The short answer
Verge undercloak is the supporting layer fixed under the last row of tiles at a gable edge. It can be a cement fibre board, a plastic strip, a timber fillet or a row of tile slips set in mortar. It carries the overhanging edge tiles, closes the gap between the roof and the wall, and gives the verge mortar or the dry-fix strip something firm to sit on. If it sags, rots or is missing, the verge above it cannot hold.
Where it sits
At a gable, the tiles on the roof finish a short distance beyond the wall so that rain drips clear of the brickwork. Beneath that overhanging edge, a strip runs along the top of the wall and under the tiles. That strip is the undercloak. It is hidden from the ground, tucked beneath the edge tile, and most owners only learn it exists when a surveyor points it out.
What it is made of
Older houses often have undercloak made from a row of plain tile slips, called tile slips, bedded in mortar. Many mid-century roofs have asbestos cement board, which was common at the time and should not be disturbed or broken, so we take care and follow HSE guidance if we meet it. Modern roofs use cement fibre board, treated timber or moulded plastic profiles. Each has its own strengths: tile slips look traditional and last well, boards are quick to fit, and plastic does not rot but can become brittle in sunlight.
What it does
The undercloak does three jobs. It supports the edge tiles, which would otherwise sag or snap under their own weight and wind load. It forms a base for the verge, whether that is a bed of mortar or a fixing strip for dry caps. And it shuts the gap at the wall top so that wind, rain and birds cannot enter the roof space. A good undercloak is straight, firmly fixed and slightly sloped back towards the roof so that water runs onto the tiles and not onto the wall. Our guide to verge repairs shows how it fits into the wider job.
How undercloak fails
Mortar-bedded slips crack and fall out, leaving the edge tiles unsupported. Timber fillets and boards rot when damp gets in, particularly where the verge mortar has already failed and let water run behind. Cement fibre can crumble at its edges. Plastic can warp or crack. Any of these faults lets the edge tiles drop, tilt or lift, and the verge mortar above loses its bed. A verge that keeps cracking despite repointing is often a verge with a failed undercloak.
Signs to look for
From the ground, watch for a wavy edge to the verge, edge tiles that tilt outward, cracked mortar that repeatedly returns or green growth along the gable edge. Wet staining on the gable wall below the verge shows water is not shedding properly. In the loft, damp timber at the gable end can show water has got behind the edge. Take photographs and send them in. Our answer on what a roof verge is explains how the parts relate.
Asbestos cement undercloak
On houses from the middle of the last century, the undercloak is sometimes a flat sheet of asbestos cement. If it is sound and left alone, it is not a hazard, but breaking, drilling or sawing it can release fibres. Careful removal of intact asbestos cement without breaking it is non-licensed work, a risk assessment and trained workers are always needed, and the HSE publishes guidance at hse.gov.uk. If we suspect it on your gable, we stop, explain what we see and agree the safe way forward before touching it. Overlaying the old board is sometimes an alternative to removing it.
Undercloak and dry verge
Dry verge systems rely on the undercloak even more than mortar does, because the fixing strip is screwed down into a firm base. If the undercloak is weak, the caps will not stay on. That is why we check it before a retrofit, as explained in our answer on whether dry verge can be fitted to an existing roof. BS 5534:2014 expects verge parts to be mechanically fixed, which assumes the base is sound.
Replacing it properly
A repair starts with taking off the old verge mortar or caps and the loose edge tiles. The defective undercloak is cut out back to firm material, and a new section is fixed to the wall top and the batten ends with stainless or coated fixings. It is set slightly sloping into the roof, so water runs back onto the tiles, and the edge tiles are relaid on it with a consistent overhang. Only then is the verge finished. Skipping this step and simply repointing is the most common reason a verge needs redoing within a couple of winters.
What we do
We check the undercloak whenever we survey a verge, and show you photos of its condition. A damaged section can often be replaced without disturbing the whole edge. The fixed written quote lists the work. The wider range of gable and ridge repairs is on our Basingstoke ridge tile repairs hub.
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