The short answer
There is no single figure, because the safe length depends on the thickness of the lead, whether it is a flat, a gutter, a valley or a flashing, and how it is fixed. The principle is the same everywhere: the longer an unbroken sheet, the more it expands and contracts, and the more likely it is to creep or crack. Industry guidance published by the Lead Sheet Association sets recommended maximum lengths for each situation, and a roofer should work within them.
Why length is limited at all
Lead is a heavy metal that expands as it warms and contracts as it cools, and it does so by more than most roofing materials. A long sheet fixed firmly at both ends has nowhere to go when the temperature changes, so the movement builds up as stress. Over many seasons, the metal may stretch downhill a little at each warm spell and not fully recover at each cold one, which is called creep. Eventually it thins and splits, usually at a fixing or a fold. Keeping the sheets short, and letting them slide freely over a base layer, spreads the movement across many small joints so that none of them has to take much.
What changes the limit
Three things move the figure up or down. The first is thickness. Heavier lead is stiffer and resists creep better, so a thicker code can be laid in longer lengths than a lighter one. The second is the position. A flat roof surface, a gutter, a valley and a vertical cheek each have their own recommended maximum, and a sheet on a steep face has to cope with gravity pulling it down as well as thermal movement. The third is the way it is fixed and jointed. Sheets on a flat roof are normally laid in bays divided by timber rolls or by welted seams, which let the metal move at each joint. The more frequent the joints, the shorter each bay needs to be. The Lead Sheet Association publishes tables for each case, and your roofer should have them or know the rules by heart.
We do not quote figures here, because the numbers depend on the code and detail, and quoting a figure out of context could mislead. If someone offers you a very long single run without joints, ask why.
How joints deal with the movement
The common joints are the roll, the welt and the lap. A roll is a lead sheet dressed over a shaped timber strip, which stands proud and lets each bay expand into it. A welt is a folded seam, often used across the fall of a roof. A simple lap, one sheet tucked over the next, is used on flashings and soakers, where the pieces are small. Each joint is placed to suit the water flow, with the higher sheet always overlapping the lower on the direction of flow. Drips, small steps in the deck, are used on flat roofs to break up the run and keep the laps above the water level. Our guide on lead bay roofs and canopies shows how this works on small flat areas, and the dormer detail is in how dormer cheeks are leaded.
What a failure looks like
A sheet that was laid too long often shows as a series of ripples or a crease running across the roof, and splits at the corners, along folds and around fixings. You may also see lead sagging away from the wall at the top of a flashing. Our page on why lead cracks goes through the causes in more detail, including poor fixing and thin sheet. If the cracks run across the roof at regular intervals, a long bay is a likely cause, and the repair is to relay the lead in shorter lengths with proper joints, not to patch each split.
Gutters, valleys and flashings
The idea carries across all the lead details on a roof, but the numbers differ. Lead gutters and valleys are laid to a fall and carry concentrated water, so they are jointed at set intervals with drips and laps. Flashings along a wall are short pieces overlapped in series, never one long strip, so each piece can move on its own. Soakers are smaller still. Where lead lines a flat roof, the combination of fall, bay width and sheet length all interact, and a change to one means adjusting the others. A roofer who only quotes a price per metre without discussing these things may not be planning the joints at all.
What to ask a roofer
Ask what code of lead will be used, how long each sheet or bay will be, where the joints will go, and what is under the lead. Ask whether the work follows the Lead Sheet Association guidance, and ask to see the plan marked on a photograph of your roof. A knowledgeable roofer will answer without fuss and will tell you if your roof needs extra joints.
We lay out the bays and joints on the survey, so that you can see how the leadwork will be divided before any lead is ordered. The lead roofing and leadwork hub lists the related pages, and our main page shows the rest of the work we cover. A phone call to 01256 213956 will book the survey.
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