Every house with a side wall that rises above a lower roof has one of these: a gable wall that stands up beside a porch or extension roof, a chimney stack that sits in the slope, a dormer cheek or the party wall between two terraced houses. At each of these, a sloping roof ends against a vertical surface and the join runs diagonally down the slope. Rain runs down the wall and across the join at every course of tiles or slates. To keep it out, roofers use two pieces together: a soaker beneath the covering and a stepped flashing above. Most people have never heard of either, and yet failures here account for a great many leaks. This guide explains how they work.
The problem an abutment has to solve
Where a roof meets a wall, there is a gap. Tiles cannot be built into brickwork like a bond, and the roof and wall move differently with temperature, damp and settlement. If the gap is left open or filled with mortar alone, water will find it. The detail has to carry water from the wall face across the join and over the surface of the tiles, accept some movement and stay fixed in a gale. It also has to be done at an angle, because the join steps down the slope as the roof falls away from the wall, and every tile or slate course meets the wall at a different height.
That is why the solution has two parts. A soaker is the piece that sits under the covering and turns up the wall. A step flashing is the piece that sits over the soaker, tucks into the wall and covers the join. Together they make a watertight chain, one piece at each course, that follows the slope.
What a soaker is
A soaker is a small rectangle of lead, bent to an L-shape. One leg lies flat on the roof, under the slate or tile at that course, and the other leg turns up the wall. Each course has its own soaker, so a slate roof with fifteen courses beside a wall will have fifteen soakers interleaved with the slates, each overlapping the one below. Water that runs down the wall hits the vertical leg, drops onto the flat leg and is carried out over the top of the slate below, not behind it. Soakers are normally made of code 3 lead, since they are covered and protected. Our guide to lead codes shows why a light code is enough there.
On slate, soakers are cut to the size of the slate, with a head lap that depends on the roof's pitch. On plain tiles they are cut to suit the tile gauge. They must be long enough to lap properly and wide enough to project out from under the covering, but cut and sized for each roof, which is why they are usually made on site and not bought ready-made.
What a step flashing is
The step flashing, also called a stepped flashing, is the lead that you actually see. It is a strip cut in a series of steps, matching the diagonal line of the roof, with each step tucked into a mortar joint in the wall and the lower edge dressed over the roof covering. At the wall, the top edge of each step is turned into a groove called a raggle, or into a raked-out joint, and wedged with lead wedges before being pointed in mortar. The lower edge of the flashing laps over the upturned leg of the soaker beneath, so any water that gets past the flashing meets the soaker and runs out onto the tiles.
The cut of the steps matters. They are set to follow the brick courses of the wall, so the lead goes into the mortar joint and not across a brick. A good step flashing looks neat and regular. A poor one, with wavy steps and gaps, looks untidy and leaks.
Where you find them
- Gable abutments. A lean-to or porch roof meeting a house wall.
- Chimney sides. The sides of a stack are stepped, while the front has an apron and the back has a gutter.
- Dormer cheeks. The roof slope meets the side of a dormer.
- Party walls. Terraces and semis where the dividing wall rises above the roof.
- Rooflight and skylight edges. Where a window frame sits in the slope, with its own set of flashings.
On a chimney, the stepped flashing is one piece of a set, which also includes the apron, the back gutter and the cover flashings. Our guide to the chimney back gutter describes the piece behind the stack.
Why they fail
Because soakers are hidden, they are the part that fails without anyone noticing. Lead soakers on very old roofs can be very old, but they may have corroded where leaves and moss collect behind the flashing, or where water has sat in a poorly drained corner. A soaker that is too short does not lap enough and lets water through, which is common on older repairs where the soakers were cut to save lead. A soaker that has slipped, or one that was never fixed, may have dropped down so the head no longer sits under the slate above. And a soaker that was replaced with a sealant bead is no soaker at all.
The stepped flashing fails differently: the pointing at the wall crumbles and the wedges loosen, so the lead pulls out. The steps crack at their corners, where bending has hardened the metal. Wind lifts an unclipped lower edge and bends it away from the tiles. Lead that has been dressed without a clip or tack can creep down, leaving a gap at the top. The usual causes and the symptoms are listed in our page on the signs of failing lead flashing.
| Part | Job | Typical code | Common failure |
|---|---|---|---|
| Soaker | Carries water out over the covering | Code 3 | Too short, slipped or corroded |
| Step flashing | Covers the join and tucks into the wall | Code 4 | Pointing failed, corners cracked |
| Cover flashing (top) | Shields the top edge of the soakers | Code 4 | Lifted or poorly wedged |
| Sealant fillet | Not a proper part of the detail | None | Cracks and hides leaks |
Tiles and slates are not the same
On slate and on small plain tiles, individual soakers work well, because the covering is laid in small overlapping units and a soaker can be slotted between each. On large interlocking concrete tiles, which are laid in wider units, individual soakers do not suit the profile. The abutment is instead formed with a continuous flashing, usually lead dressed over the tile profile with a rolled or welted edge, or a preformed metal or GRP trim designed for the tile. Those systems rely on the cover flashing and the tile design to carry the water, and the manufacturer's installation guide should be followed. When roofs are re-covered in a different tile, the abutment detail has to change with it.
Soakers on a chimney, step by step
To see how the pieces fit, follow one side of a chimney stack down the slope. At the top, where the stack leaves the ridge side, the back gutter turns water round the corner. Down the side, the first soaker sits under the top slate beside the brickwork, with its upturned leg running up the face of the stack. The first step of the flashing is tucked into the joint above and laid down over that leg. The next slate down gets its own soaker, lapped over the one above, and the next step follows, until the bottom of the stack, where the apron takes over. Each soaker is held by the slate on top of it and by a small nail or clip at its head; none is stuck down. The result is a staircase of small overlaps, each shedding water onto the next.
It is a simple idea, and its strength is that no single piece has to carry much water, and any leak at one step is caught by the soaker beneath. Its weakness is that all the pieces have to be present, correctly lapped and sound, and a missing one breaks the chain.
Replacing and repairing them
A minor fault, such as a loose bit of pointing or a lifted step, can be dealt with by re-wedging and re-pointing. A cracked step can be replaced by cutting in a new piece. A fully failed abutment needs the covering lifted along the wall, the old soakers and flashing stripped, the wall and chase cleaned and repaired, and the whole set re-laid: new soakers, new steps, new pointing. That involves taking up and replacing the first few courses of tiles or slates at the wall, and it is worth doing at the same time as any re-roofing. When each approach is worth the money is set out on the page about mending or renewing a flashing.
Sealant is not a substitute. A bead of mastic along the join is no soaker, will not take the movement and hides the fault. Our answer on repairing flashing with sealant explains why we avoid it.
Why we pay attention to the hidden pieces
When we survey an abutment, we look at the visible step flashing and then at what is likely to be underneath. If the roof is being re-covered, we recommend renewing the soakers rather than reusing old ones. They are cheap to make, and they cannot be inspected again for decades once the tiles are back. Reusing old, short or corroded soakers under a new roof is one of the most common ways for a leak to return within a few winters.
What we do
We check the flashing, the pointing and the wall, raise a tile or two where the roof allows, to see the soakers underneath, and take pictures of what is there. You see your own roof's detail. Where a repair will hold, you hear that first. If the abutment needs renewing, we quote a fixed price in writing, with the 10-year workmanship guarantee set out in the paperwork. The wider range of lead work we carry out is described under lead roofing in Basingstoke. For an abutment of your own, the line to use is 01256 213956, with WhatsApp as the alternative.
