The short answer
Ridge mortar cracks because it is rigid and the roof is not. Timber moves with damp and heat, frost works into hairline gaps, and rain washes the cement out of old joints. Marley reports that BS 5534, from its 2014 revision, treats mortar on its own as unreliable for ridge tiles. A cracked bed was once patched and forgotten; now it is a sign to check the fixings.
A hairline crack in ridge mortar looks harmless. It marks the start of a joint that no longer holds the tile as it did.
A rigid joint on a moving roof
At the very top of a roof, curved capping tiles sit over the join between the slopes, each bedded in mortar. Mortar is a mix of sand and cement. It is strong when squeezed and weak when pulled, so a small pull on the joint is enough to split it.
What makes the roof move?
Everything. Timber takes up moisture in winter and dries out in summer, and a rafter that swells by a fraction still shifts the tile above it. The roof structure shifts a little as it heats and cools. Wind sways the whole crest in a gale. None of it is dramatic, but over many years it opens the mortar along the same weak lines again and again.
That is the core of the problem. Mortar was never made to resist the small pulls that a roof applies every day, and Marley says the 2014 revision of BS 5534 made this plain for ridge and hip tiles. Once the bedding cracks, it has little left to give, and each season widens the split. Hairline cracks that are wide enough to take a coin edge deserve a closer look. Our page on dry ridge failure looks at the other side of the choice, where fixings rather than cement do the holding.
Weather, frost and washed-out joints
Water gets into a cracked joint by rain and driven spray. In a cold snap the water freezes and expands, widening the gap. A thaw follows and the cycle repeats. Each round loosens a little more of the bedding, until pieces start to drop out and daylight shows between tile and bed. Moss growing along a ridge line holds moisture against the joint and speeds the process.
Why does mortar crumble as well as crack?
Old mortar loses its cement. Rain and acidic air wash it out over decades, leaving a sandy, powdery bed that no longer grips the tile. You may see grit in the gutter or a pale stain running down the slope below the ridge. Fine sand on a patio beneath a gable is another giveaway. Both are signs that the bedding is breaking up.
Somerset's wind and driving rain find weak joints fast, especially on the open Levels and the hill slopes of the Quantocks and Blackdowns. A south-facing crest also takes the sun, which dries and shrinks the mortar quickly. The survey notes which side is worst and photographs it, so you see the actual joint rather than a general warning.
What a cracked ridge risks
A cracked joint lets in water, and water reaches the loft timber below. A tile with failed bedding can also slide, or lift in the wind, and fall. Ridge tiles are heavy, and one dropping from a gable is a danger to anyone beneath it.
Is a crack a reason to replace the whole ridge?
Not always. A few hairline cracks on a ridge that is otherwise firm can be cut back and re-pointed, with fixings added where they are missing. If the bedding is crumbling along a long run, re-bedding every tile makes more sense than patching, and our guide to re-bedding ridge tiles sets out the work.
One sign matters above all: movement. If a tile rocks when pressed, or the crest has dipped, the bedding has gone beyond a cosmetic fault. A tile that has slid out of line is a different problem again, and our page on a slipped ridge tile covers what to do first.
Why mortar alone is no longer relied on
No rule bans mortar, and a well-laid mortar ridge can serve for a long time. The standard did not outlaw the material; it changed the expectation. BS 5534:2014+A2:2018 treats mechanical fixings as the part that holds the tile, with mortar as a bedding and sealing material.
What replaces the mortar's job?
Either screws and clips added to a bedded ridge, or a dry ridge system in which the tiles are fixed to timber and a vented strip closes the gap beneath. A dry system tested to BS 8612 needs no cement and cannot crack in the same way, because nothing rigid is relied on to hold the tile. Joints between tiles can still move slightly, and that movement does no harm.
Neither method suits every roof, and we say which one fits yours after the survey. The pictures show your own ridge, and one written price covers the whole job. Our overview of dry ridge and verge work sets out both routes, and a short photo taken from the lawn will help us judge whether patching or starting afresh is the better buy.
More on cracked ridge mortar
Can I fill the cracks with sealant?
Sealant on a cracked mortar joint covers the symptom. The bedding underneath is still loose, and the tile can still move. A proper repair removes the failed mortar and refixes the tile. Filling cracks from a ladder is also risky, so we suggest leaving it to a roofer.
How do I know if the cracks are only cosmetic?
Look for movement and gaps from the ground. A ridge tile that has dropped out of line, a visible gap between tile and mortar or pieces of mortar in the gutter all point to a real fault. A survey with photos settles it either way.
The full picture: Dry ridge and verge repairs in Taunton ยท More short answers