In short
Ridge mortar cracks because the roof moves with heat and wind while the mortar does not, and once cracked it lets water in and frost finishes it. On an older roof the mortar is the only thing holding the ridge tiles on, which is why loose mortar and slipped ridges go together. Since the 2014 revision of BS 5534, ridge tiles have to be mechanically fixed as well as bedded, so a proper repair adds a fixing rather than just fresh mortar.
This is a job for our Ridge, valley and gable repointing service.
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Why the mortar gives up
A roof is not still. The tiles heat and cool every day, the timbers move with the seasons, and on an exposed slope near Wells-next-the-Sea or Cromer the wind is working at the ridge all winter. Mortar is rigid. It cracks along the bed, water gets into the crack, and the first hard frost lifts it. Marley, who make the tiles on many of the roofs we work on, note that mortar can fail within the first few years where there is movement, and that it needs a dry day between 5°C and 25°C to set properly in the first place. Plenty of ridges were pointed on the wrong day.
Why loose mortar matters more than it looks
On most roofs built before the last decade, the mortar is the only thing holding the ridge tiles down. When it goes, the tile is sitting there by its own weight. The Roof Tile Association puts it plainly: mortar can crack and fall out, loosening the tiles. A ridge tile coming off in a gale is dangerous, and the gap it leaves runs water straight into the top of the roof. That is the leak that shows as a stain along the centre of an upstairs ceiling.
What the standard says now
BS 5534, the code of practice for slating and tiling, was revised in 2014. The change that matters here is that the tensile strength of mortar can no longer be relied on to hold a ridge, and every bedded ridge and hip tile must also be mechanically fixed to the roof. Marley's summary of the revision and the Roof Tile Association's ridge details both say the same thing. In practice there are two ways to comply:
- Bed and fix. Rebed the ridges in fresh mortar and add a screw or clip through each one into a ridge batten. This keeps the look of a mortared ridge, which matters on older pantiled roofs and in conservation areas.
- Dry ridge. Take the mortar away altogether and fit a ventilated dry ridge system, where each tile is screwed down through a union and there is nothing to crack. It also lets the roof breathe at the ridge.
What we do on a repoint
We rake the old bed out rather than smearing new mortar over it, which is the difference between a repair and a paint job. Ridges are lifted, cleaned, rebedded in a sand and cement mix and mechanically fixed. The same goes for the gable ends and the valleys, which fail for the same reason and are usually pointed at the same time. On a valley we also check the lead or GRP lining under the mortar, because a cracked valley bed is often hiding a split liner that is the real leak.
If most of the ridge is loose, Checkatrade's guide makes the point we would make: once one older ridge tile has gone, the rest are following, and rebedding the whole run is better value than fixing them one at a time as they fall.
