Why Are My Clay Roof Tiles Flaking?

What spalling is

Spalling is the loss of the outer surface of a tile in flakes or chips. On a clay tile, you will see rough, pitted patches where the smooth face has come away, sometimes with fragments in the gutter. The tile underneath looks paler and more porous than the original face. In bad cases the tile crumbles at the edges or splits in layers.

It is different from simple weathering, which dulls the surface but leaves it intact. And it is different from a crack, which is a break across the tile. Spalling is a process that works from the face inward, and it speeds up once the protective surface has gone, because the soft body underneath absorbs even more water.

Why it happens

The main driver is frost. Clay is porous, and in winter a wet tile freezes. Water expands as it turns to ice, and the pressure inside the pores forces the outer skin off. Repeated cycles of freezing and thawing across many winters do the damage, so it tends to show on older roofs and on tiles that stay wet for long periods.

Quality is the second factor. Tiles that were under-fired, or made from clay that was never ideal, absorb more water and fail sooner. Handmade and well-fired tiles resist better. Salts are a third factor: soluble salts in the clay or in the mortar can crystallise just under the surface, and the growth of those crystals breaks the face off.

Where it shows first

Look for spalling on the north slope, where the sun does not dry the tiles, and under trees where moss and leaf litter hold moisture. The overlaps are damp traps, as are tiles near the eaves and valleys where water is concentrated. Tiles bedded in mortar at the ridge and hips can spall where the mortar holds water against them.

Roofs with heavy moss growth are at higher risk. Moss acts like a sponge and keeps the tile wet through the winter. Our page on how long concrete tile roofs last covers a similar issue for the other main type of tile.

Is it serious?

A few flaked tiles on an otherwise sound roof are a maintenance job, not a crisis. But spalled tiles are weaker, and they can crack or let water through. If a lot of tiles on a slope are flaking, the same process will continue on the neighbours. Check the gutter and downpipe for broken pieces, which show how much is coming off, and the loft for daylight or damp.

It is worth checking whether the flaking is the surface or the structure of the tile. If you can pick off layers with a fingernail, the tile is failing. If only the glaze or a thin film is coming off, it may be more cosmetic.

Handmade, machine-made and reclaimed clay

Not all clay tiles behave alike. Handmade tiles are formed in a mould and fired at a controlled temperature, and they tend to be more varied in texture and in how they weather. Machine-made tiles are more uniform. Reclaimed tiles have already survived decades of winters, so those that remain sound are a proven choice, though a batch with hidden frost damage can be a disappointment. A good salvage yard sorts them and rejects the soft ones.

If you are buying replacements, ask whether the tile is rated for frost resistance and where it was made. Tiles from a different clay may look similar but absorb water differently. A tile that absorbs less water is less likely to spall in a cold snap.

Preventing more flaking

Keep the roof drying. Trim back overhanging branches, keep the gutters running so water does not back up under the eaves course, and treat moss early and gently. A well-ventilated loft also helps, as warm damp air leaving through the covering can keep the underside of tiles wet. Small habits like these add years to a clay roof.

What can be done

Replace the worst tiles with matching ones. Reclaimed clay tiles of the same type are often the best match, as our answer on matching new tiles to an old roof explains. Tiles from a hidden slope can be swapped with front ones if the back is sound. Clean off moss gently with a soft brush, and avoid pressure washing, which strips more of the face.

Coatings and sealers are sometimes sold to stop spalling. We are cautious about them, because they can trap moisture and fail. When the spalling is widespread, the honest answer is a new covering, perhaps re-using the best tiles on a smaller area. We photograph the damage and tell you which applies. See the pitched roofing page for more.