The short answer
A chimney back gutter is a lead channel built into the slope just above a stack. Rain running down the slope meets the stack, and the gutter gathers it and sends it round either side onto the roof below. It fails when the lead splits, lifts or blocks with debris, and the repair is usually new lead dressed into the stack, not a sealant.
Where the back gutter sits and what it does
A stack that comes up through a sloping roof forms a barrier to the water running down. On the downhill face, flashing at the base sheds water away easily. On the uphill face, the water arrives in a rush and has nowhere to go. The back gutter solves it. Formed from sheet lead, it is a channel laid into the slope behind the stack, with a slight fall to each side so water drains away round the corners. Its upper edge is tucked under the tiles or slates, its lower edge turns up the brickwork, and the ends meet the step flashing on either side.
Some roofers call it a back gutter, others a cricket or a saddle, and the shapes differ a little. A wide stack on a steep roof may have a ridged, pitched gutter that divides the flow, where a narrow one has a simple trough. Whatever the shape, the job is the same: stop a pool of water forming against the brick and finding its way in.
Fall is the detail that decides whether a back gutter works. The floor of the channel must slope so water runs out at the sides and never lies still, and the upstand against the brickwork has to be high enough to stop a heavy downpour riding over it.
Why back gutters fail
This is the hardest-working piece of lead on the roof, and the one most often neglected. It sits where moss, leaves and silt gather, so it can hold water for days. Typical faults are:
- blocked outlets, where debris traps water and lets it overtop the lead
- cracks across folds, where lead has fatigued from movement
- lead pulled away from the brickwork by weight of silt, or by a cement fillet that has cracked
- poor original detail, such as a gutter without enough fall
- nail holes or screws through the lead that were meant to hold it
Because the gutter is hidden from the ground and often from a ladder, faults go unseen for years. Often it announces itself as a stain beside the breast on an upstairs ceiling, or damp on the wall of the room below. Our guide to chimney leaks shows how to tell the back gutter apart from the other leak paths.
How it differs from the side and front flashings
People often use "flashing" for all the lead around a stack. In practice, the parts do different things. Step flashings run up the sides, apron flashing sits across the front, and the back gutter carries the water that the roof delivers to the rear. Soakers lie under the covering at the sides. Our answer on when chimney flashing needs replacing deals with the side and front pieces, so here the focus stays at the back.
A single weak point at the back can undo good work elsewhere. A stack with perfect new side flashings and an old, split back gutter will still leak, because the rear is where the water arrives in greatest volume. When we survey a stack, we look at the back gutter first for that reason, and photograph it from above where we can.
Where a window or higher ground gives you a view of the roof, look for a ridge of lead behind the stack and check that no weeds or moss sit along it.
Repairing or replacing a back gutter
Clearing silt and debris comes first, and now and then that is all that is needed. A small split can be repaired by a lead worker with a patch, welded or dressed over, though solder or sealant used badly will not hold. Where the lead is cracked in many places, or has been repaired repeatedly, a new gutter is the lasting answer.
New lead is dressed to the profile of the roof and the stack, tucked under the covering, turned up the brickwork and wedged into raked-out joints. The Royston Lead brochure lists Codes 4, 5 and 6 among the grades used for pitched valley gutters, and the thicker grades stand up better to the weight of standing water. We choose the code at the survey. MyJobQuote's 2026 guide says correctly installed lead can last over 80 years, so a properly built gutter is a long-term repair, while mortar and sealant fillets are not.
On a stone or old brick stack, the joints receiving the lead are repointed in a suitable mortar, which is covered in our notes on DIY chimney repointing.
Cost, access and ways to reduce the problem
For flashing repairs, MyBuilder's 2026 chimney guide gives £250 to £1,200 and MyJobQuote gives £150 to £250 for a patch. Those are national ranges for flashing work as a whole, and a full new back gutter sits towards the upper end where scaffold is needed. Once we have surveyed, the quote covers scaffold, lead, repointing and tidying up.
Some owners choose to remove or lower a stack that serves nothing, which takes away the back gutter along with the stack's weight. Our guide to reducing chimney height sets out when that makes sense. If the stack stays, a yearly look from a ladder or an inspection after storms keeps the gutter clear.
We leave the site clear, and the 10-year workmanship guarantee is issued in writing. Other work on stacks is on our chimney repairs page.