Gable-End Chimney Stacks and the Verge Beside Them

Why a gable stack behaves differently

Most chimneys we think about sit in the middle of a slope, surrounded by roof covering on all four sides. A gable-end stack is built into the end wall instead, and its outer face continues straight down as the wall of the house. There is no ridge between the stack and the sky on that side, and the roof stops at the verge, the sloping edge where the covering meets the wall.

That arrangement has real advantages. Part of the stack is supported by the full thickness of the gable, and there is less roof around it to leak. But it also means that water running down the face of the stack goes straight onto the face of the wall below, and the roof covering has to be finished neatly against brickwork on one side and the verge on the other. Both of those finishes are places where the building’s weather details change direction, and a change of direction is where water finds its way in. Any movement of the stack relative to the wall shows up first at that junction, so hairline gaps between brick and verge deserve attention.

On a terrace the gable-end stacks are usually found on the end properties, which makes them very visible from the street and also very exposed. You can see how much of this is judgeable from the pavement in What to Look For on a Chimney From the Ground. What you cannot see from the ground is the junction itself, which is why it is so often overlooked until it leaks.

Verge tiles, cloaking and the end of the roof

The verge is the weakest line on any roof. It is where the covering stops, so the last tile or slate has nothing beside it to hold it, and the edge is exposed to wind from underneath as well as above. The usual finishes are a mortar-bedded verge, a dry verge with a cloak tile or a cloaking course, and on slate roofs a neat cloak of slate bedded over the gable wall. Each has its own ageing pattern, and the choice usually follows what the rest of the roof was built with. A dry verge relies on clips or fixings rather than mortar, so it fails by slipping, while a mortar one fails by cracking.

A mortar-bedded verge cracks and falls away as the mortar dries and the tiles move. Once the bed is loose, rain blows in behind it and runs onto the wall plate and down the inside of the gable. A cloak tile that has slipped does the same job less gradually. Where the verge meets a stack, this loose edge can sit directly against the brickwork, so the stack takes whatever the verge lets in.

The treatment of the last tiles beside the stack is therefore the first thing to look at. Are the cloak tiles bedded and pointed right up to the brickwork? Is there a visible gap between the last tile and the stack? A gap there is an invitation, and so is a tile that sits slightly proud of its neighbours, because wind can lift it and push rain underneath. Is It the Chimney? Working Out Where the Water Is Getting In helps separate stack problems from roof problems, which matters here because the two meet at exactly this point. Look too for moss or a green stain along the verge, which shows where water has been lying against the edge for some time.

The abutment flashing and where water gets in

Where the sloping roof meets the side of the stack, the junction is protected by an abutment flashing. In lead work this is a stepped cover flashing set into the mortar joints of the stack and dressed down over soakers or the profile of the tile. At the verge side the cover flashing has to finish neatly against the verge and the cloak, because the end of the flashing is where the water likes to enter.

The common failures are simple. The cover flashing lifts at its upper edge, the pointing above it opens up, and water runs behind the lead. Or the flashing is turned up against the stack but ends short, leaving a gap at the verge that neither the tile nor the lead closes. Or a mortar fillet has been used in place of the lead altogether and has cracked, a problem set out in Lead Flashing or a Cement Fillet? How to Tell Which You Have.

Water that gets in here tends to appear in odd places. It can show on the gable wall inside the loft, on the upper bedroom wall at the end of the house, or as staining on the outside of the wall under the verge. It is easy to take it for a chimney breast problem, which is why Damp on a Chimney Breast: Leak, Condensation or Salts? is a useful comparison. If the damp is at the gable end and not in the breast, the verge junction is a likely candidate.

How the junction is repaired and what to ask

A sound repair usually works from the covering inwards. The verge tiles or slates are re-bedded or replaced first so the edge is secure, then the abutment flashing is renewed with lead to suit the covering, and finally the pointing above the flashing is raked out and renewed so the cover is held tight. Doing the pointing before the lead is a common reason for early failure.

Because this junction shares the roof edge with the neighbouring property on a terrace, access matters too. A gable stack on an end house may stand over a side passage, a yard or a public footway, so the scaffold can need permission from the council. Local arrangements for that are summarised on the page for chimney repairs in Birkenhead. A written quotation should say which parts of the verge and flashing are being renewed, and What Should Be on a Chimney Repair Quotation? explains what to look for.

Please keep off the roof while you wait for a visit. A phone camera held at the end of the garden, or from the street, shows the gap at the verge more clearly than you would expect, and you can send it to us on WhatsApp. We work on the outside of the stack and the roof around it; anything involving the flue inside belongs with a HETAS or Gas Safe registered installer or a sweep.