Low-Pitch Roofs and Chimney Flashing: Why Margins Are Tight

Why a shallow pitch changes the rules

On a steep roof, rain leaves quickly. It runs down the covering, hits the gutter and is gone, and any weak detail gets only a brief soaking. On a shallow roof, water moves slowly, and in strong wind it can be driven sideways or even pushed up under the laps. A joint that would be adequate at a steep angle can leak at a shallow one.

The effect is greatest at a chimney. A stack is a solid block that interrupts the flow of water across the roof, so it collects water on its upslope side and forces it to move around. On a low-pitch roof that water has less slope to drain by, spends longer at the junction, and has more chance of finding a gap. Debris and moss that collect behind the stack slow it further, which is why the area behind a low stack is the first place to suspect.

Much of the newer housing in Runcorn was built with shallow roofs and short stacks. That is a sensible arrangement structurally, but it leaves the lead work around each stack with less room for error than it would have on a steep pitched roof. A detail that passed unnoticed for years on a steeper roof can start to weep here after a single wet, windy winter.

Headlap, upstands and the depth of the lead

Two measurements govern how well a flashing performs. The first is the headlap, which is how far each piece of lead overlaps the one above it, or sits beneath the covering. On a low pitch this overlap needs to be greater, because water can travel further up under it. The second is the upstand, the height the lead rises up the face of the brickwork. A taller upstand gives water a longer climb before it can get behind the flashing.

On a steep roof, a modest upstand and overlap may be enough. On a shallow one, the same dimensions leave less margin, and any shortfall shows up first in heavy rain with a wind behind it. That is why the lead apron at the front of a stack, the cover flashings up the sides and the back gutter or tray behind it are all detailed more generously on low pitches.

The lead also has to be dressed properly into the covering and tucked into a chased joint in the brickwork, rather than simply stuck on. Our hub guide on Lead Flashing or a Cement Fillet? How to Tell Which You Have explains why lead outlasts a mortar fillet in nearly every case.

Why cement fillets struggle on low roofs

A cement fillet is a wedge of mortar run along the junction between brick and roof covering. It is quick and cheap to form, and on many older repairs it was the default. On a low pitch it is particularly weak. Water sits against it for longer, it cracks as the roof moves and the stack warms and cools, and once a crack opens, water runs behind the fillet instead of off it.

The giveaway from the ground is a grey, rough-looking line where the stack meets the tiles. Over time it develops fine cracks, then gaps, then loose pieces. By the time damp appears indoors, water has often been getting in for a good while.

Replacing a failed fillet with lead is the usual repair on a shallow roof, but only if the lead can be tucked and lapped properly, which depends on the covering. That is part of why the roof type matters as much as the pitch. Where the covering is heavy and profiled, the lead has to be shaped to it as well, which is why the covering matters as much as the slope.

Reading a low stack junction from the ground

You can learn a good deal with binoculars from the garden, though you should not go up to look. Check that there is a metal flashing at all, rather than a grey mortar line. Look for lead that has lifted at the edges, slipped down the roof, or been bent back by wind. Look for gaps between the lead and the brick, and for staining below the stack that shows where water is escaping.

A short stack on a low roof is closer to the ground than a tall Victorian one, and so it is easier to see detail. That can give a false sense of security. The roof is shallow, but it is still a roof, and a fall from the eaves is a serious matter. Treat the ground as the only place you stand to look. Concrete tiles are brittle and awkward to walk on, as noted in chimney repairs in Runcorn.

If damp has appeared indoors, it is worth reading Is It the Chimney? Working Out Where the Water Is Getting In to work out whether the stack is the likely source before arranging repairs.

Access and what a repair involves

A low roof invites the idea that a ladder will do. Usually it will not. Lead work needs both hands, a board for tools and a stable platform at the right height, and a ladder against the eaves does not provide that. A small scaffold or access tower at the stack gives a safe working position and keeps weight off the covering.

The repair itself generally means lifting the covering around the stack, forming new lead flashings with the correct overlap and upstand, chasing the sides into the brickwork and pointing them in, and relaying the tiles over the top. Any loose or perished brick at the base of the stack is dealt with at the same time, because new lead fixed into crumbling joints will not hold for long. Once the lead is in, the covering is checked for lap and fixings before the scaffold comes down.

A written quotation should say what will be replaced, how the lead will be fixed and what access is allowed for. What Should Be on a Chimney Repair Quotation? lists what to expect on the page.