The short answer
Lead soakers are small, pre-bent pieces of lead, one for each course of slate or tile, that sit at the edge of a roof where it meets a wall. They lie under the stepped flashing and over the covering, so any water that gets behind the flashing is led out onto the slates or tiles below. Code 3 is the usual sheet for them.
What a soaker is and where it sits
Take a roof slope that ends at a taller wall, whether the flank of a chimney stack or a gable end: the covering cannot simply butt the masonry. Water would run down the wall and in at the joint. The answer is a two-part seal: a soaker and a flashing.
A soaker is a rectangle of lead folded to an angle. One leg lies flat under the slate or tile, and the other turns up against the wall. There is one soaker for every course, each overlapping the one below, so together they form a stepped waterproof lining up the side of the roof.
The flashing, a separate strip of lead, is dressed over the upturned leg and sealed into the wall. You can see the flashing from the ground. You cannot see the soakers, which is why they get forgotten and why a faulty one can go unnoticed for years.
How soakers and stepped flashing work together
Each part handles water the other cannot. The flashing sheds most of the rain that runs down the wall. If any gets behind it, or blows under it, the soaker takes it on a flat, fall-away surface and directs it onto the covering, which carries it to the gutter.
A soaker is fixed by the slate or tile that sits on it. It is not nailed through the visible face. That keeps the roof weathertight and lets the lead move a little with the weather. The upturned leg is held by the wall and the flashing above it.
The stepped line of the flashing follows the slope, with one step to each course, so that every soaker has a matching step above. If the courses are of different heights, as in stone slates with diminishing courses, the soakers have to be cut to suit each one. Our page on where leadwork sits on your roof shows how the parts fit together across a whole house.
Which lead code a soaker needs
Leadworx lists Code 3 for soakers and light duty, and Royston Lead's typical-use classes list Code 3 for soakers only. It is 1.32 mm thick and weighs 14.97 kg per square metre, and its roll is marked green. Code 4, 1.80 mm thick, also appears for soakers in Royston's list and is the usual sheet for the flashing above.
The choice is about weight and cost. There are many soakers on a roof, perhaps a score down one side of a chimney, and all of them are hidden. A light sheet does the job and keeps the load on the slope down. Because a soaker is protected from weather and sun by the covering, it moves less than the flashing does.
Our short answer on the lead code for flashing covers the visible strip. This one is about the part you cannot see.
Why soakers matter, and how they fail
A soaker is a backup, so a fault in it may not show for a while. The first sign is often damp at the ceiling near a wall or stack, long after the flashing looks tidy from outside.
Missing or too few soakers
Some older roofs have flashings but no proper soakers, or soakers that have been replaced by a smear of cement or a strip of felt. Those hold for a time, then fail when the cement cracks.
Soakers too short
A soaker that does not reach far enough under the covering, or up the wall, lets water bypass it. Lap is a measurement we take on site, noting how far each piece reaches under the covering and up the masonry.
Corroded or torn lead
Soakers last a long time, but not forever. Lead under a roof covering can be damaged by acidic mortar or by being moved when the slates were re-laid.
Disturbed during other work
Slipped slates, new tiles or a rebuilt stack can all knock soakers out of line. For that reason a re-roof or a stack rebuild is the natural time to renew them.
Renewing soakers during a repair
Replacing soakers means lifting the slates or tiles along the wall, a course at a time, then fitting new lead and re-laying the covering over it. It is more work than patching a visible flashing, and it is why a stepped flashing repair that ignores the soakers underneath may not last.
On a stone or slate roof, the loosened slates are set aside and re-used where they are sound, with matching replacements for any that are not. Our guide to lead sheet lengths explains why each piece is kept short. Keeping the stepped pieces small, and cutting each soaker to the height of its own course, also means one damaged soaker can be changed without disturbing the rest of the run, and the stepped line above is sealed as covered in the answer on pointing lead flashing.
The quote after the survey lists the soakers, the flashing, the slating and the scaffold as separate lines, alongside the survey photographs of your roof. Our workmanship guarantee runs for 10 years and is put in writing; the survey photographs show the wall before anything is lifted, so you can see what was found underneath once the courses are open. The service as a whole is described on lead roofing, or ring 01453 933613 with a picture of the wall in question.