Why Are My Slates Turning White?

Lichen, salts or lime: which is it?

Look at the pattern before you worry. Lichen sits on the surface as a rough, raised crust, often in rounded patches that spread outwards and can be grey, white, yellow or pale green. You can feel the texture and it will not wipe off with a thumb. Salt bloom is a thin, flat, powdery film that follows the shape of the slate and may show up after a long damp spell, then fade a little in dry weather. A lime streak is the easiest to read. It starts below a mortar joint, a pointed ridge, a verge or a chimney stack and runs down the slope in a line, usually following the line of water.

From the ground, all three look similar, which is why we photograph the slates close up on a survey rather than judging from the street. Binoculars from the garden are enough for you to see whether the marks are patches, films or streaks.

What lichen does to slate

Lichen is a mix of fungus and algae, and it grows on slow-drying, stable surfaces. It prefers north-facing slopes and roofs shaded by trees. On natural slate it does little harm to the stone itself. It holds moisture against the surface, so the slate stays damp for longer, and its roots can lift the edge of a thin or already weathered slate. It is not the same as moss, which is thicker and does more damage to a roof, and our page on moss on slate roofs covers that. Cleaning lichen off with a pressure washer is a bad idea: it strips the surface, forces water under the laps and shortens the life of the slate. Hand removal with a soft brush and a suitable biocide, then leaving it to die back, is gentler.

Location on the roof helps. Lichen favours the shaded, slow-drying slopes, so a pale crust on the north side and clean slates on the south is typical. Lime run-off follows the water, so streaks sit directly below the joint that makes them. Salt bloom is more even across a slope and tends to appear on the slates that stay wettest. If the white is confined to the edges of a few individual slates, look closer, because that pattern can mean the stone is starting to lose its surface. Seasonal change is another clue: a bloom that fades in a dry summer and comes back in a wet winter is behaving like a salt, and a patch that never changes is more likely lichen.

Salt bloom and weak slate

A white chalky film on natural slate can be calcite, a lime-bearing mineral in the stone that dissolves in rain and dries on the face. A little is normal and fades. Heavy, repeated blooming, along with rusty streaks, can mean the slate contains carbonate or iron minerals that will break down. Those slates tend to flake and split later, and we explain that fault on our page about why slates delaminate. On man-made fibre cement slates, a pale bloom on the surface is a common effect of lime coming to the face of the cement, and it is cosmetic.

Lime washing down from above

If the white runs below pointing, look at the mortar. Mortar bedding under ridge tiles, hip tiles and verges gives off lime as it ages, and rain carries it onto the slates below. Old, crumbling mortar is the usual source, and it is a sign the joint is due for attention. Modern roofs are expected to have ridge, hip and verge parts fixed mechanically rather than relying on mortar alone, as BS 5534:2014 sets out. A dry-fixed ridge stops the lime source for good.

Where mortar is the source, we can rake out and rebed it if it is sound, or replace it with a mechanically fixed dry ridge where the old joint has failed. Sorting the source first means any later clean of the slates stays clean.

When to call us

Leave it alone if it is lichen on a sound roof, or a light bloom that comes and goes. Ask for a survey if you can see white with slipped slates, flaking edges, rust stains, damp in the loft or lime streaks below crumbling mortar. We photograph the slates and the joints that feed them, name the cause, and write a fixed quote only where work is justified. If the roof has years left, we say so. Start at our slate roofing in Basingstoke page for the full picture.