Why Is the Mortar on My Ridge Cracking?

A roof is never perfectly still. Timbers shrink and swell with the weather, the tiles heat up in the sun and cool at night, and wind pushes against the ridge all year. Mortar is a rigid material, so a bed that cannot flex with that small repeated movement develops cracks, often first where one ridge tile meets its neighbour or along the bottom edge where it meets the roof tiles.

Settlement of the house itself and the shrinkage of new mortar as it cures can do the same, and a gable end that has shifted slightly over many winters can pull a ridge joint open. Hairline cracks that do not widen and leave the tiles firmly seated may be nothing more than surface crazing, worth watching but not urgent. Wider cracks that run the whole length of a joint, or open and close with the weather, are a sign that the bed has lost its grip on the tile. Our answer on loose ridge tiles explains how to treat that later stage safely.

Mortar is a mix of sand, cement, sometimes lime, and water, and the proportions matter. Too little cement leaves a crumbly bed that powders under a fingernail. Too much leaves a hard, brittle one that cracks rather than moves. Sand with too many fines, a mix left to dry too fast in hot weather, or a bed laid onto dusty, wet or algae-covered tile all lead to poor adhesion, so the mortar parts from the tile instead of holding it.

Older roofs sometimes show repair patches in a different colour, which means someone has patched over failed mortar without removing it. New mortar does not bond well to loose old material. A ridge that has been repaired more than once is a candidate for stripping back and starting again, since each layer sits on a weaker base than the one before. Ask the roofer what mix will be used, how the tiles will be cleaned beforehand, and what is being done about fixing the tiles as well. A clear answer to all three tells you the job has been thought through, and a vague answer tells you to ask again.

Once a crack opens, rain gets in. In winter that water freezes and expands, widening the crack and loosening the bed further. Wind then finds the loose tile, and a ridge that was merely cracked becomes one that rocks. The damp can also reach the timber ridge board and the felt below, which is why a small crack is worth watching. Look for stains on the ceiling near the top of the house, or green growth along the joints, as signs that water is already getting in.

The slating and tiling code, BS 5534, now expects a clip or screw on bedded ridge and hip tiles, so a mortar bed is not left to do the whole job. Working to that code is good practice rather than a legal duty. Many older mortar ridges have no clips or screws at all, which is why they lean so heavily on a bed that was always going to crack. Our answer on whether ridge tiles need mortar goes further into the standard and the alternatives.

Repointing, which means raking out the cracked mortar and filling with fresh, suits a ridge where the tiles are firm and only the surface has failed. Re-bedding lifts the tiles and relays them on new mortar with clips or screws, and it is the right repair where tiles have started to rock. Either can last well if the work is done in dry weather and onto a sound base, with the joints left to cure before rain and with any rotten ridge board replaced first.

A dry ridge takes the mortar out of the picture altogether, so there is nothing left to crack. Checkatrade's June 2026 repair table gives £50 to £70 per metre for dry ridge repairs as a national guide; the figure for your roof depends on the run of the ridge, how we reach it and what lies beneath. Our dry ridge versus mortar comparison and our notes on retrofitting a dry system help you decide, and the dry ridge and dry verge page covers the whole system. Photos of the ridge from the ground are a good place to start, and a survey on the roof itself then confirms what the tiles are sitting on, so the price you receive is for the job the roof needs and not a guess made from the ground.