The short answer
Lead soakers are small rectangles of lead, bent into an L, with the flat leg lying beneath a tile or slate and the other leg turned up against the wall. One goes in with every course of tiles. They carry rain that gets under the tile edge back out onto the roof surface, and they work with the stepped flashing laid over the top.
What a soaker is, where it hides beside a gable or stack, and why a roof with step flashing needs them.
What a soaker actually is
A soaker is a piece of lead cut to a set size and folded along its length so it forms an L. The wide leg lies flat on the roof, tucked beneath the tile or slate in one course, and the narrow leg turns up against the vertical face of the wall or chimney. Because tiles overlap in rows, a separate soaker is fitted with each course, so the soakers also overlap and step up the roof together. A run of them beside a long gable can number a good many pieces, each cut and folded by hand or to a template.
The lead is thin and easy to bend. The Royston cast lead brochure recommends Code 3, at 1.32 mm, for soakers only, and also lists Code 4. Thicker sheet suits other details but is awkward to fold under tiles. The size of the soaker depends on the tile: it must reach far enough under the tile to stay put and far enough up the wall to catch water that runs down it.
It helps to picture the job. Where a tiled slope ends at a gable wall or runs alongside a chimney, the tile edge cannot be sealed tight to brick. Water will get in at that edge. Soakers are the hidden layer that deals with it.
How soakers work with step flashing
Soakers are only half of a pair. The other half is the flashing, a strip of lead dressed over the top. Where the roof meets a rising wall, the flashing is stepped so that it follows the line of the tile courses, and its top edge tucks into a groove in the brickwork. This is step flashing, and our answer on step flashing vs cover flashing shows how it differs from a straight cover.
Together they make a two-stage defence. The flashing stops most rain reaching the joint at all. Any water that gets past it falls onto the soaker, which sheds it down and out over the surface of the tile below. Neither part alone is as reliable as the two working as a pair, and each depends on the other being tucked, lapped and fixed correctly.
Not every roof uses this method. Some modern work uses a continuous, one-piece flashing system, or a pre-formed proprietary product, instead of separate lead soakers. A roofer should say which they are fitting on your wall and why.
Why they matter, and how they go wrong
A missing or damaged soaker is a hidden fault. You will not see it from the ground, and the flashing above can look perfect. The first sign is usually a damp stain on a ceiling or wall beside a chimney or gable, after heavy rain or a gusty wind that drives water sideways.
The causes are ordinary. Old soakers can be cut too short, so they never reached far enough under the tiles. They can be left out in a rush, or lifted and not replaced when a tile was changed. Lead that is thin or tired can crack at the fold, and corroded or torn lead stops guiding water at all. After the soaker fails, damp can soak into roof timbers long before it appears indoors.
Re-tiling beside a wall means renewing the soakers at the same time, since they are laid course by course with the tiles. Reusing old ones is rarely worthwhile once they have been disturbed. Where only one or two have failed and the tiles around them are sound, a repair can be possible, but the roofer has to be able to lift the tiles without breaking them first. Old, brittle tiles often make a full strip along that edge the more sensible job.
What to ask about soakers in a quote
If you are having a wall or chimney re-flashed, ask whether new soakers are included, and what lead code they will be. A well-written quote names the code and says whether the tiles beside the wall will be lifted to fit them. Our lead sheet codes explained guide and the short answer on what code lead for flashing help you read the answer.
Ask, too, for photos taken before the flashing goes over them. Soakers are covered once the job is finished, so the only chance to see them is during it. A roofer who is happy to share a photo of the soakers laid course by course is showing you work that will be hidden for years.
We survey first and photograph the wall edge from the access we set up, and the price comes as one written total once we know what the edge needs, with the lead code for the soakers named in it. If you want the broader picture, our lead work page covers flashings, valleys and the rest of the trade.
More on this in Lead Work & Lead Flashing in Crawley
Two more soaker questions
Can soakers be replaced without re-tiling?
Sometimes, but not easily. Each soaker sits under a tile, so the tiles along that edge must be eased up course by course to slide new ones in. If the tiles are brittle or the fixings are old, lifting them can break them, and re-laying that strip is the better way to do it.
Do soakers make step flashing unnecessary?
No. Soakers carry water that gets under the tile edge, and flashing seals the gap between the tiles and the wall above. They do different jobs and work as a pair. A wall edge with soakers but no flashing, or flashing but no soakers, is leaving a gap for rain.