The short answer
A gap under the ridge is a problem when mortar has fallen away and left the ridge tiles unsupported, or when it lets in rain, wind-blown snow or nesting birds. It is normal when a dry ridge or ventilated system has been built with openings on purpose. Look at the pattern: ragged holes and loose tiles mean repair, neat even openings behind a fitted vent strip usually do not.
Three kinds of gap that look the same
The ridge runs along the crest of a pitched roof and is capped with curved or angled tiles. Beneath those, the tops of the last tiles on each side stop short of the peak, leaving a space. On older roofs that space is filled with mortar, which seals it and bonds the ridge tiles in place. On newer ones, a dry ridge system closes it with a vented roll or a strip that lets air through but keeps rain out.
So a gap can be one of three things. It can be mortar that has crumbled or shrunk away, in which case it is a defect. It can be a built-in ventilation opening, in which case it is working as intended. Or it can be a larger hole made by weather, age or movement, which is the one that attracts pests. Sorting one from another comes before any repair, and it is not always possible from the ground.
Missing or failed mortar
Mortar is brittle. Frost, heat, movement in the roof and plain age make it crack, shrink and drop out in chunks, often first at the ends of a run or around a hip. A pale line of fallen crumbs in the gutter, or a dark gap with a bright edge, are the usual signs.
Once the bedding has gone, the ridge tiles sit there on gravity alone. A strong gust can lift or shift them, and rain gets onto the underlay and timber at the top of the roof. Since BS 5534:2014, a modern ridge should have screws or clips on top of any bedding, so a repair that simply fills the gap again may leave the tile less secure than the standard intends. Our page on ridge repointing explains when refilling the joints is enough and when the tiles should be lifted, refixed and re-bedded.
Gaps that are meant to be there
A roof needs to breathe. Moist, warm air drifts up from the rooms below, and unless some of it can leave the loft it turns to water on cold timber. Many roofs are therefore built to let a controlled amount of air out at the top, and some ridge systems do this through a vented section under the ridge tile.
If your gap sits behind a neat, evenly spaced grille or a ribbed strip, and the ridge tiles are clipped or screwed down, it is very likely deliberate. Closing it with mortar would be a mistake in that case, as it could trap moisture in the loft. The same goes for mortar-bedded ridges that rely on a little air movement beneath the tile. The page on dry ridge on slate shows how an open-ended system should look when it has been fitted correctly.
Check the loft too. Daylight showing at the ridge from inside, or a draught on a windy day, points to a defect. Faint, even light along a whole ridge may be a vent. Damp rafters or stained felt near the peak suggest a gap is letting water in.
Birds, wasps and other visitors
A gap at the ridge is warm, dry and sheltered, which is just what small birds and wasps like. Starlings and sparrows tuck nests into gaps near the peak, and wasps will colonise a cavity under the tiles. Both can leave debris, droppings and damp nesting material behind, and the tapping or scratching sound in the early morning is often the first clue. The law protects bats and nesting birds, so signs of activity are checked before a gap is sealed and the work is timed to suit. Wasp nests are a separate matter, and a pest controller deals with those before any roofer closes the space over them. Droppings or straw hanging from the ridge line are worth photographing for the survey.
Closing a gap properly stops the problem returning. A bird guard or ridge fixing that leaves no entrance is far more effective than mortar pressed into a hole that will fall out again within a couple of winters.
What happens to a gap that needs fixing
A visit means looking at the ridge on the roof itself and again from inside the loft, taking photographs of each fault and recording whether a gap is lost mortar, a vent or a hole. If the tiles themselves are firm and just their joints have gone, local repointing may do. Where they have shifted or the fixing is poor, the tiles are lifted, the bed is cleaned out and the run is re-laid with a mechanical fixing. Where a roof would be better served by a ventilated dry ridge, that change is described, with the reasons.
Cost turns on ridge length, roof height and whether scaffolding is needed. As a rough national marker, MyBuilder's slate roof price guide (updated August 2026) gives £300 to £600 for new ridge tiles or a fresh bed under the old ones, which says nothing about your own house until someone has looked at it.
If you can see a gap, photograph it from the ground and send it over. The gable end of the same roof is worth checking at the same time, and our page on verge repairs covers that edge, while the dry verge colour guide helps if a new one is on the list. The wider ridge and verge dry-fix section covers every part of the top of the roof. A finished repair comes with our guarantee on workmanship, in writing and good for ten years, and the offcuts go with us.