the stack and finding its way down the flue or through the masonry. Re-leading a chimney that is already drinking from above is money wasted and a phone call you'll get again next spring. Before you touch the flashing, look up:

• Cracked or spalling crown, and failed flaunching around the base of the pots
• Perished pointing and open joints in the brickwork
• Cracked render, which holds water against the stack like a sponge
• Loose pots, or an unused flue with no rain cap or cowl

Deal with the chimney first. It also makes sense to check the flashing on a dry day with the roof covering dry, so you're not chasing phantom damp.

Mistake 5: Rough Lead Work

Lead is forgiving, but only up to a point, and it work-hardens every time it is bent. Stretching a single piece across a sharp corner thins the metal and it splits within a year or two. Dress it in stages, keep pieces to a manageable length, and use more laps rather than one heroic sheet.

Fixings matter too. Screws or nails driven through the face of the lead will pull through as it moves; use lead clips or wedges instead, and never puncture the upstand above the waterline. Avoid trapping lead hard against galvanised steel, bare zinc or copper — dissimilar metals in the same damp environment will corrode. Separate them, or use compatible fixings.

Two smaller points that catch people out: don't dress lead in freezing weather, as cold metal cracks rather than folds, and don't walk on freshly dressed sheets in warm sunshine — you'll leave low spots that hold water. If you use patination oil, apply it to dry lead, because a damp surface leads to white staining that looks like a fault.

Mistake 6: Skipping the Test, or Testing Blind

Hosing the whole roof and declaring it sorted tells you nothing except that it leaks or it doesn't. Work in sections instead, starting at the lowest point of the flashing and moving upwards, one lap at a time, with someone inside watching for the first sign of damp. That way you find the actual entry point rather than sealing everything and hoping.

And be honest about access. A chimney flashing sits above the eaves line, often above a fragile covering. Ladders hooked over gutters are not an appropriate place to be working with a grinder and a hammer. If the stack is more than a storey above the gutter, or the roof is steep, wet or frosty, that job belongs to someone with a tower or scaffold and the right insurance.

A Quick Checklist Before You Finish

Run through this before you pack up, and you'll avoid most of the callbacks mentioned above:

1. Chase cut to depth, raked clean, lead wedged in and pointed with a soft mix.
2. All laps running downhill and generous, with soakers set behind the covering.
3. Back gutter raised, falling cleanly, and free of grit.
4. Crown, flaunching, pointing and render checked and made good.
5. No fixings through the face of the lead; clips only.
6. Staged water test carried out, lowest lap first.

If any of it gives you doubt — particularly the chase, or a stack with cracking in the masonry — a surveyor or an experienced chimney specialist is worth the cost of an hour. Flashings are a small part of the roof by area and a large part of its problems. Get the detail right once and you can forget about it for decades.

Photo: stevepb / Pixabay