The short answer
Ridge mortar cracks because the ridge is the most exposed joint on the house. It swings between hot and cold, wet and dry, and it rocks in the wind. Rich or dirty mixes shrink as they cure, frost splits damp joints, and the timbers underneath flex. Under BS 5534:2014 a mortar bed is no longer treated as enough by itself, so a failed bed may also mean nothing else is holding them.
What the cracks are telling you
Ridge tiles are the capping row laid along the apex of a pitched roof, each one set in a bed of mortar with the open ends sealed by a pointed face. A hairline in that face is an early warning, not a disaster. It tells you the bed behind has started to shrink back from the tile, or from the roof covering underneath it.
Wider gaps are another matter. Crumbling edges, loose grit in the gutter and daylight showing under a tile mean rainwater now has a clear path into the bed. Water softens whatever mortar is left and carries it out in small amounts with every shower, and each wet spell leaves the joint weaker than the one before.
That is why a fine crack noticed in October can turn into a rocking tile by February. The cracking itself is rarely the end of the story, but what happens next depends on how quickly someone looks at it.
Movement, weather and moisture
The apex of a roof heats up and cools down faster than the walls below, because it is thin, high and open to sun and wind from every direction. Clay and concrete expand slightly in warmth and draw back in cold. Mortar has very little give, so it cannot stretch to follow, and over thousands of daily cycles the bond at the edge of every joint is the first thing to let go.
Moisture is the second driver. Mortar drinks rain and then dries out again, over and over. A bed that is full of water when a hard frost arrives has nowhere for the freezing water to expand except outwards, so the face flakes or splits. Wind adds a steady rocking force on tiles that stand proud of the roof, and the rafters shift a little with the seasons as the timber takes up and gives off moisture.
The same forces work on a gable edge. Our page on dry verge colour shows how a dry-fixed edge looks once there is no mortar to weather.
The mix and the workmanship
Not every crack is the weather's fault. A bed made too rich in cement sets hard and brittle, then shrinks as it cures and pulls away from the tile within the first year or two. A bed that is too lean crumbles under a thumbnail. Dirty sand, too much water in the gauge and a batch that has begun to go off before it is spread all leave joints that fail early. Mortar pressed onto a dusty or soaking tile never takes hold of it properly in the first place.
Older ridges bring a different problem. A bed laid decades ago may have been perfectly sound for its day, but it has been through all those winters, and the pointing has often been patched more than once. A new patch cannot grip a bed that is itself failing, so it comes away along the same line as the original crack.
Mix choice also depends on the building. Soft old roofs and hard modern ones do not want the same mortar, a point picked up on our page about whether mortar bedding is allowed.
Why a second fixing now matters
BS 5534:2014 expects more than a mortar bed under ridge and hip tiles, and the tiles are secured with clips, brackets or screws into a timber, and the mortar seals and supports. Plenty of older roofs have no such fixing, and the tile rests on gravity and a layer of tired cement. When the bed lets go, the tile can slide or lift in the next strong gust.
Our guide to BS 5534 ridge fixing sets out what the standard asks for. Mortar is not banned, and many good ridges are still bedded in it. The point is that cracked mortar over a ridge without any other fixing should be read as a loose-tile risk, not a surface blemish.
A rattling or lifting ridge in a gale is often the first sign of this. If you hear it, look at the pointing before the next bad forecast.
What to do once the joints open
Start with binoculars from the garden or a bedroom window. You are checking for gaps, missing pointing, tiles sitting at a slightly different angle from their neighbours, and grit from the bed collecting in the gutter or on the drive. Leave the climbing to us.
MyBuilder's guide, updated in August 2026, gives national figures of £200 to £400 to repoint a ridge or hips, against £300 to £600 to re-bed or swap ridge tiles. Treat those as a national guide only; your own figure comes after a survey. Repointing suits a bed that is still sound under a tired face. Where the bed has failed, repointing only hides the problem and a full rebed or a dry-fix system is the proper repair. A dry hip system applies the same idea along the hips.
Expect photographs of your own ridge from the survey, then a written quote. Our repair work carries a 10-year workmanship guarantee in writing, and we keep the job to what the fault needs. Start from the ridge and verge page, call 01793 626096 or message us on WhatsApp.