Cracked Mortar Along The Roof Edge

Key takeaways

• The verge is the sloping edge of the roof at a gable end, where the covering meets the top of the wall.
• Its mortar both weatherproofs the edge and mechanically restrains the end tiles.
• It is the second most wind exposed line on a roof after the ridge, and it fails for the same reasons.
• Missing sections let wind driven rain into the wall head and into the roof space.
• Dry verge systems replace the mortar with a clipped plastic or metal trim and do not crack.

What a verge is and where to find it

Look at the end wall of a house, the triangular one under the slope of the roof. The line where the roof covering stops and the wall begins, running diagonally up to the ridge, is the verge. On most older houses that line is finished with a bed of mortar, sometimes over an undercloak of slate or fibre cement board, which seals the gap and locks the edge tiles into place. On a terraced house you will only have verges at the two ends of the terrace, which is why mid terrace owners often have no idea what the term means, while the end of terrace next door has an expensive problem.

The two jobs it is doing

Weatherproofing is the obvious one: the verge closes the gap between the last tile and the wall so wind driven rain cannot get behind the covering. The restraint job is less obvious and more important. The tiles at the very edge of a roof have no neighbour on one side, so they rely on the verge bed to hold them down. When that bed goes, those tiles are held by whatever nails were used and by gravity alone, at the exact position where wind lift is strongest. This is why verge failure and storm damage are so closely related, and why a length of missing verge mortar is more serious than the equivalent length of missing mortar anywhere else.

Why it cracks

Mortar is rigid and everything around it moves. The roof expands and contracts with temperature, the timber beneath shrinks over decades, and the wall itself moves slightly with moisture and season. The bed absorbs that movement until it cannot, then it cracks along its weakest line, which is usually the joint between mortar and tile. Water then gets into the crack, freezes, expands, and takes a piece with it each winter. Older properties were built with lime based mortars that could flex; many have since been repaired with hard cement mixes that cannot, which is why a repointed verge frequently fails faster than the original did.

What you can see and what it means

From the ground at the gable end, look along the diagonal line of the verge against the sky. A sound verge is continuous and even. A failing one shows hairline cracks, then open gaps, then visible chunks missing where you can see daylight or the edge of the tile underneath. Look for mortar debris on the ground at the base of the gable wall, which is the most reliable early evidence. Look at the wall itself for a damp stain or a green streak below the verge line, which shows water has been running down inside rather than off the edge. And look at whether the end tiles line up with their courses, because a tile that has crept outward is a sign the restraint has gone.

What happens if it is left

Three things, in order. Water enters the top of the wall and travels down inside it, which typically shows up as damp on an upstairs internal wall at the gable end. Wind gets under the edge tiles, which lifts and eventually removes them, usually in the first serious gale after the bed has failed. And the exposed timber at the roof edge, the batten ends and any undercloak, starts to rot because it was never designed to be weathered directly. The last of those is the expensive one, because replacing rotten batten ends means lifting tiles back along the slope rather than working at the edge alone.

Mortar again, or a dry verge

Rebedding in mortar is the traditional repair and it is legitimate, particularly on a period property or in a conservation area where appearance is controlled. It should be done with the right mix, over a proper undercloak, and it will need doing again in twenty or thirty years. A dry verge system replaces the mortar with a clipped trim, usually plastic or metal, which fixes mechanically to the batten ends and does not crack, does not wash out and does not need repointing. Most new work uses dry verge for exactly those reasons. The trade off is appearance, since a dry verge reads as a defined plastic edge rather than a mortar line, which some owners dislike on an older house.

How long a failing verge can be left

More than a slipped tile, less than a loose ridge. A verge with hairline cracking and no missing sections is early stage and can be planned. A verge with open gaps, missing lengths or visible daylight should be dealt with before the next windy season, because the failure mode is not gradual: the tiles hold until they do not, and then several go at once. Bring it forward immediately if the end tiles have visibly moved, if there is damp appearing on the inside of that gable wall, or if the gable overlooks somewhere people walk. As with the ridge, the danger is a heavy object falling from height rather than the water.

Doing it once rather than twice

Verge work needs edge access along a sloping line, which means a scaffold or tower rather than a ladder, and that access is most of the cost on a small job. Since the ridge sits at the top of the same gable and is usually the same age with the same weather exposure, it is worth having both assessed together and, if both are near their end, done in one visit. The same applies to any pointing on the gable wall itself. Paying for access three times in five years for three small jobs on the same elevation is the most common avoidable expense in domestic roofing. There is one more thing worth checking at the same time, which is the undercloak beneath the verge mortar. On many roofs it is a strip of fibre cement or slate that supports the bed and closes the gap over the wall. Where it has broken up or was never fitted, a rebedded verge has nothing solid to sit on and will fail again faster than it should, so it is worth asking whether the undercloak is sound before agreeing to mortar alone. Where it has failed, replacing it is a small addition to a job that already has the access in place.

Frequently Asked Questions

What is the verge on a roof?

The sloping edge where the roof covering meets the top of a gable wall, running from the eaves up to the ridge. On older roofs it is finished with a mortar bed, often over an undercloak, which seals the edge against rain and holds the end tiles down.

Is cracked verge mortar serious?

More serious than it looks. Beyond letting water into the wall head, the verge is what restrains the most wind exposed tiles on the roof. Once sections are missing, those tiles are held by little more than gravity in the position where wind lift is greatest.

What is a dry verge?

A system that replaces the mortar edge with a clipped trim, usually plastic or metal, fixed mechanically to the batten ends. It does not crack or wash out and does not need repointing, which is why most new work uses it. The trade off is that it looks like a defined trim rather than a mortar line.

Can the verge just be repointed?

Where the bed underneath is still sound and only the surface has weathered, repointing is reasonable. Where sections are missing or the tiles have moved, the bed itself has failed and pointing over it leaves the restraint problem unsolved while making it harder to see from the ground.

Why is damp appearing on my gable wall inside?

A failed verge is one of the most common causes. Water that should run off the roof edge instead enters the top of the wall and travels down within the masonry, emerging on the inside face of the gable, usually at the upper level of the house rather than near the floor.

Do both gable ends need doing at once?

Not necessarily, since exposure differs. The elevation facing prevailing weather, generally the west or south west in Greater Manchester, usually deteriorates first. It is still worth having both assessed in the same visit, because the access cost dominates and a second visit later is rarely cheaper.