Why must lead be fitted in short lengths?

Every metal changes size with temperature, and lead does it more than most roofing materials. In sun, a lead roof gets noticeably warmer than the air above it, and at night it falls back. Each swing makes the sheet a little longer, then a little shorter.

The amount a piece moves depends on its length. A short piece shifts a small distance and a long one shifts a larger distance. If the metal is free to travel, nothing happens. If something stops it, the stress concentrates in one spot, and the lead stretches there, thins and eventually opens.

Lead is also soft and slow to recover. Each time it is pulled out, it does not spring back entirely, so a long, tightly held piece creeps in one direction over the years. It slides down a slope, ripples across the middle or pulls away from its top fixing. Short lengths stop the creep from building up.

MyJobQuote's guide to replacing leadwork on a chimney says the correct code should be fitted in 1.5 metre lengths. That figure is for flashings, the narrow strips at walls and stacks, and it is a sensible rule of thumb, not a legal limit. Flat roofs and gutters are laid out in their own way, with rolls and drips that break the lead into bays.

Treat it as a maximum for each flashing piece, not a target to stretch. A flashing running 4 metres up a gable is not laid as one strip; it is cut into pieces no longer than the guide, each overlapping the one below. On a stepped flashing, the steps do much of this naturally, since each piece is shorter than the run.

Code and length go together. A thicker code is stronger, but it still moves, so the piece length still has to suit the position. Ask for the code and the piece length together on any quote. Our answer on the lead code for flashing names the usual sheet for each part.

Short pieces would be a problem if every joint leaked. The answer is the lap. Each piece overlaps the one beside it by enough that water cannot run sideways under the edge, and the pieces are not soldered or sealed to each other, so they can slide.

On a pitched slope

The upper piece laps over the lower, in the direction of the fall, so the water sheds off the joint. The lap is set by the slope and the exposure, which is why a shallow, wet position needs more overlap than a steep, sheltered one.

At the fixings

The top edge of each piece is held, usually by nails or wedges into a chase, and the lower edge is held by clips that stop it lifting but let it slide. A sheet fixed hard on all four edges cannot move, and that is the standard route to a crack.

Where the joints have been fixed with sealant or solder, we see splits form beside the join, because the lead has nowhere else to give. A lap that is left free is simply a place where two pieces can slide past each other, and it is what keeps a long run of lead intact across summers and winters.

Over-long lead shows a few familiar signs. The sheet ripples across its width, as if it were wrinkled. The top edge pulls out of the wall, or the bottom edge slides down and leaves a gap. A crack opens along a fold, or through the sheet beside a nail.

Each of these is the same fault seen from a different side. The metal tried to move, and either the fixing held it or the length was too great for the lap to take up. Our answer on whether a lead roof can crack and why covers the other causes: the wrong code, hard fixing and age.

Length also affects how long the work lasts. MyJobQuote says a correctly installed flashing can last over 80 years. That depends on details such as the lap and the length being right from day one.

One further point: long runs are more at risk from theft, since they come off in larger pieces. See the notes in our answer on stolen lead and insurance for what happens next.

A lead roof laid in sensible lengths ages evenly, and a pale grey surface film develops over time. New lead can also shed a white deposit in its early months, before that film has formed, and rain carries it onto the wall below as a stain. Our answer on white staining below lead explains the cause.

On a survey we measure each run, count the laps and look at how the fixings are placed. We photograph each piece from your roof and note which are over the guide length. The written quote then prices a repair or a renewal in shorter pieces on separate lines, and the 10-year workmanship guarantee we give is written down.

If you can see rippling, slipped lead or a gap at the top edge, send a photograph through the enquiry form, taken from the ground or from a window, and tell us roughly how long the run is. For the whole service, see lead roofing in Stroud.