What is the minimum pitch for roof tiles?

A minimum pitch is the shallowest slope on which a tile has been shown to keep out the weather. Water has to run off the tiles before wind or capillary action can push it through the joints, and a tile that is large and interlocks manages that better than a small flat tile held only by overlap. So the figure belongs to the tile, not to the material in general.

It is stated in degrees from the horizontal, and it assumes the headlap, battens and underlay are all as the manufacturer specifies. Exposure also matters. The published number is a starting point for a moderately exposed site, and a windy plot may call for a steeper slope or a different tile. Tiles that work below their minimum often seem fine for months, then leak at the first prolonged storm.

Headlap links pitch to safety. It is the overlap between a tile and the course two rows below it, and the shallower the slope, the more overlap is needed. The slate tables show the principle clearly: for a 500 by 300 mm slate in moderate exposure, the headlap is 115 mm at 20 degrees, 95 mm at 25, 80 mm at 30 and 60 mm at 40. Every tile family follows the same logic, which is why a roof cannot simply be tiled more thinly to save money.

Plain tiles are the most demanding. Marley lists its clay plain tiles at 40 degrees for Canterbury, 35 for Ashdowne, and 30 for both Hawkins and the Acme Single Camber. Its concrete plain tile needs 35 degrees, with a 65 mm headlap and a laid weight of 73.8 kg per square metre at a 100 mm gauge. The gauge is the spacing between battens.

Plain tiles are small, thin and rely on a double layer of overlap, so a steeper slope helps the water clear quickly. They are common on older cottages and houses, where the steep roof was part of the design. If your roof is plain tile and below 30 degrees, the tile may sit below its maker's published limit, and the roof deserves a closer look. We would check the pitch and the construction before doing anything else.

Clay plain tiles are the easiest to underestimate. The Canterbury at 40 degrees is steeper than many ordinary suburban roofs, so a house that looks as if it wears clay plain tiles may well be carrying a different tile, or may have been re-covered with one that does not meet the maker's limit. The profile and the maker's mark on the back settle the question quickly.

Interlocking tiles give the most freedom. Marley's published minimums run as follows: Mendip 12.5 at 12.5 degrees, Mendip smooth and Wessex at 15, Edgemere and Modern and Duo smooth at 17.5, Double Roman, Ludlow Major, Ludlow Plus and Ashmore at 22.5, and Anglia smooth at 25. The Lincoln clay pantile needs 15 degrees.

These numbers explain why extensions, porches and lean-to roofs are usually covered with an interlocking tile or pantile. You can read more on shallow roofs in our page on plain versus interlocking tiles. For slate, BS 5534 gives a minimum of 20 degrees for a 500 mm slate in moderate exposure with a 115 mm headlap, and hook fixings are not used on any pitch below 25.

Standing on the roof is not needed. A phone with a level app held against a rafter in the loft gives a good reading. Measure a few places, as old roofs sag and the pitch can change along a slope.

If you cannot tell which tile you have, the profile tells you most of it. Flat and uniform suggests plain tiles, a large rolled shape suggests interlocking, and an S curve suggests a pantile. Our page on matching roof tiles shows how to identify a tile. Send us photographs and the measurement, and we will check them against the maker's figure.

Below its minimum, a tile roof depends on the underlay to keep the building dry, and the underlay is not designed for that. Water gets under the tiles in strong wind, battens stay damp, and rot follows. The options are to use a tile with a lower minimum, or to change to a continuous covering such as EPDM or GRP, which is what flat roofs use.

Raising the pitch is possible in theory, but it is major work and a planning question, so it is rarely the economical route. Look at the underlay too. Our guide to the roof underlay explains the types, and ventilation matters more as the pitch falls. For roofs at 15 degrees or less, the BS 5250 guidance reproduced by Marley asks for more eaves ventilation than on steeper roofs.

Before any tiles are ordered for a roof, someone should read the data sheet for the exact product, measure the pitch and leave a margin above the minimum. That check is part of our survey, which produces pictures of your slopes and, after it, a written quote. Finished work is guaranteed for 10 years, with the paperwork to prove it.

If you are planning an extension, tell us the angle you have in mind before the roof is framed. A change of a few degrees at design stage can open up a much wider range of tiles, and it costs nothing on paper, whereas fixing a roof built too shallow for its covering costs a great deal afterwards.

Related questions are in the pitched roofing hub, where each answer starts from the same facts about your slope and your tile.