What Is a Chimney Back Gutter?

Water runs down a roof slope until something stops it. The back of a chimney stack is that something. Without a channel, rain and leaf litter bank up against the brickwork and find any weak joint. The back gutter is a shallow lead trough on the high side, running across the full breadth of the stack and a little beyond it.

How does water get away from it?

The gutter slopes slightly so water travels to the sides, where it runs onto the roof covering past the stack. That is why it is often described as a small valley, the angled channel where two roof slopes meet, laid flat across the slope instead. The sides of the gutter turn up, the back edge is lifted behind the stack, and the front edge sits over the tiles or slates beneath.

Because it lies upslope, the back gutter is the piece most affected by moss, bird nesting material and windblown leaves. A trough that cannot drain holds standing water, and standing water finds every pinhole. On a stack set well down the slope, the volume of water arriving at the gutter is larger still, so the lead has to be formed generously and kept clear.

Most failures come from a few repeating causes. Thin lead is the first. A back gutter works hard and moves with temperature, so light lead splits along its folds. Poor shaping is the second: a gutter without enough lift behind the stack lets water overtop in heavy rain.

Is a leaking gutter always the lead's fault?

Not always. Blocked outlets, a gap where the gutter meets the step flashing at each side, or cracked mortar above the lead all let water bypass perfectly good sheet. Old repairs with bitumen or roofing sealant often hide the real fault for a season, then make a proper repair harder, as our answer on sealant on lead flashing explains.

A stained ceiling next to the chimney breast, or damp on the breast itself, points to the back gutter more often than to the apron at the front, because the gutter holds water the longest. Clearing leaves from the gutter twice a year, from a safe position, costs nothing and slows the damage. Anyone who needs a ladder to reach the stack should leave the lead alone and call a roofer, because a slip from that height is serious.

Lead comes in numbered codes by thickness. The Royston Lead brochure lists a pitched valley gutter, the closest comparison to a back gutter, under codes 4, 5, 6, 7 and 8. Code 4 is 1.80 mm, code 5 is 2.24 mm, and code 6 is 2.65 mm. A back gutter is a long, flat-lying piece, so we lean towards the heavier end of what is listed.

Does thicker lead always mean a better gutter?

Not on its own. A thick sheet that is cut too long still buckles as it warms and cools. What matters is a code matched to the breadth of the stack, and a gutter formed from sheets sized for lead's natural movement. Our answer on why lead has size limits covers that movement.

The quote should say which code is going in. We tell you after the survey, and it sits inside the one written price. A code named on paper is a useful check that the lead delivered to site is what was agreed.

Lead can last for over 80 years, and a back gutter that is correctly formed and kept clear gets closer to that figure than one left to silt up and hold water through every winter.

Small splits can sometimes be patched with a welded or soldered repair, though that suits only sound surrounding lead. If the gutter has lost its fall, or the lead has crept and thinned, the stack is better done properly from a clean start.

What does a rebuild involve?

The old lead comes off, any rotten boarding or timber under it is checked, and a new gutter is formed to fit. The sides are tied into the step flashings, the back edge is turned up and chased into the brickwork, and the front laps over the covering. Chasing means cutting a slot into the mortar joint and wedging the lead in. Our answer on how lead flashing is fixed into a wall shows how.

The apron on the front face is often renewed in the same visit, because disturbing one piece loosens the next. Our lead work page sets out the pieces that make up a chimney. A photo of the stack is all we need to begin: after the survey you see pictures of the gutter itself, and receive one written price covering the whole rebuild.