The short answer
Mortar falls from a roof when a bedded joint has cracked and crumbled, usually at the ridge, along a gable edge, on a hip or around a chimney. Frost, water and age break it down. A few crumbs may be minor, but pieces of any size mean tiles or flaunching may be losing their hold, so it is worth finding the source soon.
Where the mortar is coming from
A pitched roof has several places where tiles or masonry are bedded in mortar. Working out which one is shedding it comes first, and the size and colour of the pieces often tell you.
- The ridge. Mortar from along the top of the roof falls straight down the slope and tends to land in the gutter or near the eaves. The pieces are often wedge-shaped, with a curved face from the underside of a ridge tile.
- The verge. Pieces from a gable edge land beside the gable wall, and are often long thin slivers of the pointed fillet.
- The hips. Mortar from the diagonal corners lands along the line below them.
- The chimney. Flaunching is the sloping mortar cap around the chimney pots that sheds water off the stack. When it breaks, pieces fall onto the roof and slide into the gutter. The stack's pointing can drop too.
- Walls and parapets. Mortar from nearby brickwork or stone joints can be mistaken for roof mortar, particularly on terraces.
Look at the colour and texture of what you find. Sandy grey, crumbly pieces are typical of old bedding that has simply aged out, while the debris of a mortar bed with a mix that was strong in cement tends to be hard and bright. Hard, bright grey chunks may be cement-rich mortar from a more recent repair, which can be too hard for the old materials around it.
Why mortar breaks up
Mortar is a blend of sand, lime or cement and water. It sets hard but stays slightly porous, and that is where the trouble starts. That porosity is its weakness. It takes in water, and when the water freezes it expands and forces the surface off. Repeated across winters, the joint opens, loosens and lets more water in.
Movement is the other cause. Roofs flex a little in wind and as timbers dry and swell. A rigid mortar bed can split at the exact spot where the flexing concentrates, particularly if it is stronger than the tiles it joins. Cracks fill with water, birds peck at them and moss roots into them.
Age plays its part. Bedding that nobody has revisited has had many seasons of weather. Pieces may fall without any sign on the tiles themselves. When you find fragments, assume the bed has worked loose somewhere above, and not that it was a one-off.
How serious is it
A scatter of small crumbs after a frosty spell is a prompt to look, not a cause for alarm. Larger pieces, repeated falls, or fragments that carry a flat tile-shaped face are different. They mean a bed that has let go of a tile, and a tile left without its bed is held by little more than its own weight.
In wind that tile can lift or slide. On a ridge or verge that is a hazard to anyone underneath, and once one tile goes the neighbours lose their support. Water that gets in through the gap can wet the underlay and timbers, which is a more expensive problem than the mortar.
The tiles themselves are only part of the picture. Under BS 5534:2014, mortar by itself is no longer accepted as the means of fixing ridge, hip and verge tiles. An old roof held only by a bed that is now dropping out therefore has nothing in reserve. That is why we take mortar on the ground seriously.
What to do when you find it
Start by keeping people and pets clear of the area under the source. Stay on the ground. Binoculars will show you a great deal: gaps along the ridge line, a hollow-looking verge, a broken-looking chimney crown.
Note where the mortar landed and photograph it with the area around it, because the position points to the source. Check the gutter, as mortar and moss can block it, and clear what you can reach safely from the ground or from a firm ladder.
If you have noticed damp or a stain upstairs, tell us, since water may already be entering. Wet loft or stained ceiling? Say so when you call, because active leaks are handled first and made safe before the bigger repair is planned.
The repair and the choices in it
The lasting repair rebuilds the bed, because patching the pointing leaves the weak layer underneath. On the ridge, our answer on how often a ridge needs rebedding explains the timescale. The ridge tiles are lifted, the spent mortar is cleared away, and each tile is reset on new bedding and screwed or strapped down. Where mortar is falling from the verge, the seat beneath is the first thing to inspect, and the page on the verge undercloak explains why.
Some owners choose to change from mortar to a manufactured system. A dry verge or dry ridge removes the wet bed altogether, and our guide to dry verge systems compares them with a traditional finish. On an older or listed house, a mortar finish may be the better choice for appearance, and if consent could be involved, B&NES planning will tell you.
The survey is free and you receive photographs of what we find. A national typical figure for mortar re-bedding is not published in the sources we use, so we do not quote one; instead, the price you receive is a fixed one, agreed on paper after the visit. The whole group of jobs sits under ridge and verge repairs.
