Tiles on a low-pitched roof: why they leak

Tiles do not make a roof waterproof by themselves. They throw off most of the rain, and each course tucks under the one above to deal with what is left. That overlap is the headlap. On a steep slope water runs off quickly and a modest headlap is plenty. As the slope flattens, water moves slowly, a gust can push it sideways and upwards, and surface tension can draw it across the joint between two tiles.

The result is a roof that leaks in some storms and not in others, often with no slipped or cracked tile to point at. Stains tend to appear in the loft along one wall, or beneath a particular course, usually after rain from the same quarter. If a roof has done that more than once, the pitch deserves measuring before anyone spends money on replacement tiles.

Repeated patching is the usual sign of this fault. A new tile slotted into a course that was never going to shed water at that angle simply moves the weak point to its neighbour.

A minimum pitch belongs to a product, not to tiles in general. Marley's published figures give 15° for its Lincoln clay pantile and for Mendip concrete tiles, 17.5° for Edgemere, 22.5° for Double Roman and 35° for its concrete plain tile. Its clay plain tiles run from 30° to 40° depending on the type. Sandtoft states 30° for its Old English Pantile. The only reliable check is the technical sheet published for the precise tile on your slope, which we can usually identify from a photograph or a loose tile.

Most owners have never measured their pitch, which is given in degrees. We take it on the survey, together with the rafter length and the headlap really achieved, and put the figures in your report. Low pitches hide in places: a dormer cheek, a rear extension or a porch roof can be well under the main roof's angle, so the part that leaks may not be the part you would suspect.

Exposure counts as well. A tile at its bare minimum on a sheltered slope may cope where the same tile on an open elevation does not, so treat the published figure as a floor rather than a target. Our answer on the minimum tile pitch sets the numbers out tile by tile.

Raising the pitch means rebuilding the roof structure, which is seldom sensible as a repair. The usual route is to leave the angle alone and change the tile. Interlocking and pantile-profile tiles have deep side channels and a long overlap, and some are rated down to about 15°, as the figures above show, where a plain tile is not.

A different covering has consequences. A concrete tile weighs more than a clay plain tile, and a covering 15% heavier or more brings in a Building Regulations application and a structural check, per the Planning Portal. The same guidance also asks for a check where the new covering could perform very differently in a fire. A change that alters how the house looks can also raise a planning question, and our answer on changing roof tile colour and material sets that out.

The pantile profile is the common candidate for shallow slopes, and that page explains why the shape copes where a plain tile struggles.

Where the slope is too flat for any tile, the honest description is a low-slope or flat roof, and it needs a continuous waterproof covering. Typical choices are EPDM (rubber sheeting), GRP (glass fibre set in resin), felt or single-ply. MyBuilder's 2026 flat roof guide gives EPDM at £90 to £130 per m² and GRP at £95 to £150 per m², as typical UK ranges.

A membrane also needs drainage built in. BS 6229 guidance is to design for a fall of 1:40, which leaves a finished fall of at least 1:80 once tolerances are allowed for. A membrane in place of a visible tiled slope changes the appearance of the roof, so check with the council first if the building is listed or sits in a protected area.

This is a larger decision than re-tiling, and we would rather lay out both options after a survey than steer you to one by default. Sometimes a modest membrane over a small section is all the roof needs.

Occasionally the tile suits the pitch and the roof still leaks because of how it was laid. A headlap that is too short, battens set at the wrong gauge, or an underlay that has torn or sagged will each let water through on a roof that ought to work. A re-cover strips the slope, renews the underlay, re-battens to the maker's gauge and lays the tiles again, normally reusing the sound ones.

Making a leak safe first

A leaking roof comes first. We make it safe, with a temporary cover where needed, then survey. On MyBuilder's 2026 figures a roof survey costs about £150 to £500, and the quote that follows takes in the whole re-cover, scaffold included. If the job is notifiable we explain what consent is needed and handle the Building Regulations notification, and a 10-year workmanship guarantee goes to you in writing when it is done.

Our pitched roofing page walks through the stages from first look to cleared site.