What is a verge undercloak?

On a mortar verge, the sloping gable edge of the roof is finished with pointed mortar rather than a clipped dry-fix unit. Underneath that pointing, the roof tiles stop a little short of the wall, and a narrow strip is bedded on top of the wall head, sticking out slightly past the face. That strip is the undercloak.

Its job is to form a base. The last tiles lie over it, and the mortar is pointed on top of it, so the undercloak gives the mortar something solid to sit against and sets the line of the verge along the gable. Without it, the mortar would be a bare fillet bridging a gap, with nothing underneath to stop it falling away.

The overhang matters too. The strip projects past the face of the wall, so rain running off the verge drips clear instead of tracking down the brickwork or stonework. On Cotswold stone walls and rubble gables, where the wall head is uneven, the strip is also what evens out the bed so the tiles above lie true.

Several materials turn up, and a roofer should recognise all of them.

• Slate. Strips of natural slate give a thin, durable base and match slate roofs well.
• Plain clay tiles. Two courses of plain tile, often called a tile-and-a-half or double-course, are a traditional undercloak on clay roofs.
• Fibre cement. A flat, thin fibre-cement strip is a modern, lightweight choice. Older asbestos cement strips may still exist on a roof, and careful handling matters if one is disturbed.
• Plastic or PVC. Some newer undercloaks are preformed plastic channels, usually used with a mortar-free or mixed detail.
• Stone. On a Cotswold stone roof the verge may be formed by a thin stone slab instead.

The right choice follows the covering and the period of the building. A slate strip under a clay-tile roof, or the reverse, can look wrong and wear differently. On a listed or period gable, the strip is part of the look of the building, so check with Stroud District Council before altering it.

When an undercloak is sound, the verge is a stable three-part build: the wall, the strip, and the mortar and tiles bedded on it. When it cracks, slips or rots away, the mortar above loses its seat and falls out in lumps. That lets rain in at the batten ends and the wall head, and the problem spreads along the gable.

The undercloak also carries part of the weight at the edge. Mortar pointing is brittle, and an undercloak that has moved away from the wall lets the pointing crack along its length. Cracked verge mortar is often a symptom of the strip underneath, which is why a repair that only repoints the surface can fail within a few seasons. Frost makes it worse: water that gets behind a cracked joint freezes, expands and pushes the pointing further off.

BS 5534:2014 asks for verge tiles to be fixed mechanically, not just bedded, and a sound base gives those fixings something solid to hold. Our guide to mortar verge repairs in Stroud covers the whole repair, including pointing and cracks.

From the ground, look along the gable edge for the line of the verge. These are the usual signs.

• Mortar missing in lumps, or a crack running along the length of the verge.
• A visible gap between the verge and the wall.
• A broken or slipped strip showing at the edge, or one hanging below the line of the wall.
• Green growth or damp staining down the gable wall under the verge.
• Verge tiles that sit uneven or rock when pressed, which a roofer will check from a safe position.

From the loft, you may see daylight or damp near the gable end rafter. If a leak shows at the top corner of a gable ceiling, the verge is worth checking. Our answer on how a leak can come through the ridge shows how to tell a verge or ridge leak from other causes.

Renewing an undercloak means taking off the verge mortar and the last tiles, removing the damaged strip, bedding a new one square to the wall with the right overhang, and relaying the tiles and pointing. It is careful, slow work, and it needs scaffold or a tower at the gable.

If the verge is changing to a dry-fix system, the old undercloak is often removed or kept as a base, depending on the kit. Mortar is no longer the main support in a dry verge, so the strip's job changes. The tested systems use a unit that clips to the tile edge instead. Our answer on dry ridge on slate includes the related slate edge details.

The undercloak is a small part of a verge job, so its cost depends mostly on access and on how much of the run needs redoing. After a survey, the written quote covers access, the strip, the tiles, the mortar and labour, so you can see what the job involves. We photograph the verge from our survey position and send you what we find.

A 10-year workmanship guarantee in writing covers our repair. For a first look, send photographs on WhatsApp or phone 01453 933613, and see the main page on ridge and verge repairs.