Why does ridge mortar crack and fall out?

Mortar sets hard. The roof underneath does not stay still: timber takes up and gives off moisture, tiles warm in sun and cool at night, and gales push and pull at the top of the house. Because the bed cannot flex, the strain goes into its weakest line, usually the joint between mortar and tile or a thin patch in the middle.

A hairline crack is the first stage, and it can sit there for a long while without doing harm. It becomes a problem once water gets in, because wet mortar that freezes and thaws loses its grip and breaks into pieces. Those pieces drop out first at the ends and along whichever side of the roof meets the weather head-on, which on this coast is often the side facing the sea. Look at where the cracks fall: lines at the ends of a run, or at a hip junction where several tiles meet, often show movement gathering at the point where the roof changes direction.

Not every crack is the roof's doing. A mix that is too sandy, too dry or too wet will set weak. A bed laid thin, on a dirty or damp surface, or over tiles that were never cleaned, will not key properly. Mortar left too long in the bucket, or loosened with extra water to make it spread, loses strength before it reaches the roof. Cement and lime mortars behave differently, and the wrong one on the wrong roof can leave a bed that splits at the first movement.

Age plays its part too. A bed that has stood for decades has seen every winter, and the mortar is simply worn out. Compare a crack that is clean and sharp, which can point to recent movement, with mortar that is soft, sandy and crumbling, which points to age and water. The photographs from the survey show which of the two you have, and that decides how far the repair needs to go.

Marley explains that mortar's tensile strength on its own can no longer be trusted with the job of holding ridge and hip pieces, which is why the 2014 revision of BS 5534 asks for mechanical fixings on bedded pieces. A ridge laid to current practice is therefore held by a fixing as well as by the bed. Many older ridges have mortar and nothing more.

On those roofs the bed does everything: it seals the gap and it holds the tile against a gale. When it cracks, the tile has no second defence, and a loosened piece can go in the next blow. For that reason cracked mortar deserves attention even where the ridge still looks level from the road. Our guide to BS 5534 for ridge tiles sets out what changed and what it means for older roofs.

Fallen mortar tells a story. Fine pale grit after a windy night suggests the surface is wearing. Larger lumps, especially with a clear tile-shaped face, mean the bed has come away in sections and the tile above may be loose. A scatter on a porch roof or lean-to below shows you what is falling, and also how near it lands to a path or door.

Finding out more does not mean going up there. Sweep up what lands on a path, as pieces underfoot are a hazard. From the ground, binoculars show whether the tiles sit level and whether any gap shows under them. Daylight, or a tile that has dropped, makes the job more urgent. If rain is coming through the loft as well, tell us when you ring, and we will deal with the leak before the long-term fix.

The right repair depends on how far the damage goes. Where the tiles are firm and only the face has failed, repointing may do. Where the bed has gone, the capping comes off, the spent mortar is raked out and the ridge is rebedded with a fixing added to each piece. Where mortar keeps failing, a dry-fixed system tested to BS 8612 takes the mortar out of the equation.

Once the survey is done, the written quote carries one overall figure, and the route we suggest is the lightest one that will last. Repair time is a good chance to inspect the verge ends and the hips, because the same bed usually runs through them and has aged at the same rate. Our answer on whether you can just repoint ridge tiles weighs that choice in detail, and the ridge tile repairs page covers the wider work, including the finish at each end of the ridge.