The short answer
Split lead can be repaired when the split is a single, clean fault in sheet that is otherwise sound and of the right thickness, usually by cutting out the damaged section and dressing in a new piece. The whole bay should be renewed when there are several splits, the lead has crept or thinned, or the pieces are too long to move freely. A repair cannot fix a design fault.
Why lead splits in the first place
Before deciding to repair or renew, it helps to know why the lead split, because the answer points straight at the remedy. Lead is soft, and it swells slightly when warm and shrinks when cold. A sheet that is free to slide takes this in its stride for a very long time. A sheet that is held fast at both ends, or cut too long for the job, cannot move, and the stress builds up at its weakest point. That point is nearly always a fold, a drip, a nail line or an unsupported edge.
Metal fatigue is the result. Each cycle of warming and cooling bends the lead slightly at the same spot, and eventually the surface cracks. The crack then lets water through and, in winter, lets frost pry it wider. You can read more about this mechanism in our answer on why lead cracks.
Other splits have simpler causes. A boot or the foot of a ladder resting in the gutter can put a kink in the sheet. A nail punched straight through the lead sheet, a poor weld or an old solder joint can open up. Someone may have patched a nearby leak and left the sheet under strain. If we can see a single, obvious cause, the repair has a good chance. If the lead is splitting for no clear reason, the cause is built in.
When a repair holds
A repair is sensible under these conditions.
- The split is isolated. One crack in a bay that is otherwise flat, clean and free of ripples.
- The lead is the right thickness. A code 5 or heavier gutter that has simply worn at one point is a good candidate. Code 3 is soaker weight and a poor choice for a gutter. A lead code is the numbered grade that sets sheet thickness and weight, running from 1.32 mm at code 3 to 3.55 mm at code 8.
- The length is within reason. Flashings are normally cut no longer than 1500 mm in codes 4 and 5, according to the Royston Lead brochure. If the damaged piece is longer than that, the cause may be the length.
- There is no creep. Creep is the slow downhill drift of lead on a slope, which leaves ripples at the top and a thinned stretch below.
- The deck below is sound. Timber that is dry and firm can take the new section. Damp or soft boards mean there is more to do.
In those cases we cut out the damaged section, prepare the overlaps and dress in a new piece of the same code, secured with clips rather than face nails so the sheet can still move. The joint is formed as a lap, a roll or a welt, depending on where it is, rather than relying on sealant to hold the water.
When the whole bay should be renewed
A bay is the length of lead between two drips or two joints. We recommend renewing a whole bay, or several, when the evidence points to fatigue across the sheet rather than a single accident.
Look for cracks along more than one fold, a bay that has crept so far that the top edge is visibly stretched, and a surface that shows pinholes or has gone thin and dull. These mean the metal has hardened or worn through. A new piece welded or lapped into old, tired lead will tend to fail at the join within a few winters, because the old sheet around it is still moving more than it should.
Renew too if there are no drips in a long gutter, or if the fall is wrong. Manufacturers working from BS 6229 recommend designing flat roof surfaces, gutter beds included, with a fall of 1:40 so that the finished drainage reaches 1:80. A gutter that holds a puddle is not helped by patching the lead if the water never leaves. See our guide to why a parapet gutter leaks for how fall, outlets and drips work together.
Finally, renew where earlier repairs have piled up. Several layers of patching, bitumen tape and silicone mean the original fault has not been dealt with, and the extra layers hide any new movement from view.
Why sealant and patches rarely last
It is tempting to fill a hairline split with sealant, a lead-coloured paste or a strip of self-adhesive flashing tape. These products have their place as temporary measures to stop water coming in tonight. They are not repairs.
The reason is movement. The lead on either side of the split goes on expanding and contracting with the weather. A sealant skin is much less flexible than the metal, so it detaches along the edges within a season or two. Water then runs behind it, where you cannot see it, and the leak is worse than before because the damage is now hidden. Tape has the same trouble and, once pressed onto a dirty surface, is hard to remove cleanly without damaging the lead beneath.
On a listed building there is a second concern. Visible sealant and bitumen look wrong against traditional leadwork, and in Bath the work may need discussion with the B&NES conservation team. Listed buildings require a consent of their own, quite separate from planning permission, and B&NES treats unauthorised work as a criminal offence, so ask before any change that goes beyond like-for-like repair.
How we decide, and what to expect
We repair where repair works. At a survey we look at the sheet, not just the split: its thickness, its length, the drips, the fall and the state of the boards under it. If a repair will hold, we tell you and price it as a repair. If it will not, we show you the photographs of why and give you the alternative.
On a box gutter, the channel set between two roof slopes, the same logic applies. Our page on box gutters explains how their construction differs from a parapet gutter. For what a full replacement costs, read box gutter relining cost.
If the split is leaking now, a bucket under the drip and a dated photograph of the stain help us judge how long water has been getting in. The other lead jobs, from flashings to flat roofs, sit under lead roofing.
