Sealant is not a lead repair because it covers a leak without curing what causes it. A flashing leaks because its lead has lifted, split, dropped out of its groove or lost its fixings, and a line of silicone, mastic or bitumen across the gap does nothing to put any of that back. It gives a seal that lasts until the next hard frost or hot afternoon, then cracks, and water goes behind it as before.
The pull of sealant is easy to understand. A tube costs a few pounds, it takes minutes to apply, and the leak seems to stop. It is the favourite of anyone who wants the stain on the ceiling to go away this week. The trouble is that the stain returns, the lead underneath stays wet and the roofer who looks next has a flashing smeared with old sealant that hides what is under it.
This guide explains how sealant fails on lead, what a proper repair does instead, how to spot sealant that was put on in the past, when sealant has a legitimate part in the work and what the survey does about it. The advice is general, and the roof in front of you decides the detail.
Why sealant fails on a flashing
Lead is never still on a roof. It expands in sun and shrinks in frost, and a flashing a metre long moves by a small but real amount every day. Anything stuck to it has to move too, or it tears away.
Movement and fatigue
Sealants are flexible, but only within a limit, and a joint that opens and closes daily wears the sealant out. It splits along its length or pulls away from the lead or the brick at one side. A thin film smeared over a gap has no depth to stretch, so it fails soonest.
Sunlight and weather
Ultraviolet light and rain age sealants. Silicone chalks and peels, mastics harden and crack, and bitumen dries and crazes. Each loses grip as it ages, long before the lead beneath has lost anything.
A poor surface to stick to
Lead forms a thin grey oxide skin, and it is often dusty or damp on an old roof. Sealant needs a clean, dry, sound surface, and a roof in winter seldom provides one. Brick and mortar crumble at the edge, so the bond is to dust.
Water gets under the bead
Even a sealant that looks intact can let water in behind it at an edge. The water then runs into the wall, the roof edge or the timber, and the surface stays dry, so nobody sees the harm.
MyJobQuote's leadwork page gives lead flashing over 80 years, as a lower bound. A flashing that needs sealant to survive has already lost the fixings behind that long life.
What sealant hides
The worst thing about sealant over a failed flashing is not that it fails. It is that it conceals the faults that a repair would put right.
Loose wedges and a dropped edge
When the top edge of a flashing falls out of its groove, sealant bridges the gap but leaves the lead hanging, unsupported. Wind then lifts it further, and the sealant tears.
Split and cracked lead
A crack in the lead may be sealed over, but the metal still moves, and the crack opens again. Meanwhile the sealant makes it harder to see how far the crack runs.
Missing soakers and laps
Where a soaker or a step flashing has failed, a line of sealant at the surface cannot replace a piece of metal under the tile. Rain gets past the edge, and the timber beneath rots.
Rotten timber and wet masonry
Because the leak seems to stop, the damp behind the flashing is forgotten. Wet masonry stays wet for months, and rafter ends, battens and wall plates can rot unseen.
The effect on a later repair
A roofer who strips a flashing that has been smeared with sealant has to scrape it off first, because new lead and pointing will not bond to old sealant. That adds labour. Our guide to lead flashing repair describes the proper work, and it begins with removing what should not be there.
Sealant also hides the clues. The pattern of staining and lifting that a surveyor reads is covered, so the true cause takes longer to find.
What a proper repair does instead
A lasting lead repair works with the metal's movement and restores its fixings. It rests on a few simple principles.
Restore the fixing
The lead's upper edge sits in a groove cut into the wall, with small lead wedges at intervals keeping it in place. Fitting new wedges where they are missing pulls it back firm. The lower edge is held by clips, so wind cannot lift it.
Re-dress, do not cover
Where lead has lifted, it is re-dressed with hand tools back to its proper shape, closing gaps against the wall and over the tiles. Where it is split or worn, the damaged length is cut out and new lead, matched in code, is let in with a lap.
Point the groove properly
After wedging, the groove is pointed with mortar that suits the wall, or filled with a flexible compound pressed into the joint itself. That is a filler inside a prepared joint, supported by wedges, and not a smear over an open gap.
Deal with the cause
The repair looks for why the flashing failed: a leaning chimney, a moving wall, a short upstand, a soaker that has slipped. A fix that leaves the cause in place fails again. Our answer on lead flashing coming away from the wall goes through the usual causes.
Allow for movement
Lengths of lead are limited, and laps and folds allow each piece to move without tearing. That is what a lead repair gives that a sealant cannot.
The result is a flashing that is held, lapped and pointed, and the proof is in the photographs taken before and after.
Signs that sealant has been used before
You can often tell from the ground, or from a photograph, whether a flashing has been patched with sealant. The marks are fairly distinctive.
What to look for
- Beads and smears of grey, black, white or clear material along the join between lead and wall or lead and tile.
- Thick black bitumen brushed or trowelled over the lead, sometimes with a fabric strip bedded in it.
- Layers: new sealant on top of old, showing several repair attempts.
- Cracks and gaps in the sealant, with stained brick or lead beneath it.
- Peeling edges where the sealant has pulled away from one side.
- Foam or filler squeezed into gaps at a chimney or wall.
What it tells you
Sealant in several layers means the leak has been chased more than once and never cured. The stain inside may have returned each winter. A sealant bead is also a marker for where the original fault lay, so make a note of where it was applied.
Checking safely
You can look from the ground with binoculars, or from a window. Do not climb onto the roof or lean from a ladder at a chimney. A roofer with proper access sees more, and photographs it. Our answer on signs of a leaking flashing lists the clues to check.
On a survey we photograph each patch, remove a small test area where we need to, and show what was under it.
Where sealant has a legitimate place
It would be wrong to say sealant has no use on a roof. The point is where and how it is used, and what it is asked to do.
As a joint filler
A flexible lead-compatible sealant can be used to fill a prepared groove after the lead has been wedged, instead of mortar, particularly where a mortar joint would crack with movement. Here it supports the flashing's top edge and seals the narrow gap, and the lead is held by wedges, not by the compound. That is a recognised detail, and it differs from smearing sealant over an open fault.
For small, protected details
At a minor junction under a cover, or around a pipe, a sealant or a prepared fitting may be a reasonable part of the detail, as long as the main weathering is done in metal.
As a short-term make-safe
In an emergency, such as a leak in a storm, a temporary seal can be a sensible measure to buy time. It should be described as temporary, with a note of the proper repair to follow, and the damaged lead should be photographed before it is covered. Our guide to temporary roof repairs explains the principle.
What the test is
Ask whether the sealant is helping a metal detail to work, or standing in for it. If the lead is held, lapped and wedged, a sealant filler in the groove is fine. If the sealant is the only thing keeping rain out, it is a bodge and it will return as a leak.
What the survey does and what you can ask
A survey of a flashing that has been sealed starts by finding out what lies beneath, because that is what the repair has to deal with.
On the roof
We photograph the sealant and its position, then check the lead around it for lifting, cracks, thinning and movement. Where needed, a small area is cleaned back to show the edge of the lead and the groove. We look at the chase, the wedges, the pointing and the brick behind, and at the roof covering and soakers beside the flashing.
Inside the roof
Where the loft can be reached, the timber near the flashing is checked for stains, softness and daylight. Wet timber behind a flashing that seems to have stopped leaking is common after sealant.
What we advise
The report says what was found, whether a repair will cure it and what it involves. Where the lead is sound, a repair is usually modest. Where it is badly worn, a new flashing is the answer, and the photographs show why. Either way, you get a single written total.
Questions to ask a roofer
- Why has the flashing failed, and what will you change about the cause?
- Is the lead held by wedges and clips, and is the groove pointed?
- What code of lead will you use, and how will it be lapped?
- Will the old sealant be removed first?
If the answer to any of these is a tube of sealant, look for a second opinion. For the whole range of leadwork, see our lead roofing page.
