The short answer
Drips and rolls are the joints that break a lead roof into bays. A drip is a step down in the roof deck that carries water across a joint, and a roll is a timber strip wrapped in lead that joins two sheets along the fall. Both let the lead expand and shrink with the weather instead of tearing itself apart.
How lead moves, what drips and rolls do about it, and why a lead roof laid in one big sheet is heading for cracks.
Why lead needs joints that move
Lead grows in the sun and shrinks in the cold, and a sheet on a flat roof can go through a large temperature swing in a single day. A big sheet fixed hard at both ends cannot stretch, so it stresses at the weakest point and splits. Over a few winters the same sheet can also creep down a slope under its own weight. The cure is not to stop the movement but to allow for it.
That is what the joints do. They divide the roof into smaller pieces, each free to move a little, and they are built so water cannot get underneath as the lead slides. The sheets are held at the top by clips or nails hidden under the joint, never nailed through the visible face, so each sheet hangs and moves from a single point.
That is the reason lead decking over a flat roof or a bay is laid in separate pieces, each a set length, rather than one wide sheet. A lead roof lasts as long as its movement allowances do, and those are cheap to build in and costly to put right afterwards. Missing joints are the commonest reason a young lead roof splits.
Drips and rolls, one by one
At a drip the deck drops by a step, formed by the timbers beneath, so the lead can be dressed down over the edge of the upper bay and under the lip of the next, with an overlap that sheds water. It runs across the fall, which means it also gives the roof its slope in stages. The lead at a drip is folded so the movement can happen at the step.
A roll runs with the fall, up and down the slope, and is made over a rounded timber batten. One sheet is dressed up and over the batten, the neighbouring sheet is welted round it, and the result is a raised seam you can see from a window above. A roll can be a solid roll with a cap or a hollow roll, and both let the sheets on either side move without opening the seam.
You often see rolls on a long flat roof and drips on a roof with a pitched profile or a gutter. Neither is decoration. Where you see the seam lines on an old lead roof, you are looking at the design that has kept it watertight.
How far apart they go
Spacing depends on the weight of the lead, called its code, and on the fall of the roof, so there is no single figure. A thicker sheet is stiffer and carries bigger bays than a thin one. Royston Lead's brochure sets out the code weights, from Code 5 at 25.40 kg per m² to Code 8 at 40.26 kg per m², and lists the heavier codes for parapet gutters, box gutters and tapered valleys, where the lead has to work hardest.
On a survey we measure the roof, note the fall, and set out the bays and joints on paper before any lead is cut. The written spec says what code is going on and where the drips and rolls fall. If a quote for a flat lead roof makes no mention of them, ask. A roof laid with the bays too long, or the roll positions ignored, can look fine for a couple of winters and then starts to crack.
The fall is separate. A flat roof still needs a slope so water runs off, and the drips are one of the tools that deliver it. Our guide to lead sheet codes goes into the weights.
Spotting splits and putting them right
A lead roof that was laid without drips, or with them too far apart, usually shows its problem along fold lines and at the corners first. Look for a hairline crack running across a sheet, usually at right angles to the fall, or a ripple that has built up near a fixing. A split sheet lets rain through, and the first sign inside is often a tea-coloured patch on the ceiling underneath.
A small crack can sometimes be dressed over or patched, but the cause matters. If the bays are too long, a patch will crack again next to itself. The lasting fix is to cut the roof into correct bays, put in the missing joints, and relay the lead on the right fixings. That is more work than a patch and the written quote reflects it.
Our answer on why lead flashing cracks explains the wider causes, and lead flat roofs covers the whole build-up. For the main service, see lead work.
More on this in Lead Work & Lead Flashing in Crawley
Two questions on lead joints
Do drips and rolls make a lead roof leak?
Properly built, they do the opposite, because they are where the roof is designed to move without opening. Leaks at joints come from poor workmanship, such as a roll with a loose cap or a drip lapped the wrong way, and those are put right by rework at the joint rather than by leaving the lead alone.
Can I add a drip to an existing lead roof?
Only by lifting the lead and altering the deck, since a drip is a step in the timbers underneath. That can be worth doing where long bays are splitting. We would survey the roof first, check the fall, and put one total in writing for relaying it in correct bays.