Where the back gutter sits and what it carries
Stand in the loft or look at a stack from a ladder and you can see the problem the back gutter solves. A chimney is a wall that rises through a sloping roof. Rain and meltwater run down the slope towards it, and the uphill face of the stack acts like a dam.
The back gutter is the answer. It is a trough of lead, wider than a flashing, laid into the slope against the uphill face of the stack, with its edge turned up the brickwork and its fall running to both sides. Water that reaches it travels sideways, past the corners, and rejoins the roof covering.
You will sometimes hear it called a back tray or a saddle. The names vary by trade and region, but the job is the same: stop the stack from being a barrier.
On a Thanet terrace with a stack set low on the slope, the back gutter is often the least visible part of the chimney. That is exactly why it is the part owners forget about, until a brown stain appears on the chimney breast below it.
Why a back gutter blocks and leaks
A back gutter keeps water standing for longer than a free slope does, because its fall is gentle and the channel is hemmed in by brickwork. That has two effects.
- Moss, leaves, grit and flakes of crumbled mortar settle in the channel and trap more behind them. When the channel is full, water spills over the top edge and into the stack or the loft.
- The lead flexes with temperature, and the folds in it work hardest. If the trough is formed from one oversized piece, cracks open at the creases where the metal has moved most.
A third cause is shape. A gutter with too little fall holds water, and one that is too narrow cannot carry a heavy downpour round the corners. Both are faults of the original build, and no amount of cleaning cures them.
Look for staining on the chimney breast below, damp patches on the ceiling beside the stack, or a green line of moss across the uphill face. Clearing the channel is a fair first step when the lead is sound. If the lead is cracked, clearing it only reveals the leak, and the next heavy shower will find it again.
Replacing it with the rest of the stack's leadwork
A back gutter does not stand alone. It meets the apron (the flashing across the bottom face of the stack), the step flashings that climb the sides, and the soakers, the folded pieces that lie under each tile or slate where it meets the brickwork. If one piece is worn out, its neighbours are the same age and have had the same weather.
For that reason we renew the leadwork of a stack as a set. The old pieces come off, the brickwork joints are checked, new lead is cut and dressed, and the top edges are wedged into chases and pointed. The Royston Lead brochure names Code 3 for soakers only. The thickness for the gutter and the other pieces is written into the specification.
The price of the whole set depends on the stack: how many faces, how high, how easy to reach and what state the brickwork is in. In September 2026 MyJobQuote gave "between £350 and £920" as a typical UK range for replacing the leadwork on a chimney. Your own total is written after the survey, once the stack has been seen up close.
Questions to ask before the lead is ordered
The back gutter is a place where a shortcut shows within a couple of winters. These questions separate a thorough job from a hurried one:
- Is the gutter made of one piece or joined, and where do the joints fall?
- How much fall does it have, and does it reach both corners?
- Which lead code is it, and is that written in the quote?
- Will the brickwork be repointed where the top edge sits?
- Is the stack's other leadwork being replaced at the same visit?
If the stack is old enough that its mortar is crumbling, ask whether the stack itself needs attention first. New lead pinned to loose brick will not last, and a roofer who cares about the result will say so before the lead is cut. Ask also whether the lead will be lifted in sections or taken off in one go, because a stack left open for days in wet weather invites the very leak you are paying to cure.
Related reading: what a lead apron is and stepped lead flashing, the two pieces that meet the back gutter. The wider service page is lead roofing.
