The short answer
Verge undercloak is the strip laid along a gable edge, under the last row of tiles, to support the verge mortar and keep rain out. It is made of fibre cement, plastic, slate or tile, and sometimes timber on older houses. It fails by cracking, rotting or washing out, and then the verge mortar loses its base and drops away.
Where it sits and what it does
At a gable end the roof finishes against the wall along a sloping line, and the tiles there overhang the brickwork slightly. They are laid on mortar along that edge, and the undercloak is the narrow strip beneath them. It gives the mortar a firm base and carries water away from the top of the wall instead of letting it run in.
Because it is tucked underneath, you rarely see it unless something has gone wrong. A visible strip with mortar missing above it, or a dark gap between the tile edge and the wall, usually means the verge has already begun to fail.
It is one of those details that does a great deal of work for very little visible presence, which is exactly why it gets forgotten until the verge starts to drop.
Some gables have a bargeboard or a small overhang with the undercloak hidden behind it, which makes any fault harder to see and easier to ignore.
What it is made of
Modern undercloaks are commonly fibre-cement or plastic strips cut to the length of the verge. On older and better roofs it may be a course of slates or plain tiles laid under the verge tiles, which looks neater and tends to last longer. Timber was also used and can rot, particularly where it has been wet for years.
The material matters because the undercloak is held in place by the mortar and the overhanging tiles. If the bed is too thick or the strip too thin, it sags under the weight. Our page on types of ridge tiles covers the matching shapes along the top of the roof, which meet the verge at the apex and need to line up with it.
A strip that was fine when it was fitted can be brittle after decades of sun and frost, so age counts for as much as the material.
How undercloak fails
Failures tend to follow a pattern. Fibre-cement strips crack and break along the length. Plastic strips warp or turn brittle in sunlight. Timber rots where moisture sits on it, and slates or tiles used as undercloak can slip if the bed behind them loosens.
When the strip drops, the mortar loses its support and cracks away. Rain then runs behind the verge into the head of the wall and the roof timbers, causing damp in the gable wall, staining on the ceiling below and eventually loose or lost verge tiles. Check the gable from the ground for mortar fragments on the path, tiles pushed out of line or staining on the brickwork.
A leak that shows near the ridge can trace back to a bad verge rather than the ridge itself, a point explored in our page on a leaking ridge.
Repair, rebed or dry verge
If only a short length has failed, it can be cut out and replaced and the mortar re-formed. If the strip has failed all along the verge, the fix is to lift the verge tiles, replace the undercloak, rebed the tiles on fresh mortar and fix them mechanically. Where the gable is exposed, a dry verge system avoids the wet bed and the cracking altogether.
Undercloak is priced on survey as part of the verge repair, and your figure arrives with the written quote. Our page on how long ridge repair takes shows how a typical visit is planned, and the same thinking applies to a gable.
Mixing materials is a common fault. A cement-fibre strip under clay tiles on a very old house may be the wrong match for the building.
Where a gable has been repaired before, a second look at the old work is wise. Patched sections sometimes hide a strip that has failed on either side of the repair.
Spotting trouble from the garden
A good deal shows up from ground level. Stand back from the gable and look along the verge. The line of tiles should be straight, with an even overhang beyond the wall and a continuous band of pointing beneath it. Gaps, sagging sections or tiles that tilt outwards suggest the support has softened.
Look at the brickwork below, too. Dark streaks running down from the verge show that water is getting behind it. Green growth or crumbling mortar at the wall head is the same warning. Inside, a stain high on the gable wall at ceiling level, or flaking paint near the roof edge, may trace back to the same fault.
None of this proves the undercloak has failed, but it is enough reason to have the verge looked at before the next hard winter.
On a long gable, the lower half sheds the most water and tends to show problems first.
Winter, and what we do
Verge repairs are best done in dry spells, since fresh mortar and heavy rain do not mix. Our winter ridge repairs page covers what can be done in cold weather and what has to wait.
At the survey we photograph the verge on your own roof, check the undercloak and the bedding, and put a written quote together. We aim to propose only the repair the fault needs, and what we do is covered by a 10-year workmanship guarantee in writing. Afterwards the site is left clean. See the ridge and verge page for the other options, or ring 01793 626096 and send a photo of the gable on WhatsApp.