What Is a Chimney Back Gutter?

Why a chimney needs one

A chimney stack that rises through a sloping roof is like a dam across a stream. Rain running down the slope reaches the back of the stack, which is on the high side, and has nowhere to go but to pile up against the brickwork. Without a gutter, that water would find the joint between the brick and the tiles and run into the roof. The back gutter solves it by giving the water a channel to follow. It spans the width of the chimney, falls from the centre to each side, and ends over the roof covering on either side of the stack, so the water leaves in two streams and carries on down the roof. Without it, the uphill face of the chimney would be a constant leak.

How it is built

The base is a board or a shaped timber, set between the rafters at the back of the chimney. It is cut to a gentle fall from the middle to each side, often as a shallow double slope, with a solid support so it does not sag. A layer of underlay or a separating membrane goes over the board. The lead lining is then cut and dressed over it, turned up the back of the chimney, and lapped up the roof slope on the uphill side, where it passes under the tiles or slates. At the sides, it runs into the stepped flashings that seal the sides of the stack. At the front of the chimney, where the roof meets the stack on the downhill side, an apron flashing is used instead of a gutter. The pieces of lead overlap in a set order so that water can only run over the top of a joint, not into it. The side pieces are explained under soakers and step flashing, while the wall fixing is described in the page on lead flashing set into brickwork.

You may also hear this called a saddle or a cricket on some roofs. A lead saddle is a related piece that sits over the junction at the upper corner of a stack or a ridge, which we describe in what is a lead saddle.

Why back gutters leak

The back gutter collects a lot of water and leaves, moss and grit in a small space, so it is a place where trouble builds up. A blocked gutter fills and overflows into the roof. A gutter with too little fall holds water, and standing water finds any pinhole. Lead that was laid in a single long piece can split with movement; see why lead cracks. Board that has rotted underneath lets the lead sag into a dip, which then ponds. Poor lapping at the corners, where the gutter meets the side flashings, leaves open paths for water. Sealant smeared over the whole area is often a sign that someone has already tried to stop a leak without success.

A back gutter is also easy to miss. It is behind the chimney, out of sight from the ground, so leaves can build up for years before anyone looks. A stain on the ceiling or the chimney breast below, especially after heavy rain, is often the first notice.

Keeping it clear

A little maintenance goes a long way. Ask a roofer to look at the back gutter whenever the chimney is being repointed or the roof is inspected, because that is when the stack is already accessible. Leaves from nearby trees collect there quickly, and moss thrives in the damp. Clearing the gutter once a year is cheap compared with repairing rotten board and ceilings. Look for signs from indoors too: a damp patch on the chimney breast in the loft, or a tide mark on the ceiling beside the stack, suggests the gutter is overflowing or leaking.

Repair or rebuild

If the lead is sound and the gutter is simply blocked, clearing the debris and checking the laps may be enough. If the lead has split in a few places and the board is firm, patching or welding may do. If the board is soft, the fall is wrong or the lead is thin, the honest answer is to strip and rebuild the gutter, replacing the board, relaying the lead in the right order and reconnecting the side flashings. That often goes together with repointing the stack and checking the flaunching around the pots. Our page on lead flashing repair or replace sets out how to weigh the choices.

Access matters, because the work is at the top of the roof, and scaffolding may be needed. We inspect the gutter, photograph it, and tell you what it needs. The lead roofing and leadwork hub has related pages, our homepage shows the other work we do, and the number is 01256 213956.