The short answer
Lead cracks mainly because of thermal movement: it swells and shrinks with the weather, and sheet that is cut too long, fixed rigidly or laid in too light a code cannot take that movement. The stress collects at folds, drips and nail lines until the metal fatigues and splits. Poor support underneath and foot traffic add to the damage. Correct lengths, fixings and codes prevent it.
Thermal movement is the cause behind most cracks
Lead is not a stiff metal, but it does respond to temperature. On a warm, sunny day a sheet on a south-facing slope grows slightly. After a cold night it shrinks back. Over a single day the change is small. Over years of weather it adds up to countless small movements, and the metal has to take up every one of them.
If the sheet is laid so that it can slide a little, the movement spreads across its length and does no harm. Roofers have relied on lead for centuries because it copes with this so well. The problem starts when the sheet cannot slide. Then each warming and cooling puts the same stress on the same few points. The metal work-hardens there, becomes brittle and finally cracks, usually along a fold or beside a nail line. This is fatigue, the same thing that breaks a paperclip you bend back and forth.
A crack caused this way tends to look like a clean, fine line across the sheet at a right angle to the direction of movement. You may notice it first as a damp stain below, or as a slight whitening along the line where water has been sitting. Our answer on repairing split lead explains when you can repair such a crack and when you should not.
Pieces cut too long
The most common mistake is a piece of lead that is simply too big. The longer the sheet, the more it expands and contracts at its ends, and the more strain it puts on whatever is holding it. Trade guidance sets maximum lengths for this reason. The Royston Lead brochure gives 1500 mm as the maximum for flashings and for valley gutters on a pitch, in codes 4 and 5. A flashing is the lead strip that seals the line at the point a roof butts a wall or chimney, and a valley is the sloping channel where two roof slopes meet.
Across a flat roof or along a gutter, the matching control is the spacing of drips, the small steps that break a long run into bays. The Lead Sheet Association, quoted on the Building Conservation website, gives 500 mm as the greatest spacing for the wedges that hold cover flashings in a wall. Both limits exist so that the metal has somewhere to go. Roofs where lead has been laid in one long run, or where drips are missing, are the roofs that crack.
Repair work sometimes makes the problem worse. Someone replaces a failed piece with a longer one to save on joints, and the new sheet starts to crack within a couple of winters. Fewer joints do not mean a better roof if the sheet cannot move.
The wrong code of lead
Lead sheet is made in codes, numbered by thickness and weight. Code 3 is 1.32 mm thick and weighs 14.97 kg per square metre. Code 5 is 2.24 mm and 25.40 kg. Code 8 is 3.55 mm and 40.26 kg. These figures come from Leadworx, and the uses come from Royston Lead.
Royston's guidance puts code 3 as soaker-only material, a soaker being the small piece of lead set under a tile or slate at an abutment. Flashings and pitched valleys are code 4 or 5. Parapet and box gutters are code 5 or heavier, with codes 6, 7 and 8 used where more is asked of the sheet.
A lighter code than the job needs is thinner and easier to bend and crack. It also tears more readily at a fixing. The result is a roof that may look fine for a few years and then splits in the first hard winter. Using the right code costs more in metal but usually saves money over the life of the roof. If you have a quote that does not mention the code, ask for it. Our page on lead codes gives the full table.
Fixing, support and everyday damage
Good lead is held with clips and secret fixings that let it move. Bad fixing pins the sheet in place. Nails driven through the face of the lead, a fixing line too close to the edge, or a sheet stuck down with sealant all stop the metal sliding. The sheet cracks beside the fixing rather than moving under it.
Support matters too. Lead needs a firm, flat, clean deck, with a separating layer between it and the boards so that it does not catch on rough timber or react with the wood. Sheet laid over a gap, a nail head or a ridge in the boards will thin out over that point, and eventually crack there.
Everyday damage finishes the job. Someone stands in a gutter to clear leaves. A television engineer rests a ladder on a flashing. A tree branch rubs back and forth in the wind. Each leaves a small crease, and creases are places where fatigue starts. Foot traffic should be kept off lead wherever possible, and boards should be laid in a gutter if access is needed regularly.
Not all damage comes from the roof. Frost, trapped water and acid from some timbers or lichens can eat the surface of the metal over time, which thins it and makes a crack more likely at the next movement.
What a lasting fix looks like, and when to do it
You cannot cure fatigue by patching over it. If the cause is movement, the fix has to let the lead move. That usually means cutting the affected sheet back to sound metal, replacing it with the correct code, dressing it in lengths that suit the location and fixing it with clips. Where bays are too long, extra drips or a change of detail may be needed, and that can mean a wider job than the crack suggests.
Winter conditions affect how and when lead is laid. See our answer on leadwork in winter for what is and is not practical in cold or wet weather. For the same principle applied to valleys, see our page on lead valleys explains how the sheet is laid so that it can move.
Surveying cracked lead costs you nothing, and our pictures of your roof show which cracks are isolated and which are signs of a design fault. Where repair works we repair, and where a bay has to be renewed we explain why. If a crack is letting water in now, a temporary patch can hold until a dry, mild spell arrives for the proper repair, as lead is easier to dress when it is not cold. For other lead questions, return to lead roofing.
