How to lead flash a chimney

Inspired by a helpful UK roofing guide. This walk-through draws on the video “Lead Flashing A Chimney”, a detailed UK tutorial covering the removal of old flashings from a chimney stack and renewal with code 4 sheet lead from start to finish. The sections on dressing lead around the roof tiles are particularly clear and worth watching before you order materials.

1. Assess the chimney and plan the work

Get up to the roof on a dry, still day and look at what you have. Most chimneys need three types of flashing: step flashings up each side, a back gutter behind the stack, and a front apron across the downslope face. Some also have soakers — individual lead pieces that tuck under each tile course and fold up the chimney face, with the visible step flashing covering them from above.

Code 4 lead sheet (1.8 mm thick) is the standard grade for most domestic chimney flashing work. Order enough to cover all three sections, plus a useful offcut for patching. You will also need lead wedges or rolls for fixing into the mortar joints, and lead sealant or mortar for pointing in the top edge.

2. Remove the old flashings safely

Work from the top down. Use a bolster chisel to carefully lever the existing lead out of the mortar chase (the groove cut into the brickwork). Old lead that is still in reasonable condition can sometimes be reused, but it is usually not worth the effort — new lead is relatively cheap and gives you a clean start.

As you remove each section, check the mortar joints underneath. Rake out any loose pointing back 25 mm ready for the new lead to sit in. If the brickwork itself is crumbling, repair it with matching mortar and allow it to cure before fitting the new flashing — bedding lead into soft mortar is a recipe for it falling out again inside a year.

3. Cut and dress the lead sheet

Mark out your lead pieces on the sheet using a scribe or felt-tip pen. The front apron should cover at least 150 mm down onto the tiles and 75 mm up the face of the chimney. For step flashings, each piece should overlap the one below by at least 65 mm. Use tinsnips or a sharp knife and straight edge to cut the lead cleanly.

Lead dresses readily with a bossing mallet and bossing stick — work it gently to shape without thinning the metal. For the apron, fold the top edge back about 25 mm to form the wedge piece that goes into the mortar chase. Dress the corners neatly; a scruffy corner is where water finds its way in.

4. Fit the back gutter first

The back gutter sits in the valley between the top face of the chimney and the rising roof slope. It needs to collect any water tracking down the back of the chimney and channel it sideways off the roof. Cut the piece with upstand sides and a flat base, dressing it so it sits snugly against the chimney face and the tiles.

Fix the top edge into a mortar chase by turning it 25 mm into the joint, packing with lead wedges, and pointing over with mortar. Make sure water cannot collect behind the upstand and sit against the brickwork.

5. Fit the step flashings up each side

Step flashings go on each side of the chimney, stepping up one course at a time to follow the line of the tiles. Work from the bottom upward. Each piece sits over the tile and tucks into a mortar chase in the side of the chimney. If you are using separate soakers under each tile, the step flashing sits over them and only needs to go 25 mm into the mortar joint.

Fix each piece by turning the top edge into the mortar chase and packing with lead wedges. Do not use mastic or sealant at this stage — the wedges hold the lead, and the mortar pointing seals it. Mastic works loose over time and looks untidy from a ladder.

6. Fit the front apron

The front apron covers the junction between the downslope face of the chimney and the tiles below. Fix the top edge into a mortar chase and dress the bottom over the tiles, making sure it overlaps the top tile below by at least 150 mm. The sides of the apron should tuck under the lowest step flashings.

Once all sections are in place, point over all the mortar chases with a 3:1 sharp sand and cement mix, working it firmly in behind the lead. Finish with a weathered profile to shed water. Wipe away any mortar smears from the lead face before it sets.

7. Test with a hose before you come down

Run a hose slowly over the chimney at a rate that simulates heavy rain while someone watches the ceiling below. Check around the base of all the step flashings and behind the apron. Any gap shows up quickly with running water and is far easier to fix while you are still on the roof than after the scaffolding has come down.

Once you are satisfied, clear your tools from the roof, check the tiles have not been disturbed, and come down carefully.

When to call a handyman

Call Richard if you are not comfortable working at roof height, if the stack needs repointing before the flashing can go on, or if the job involves scaffolding that you would need to hire and erect. Lead work at height is one of those jobs that rewards experience — a neat flashing job done well once is far better value than a quick patch that needs redoing in two years.