The short answer
It depends on the tile. Marley's published minimum pitches run from 12.5 degrees for its Mendip 12.5 concrete tile up to 40 degrees for its Canterbury clay, while slate follows BS 5534 at 20 degrees. Those are examples, not a rule for your roof. Only the maker's data sheet for the product itself, with its fixing and lap, gives the figure that applies.
Why a tile has a minimum pitch
A tiled or slated roof sheds water by overlap. Each course sits over the one below, and rain runs down the surface and drops over the lap. The system relies on gravity, so the steeper the slope, the faster the water leaves and the less likely it is to run back under a tile.
On a shallow slope, rain moves slowly. Wind can push it up the lap, and water can travel by capillary action, creeping along a narrow joint against the pull of gravity. Each type of tile has a profile and a headlap, the overlap of one course over the next, that resists this up to a point. Below that point, the design no longer works.
That point is the minimum pitch, and it is set by the maker after testing the tile. It is a property of that product, laid at its stated headlap with its stated fixings. It is not a general rule for all tiles, so it varies a lot from one product to another.
Pitch is measured in degrees from horizontal. A porch, an extension or a dormer roof is a place where the slope can sit close to a tile's limit.
Examples from Marley's data
Marley publishes minimum pitches for its products. Among its clay tiles, Lincoln pantile has a minimum of 15 degrees, Acme Single Camber and Hawkins 30 degrees, Ashdowne 35 degrees and Canterbury 40 degrees.
Among its concrete tiles, Mendip is listed at 15 degrees for the smooth version, and the Mendip 12.5 at 12.5 degrees. Wessex is 15 degrees, Edgemere 17.5 degrees, Modern-Duo 17.5 degrees for smooth, Double Roman 22.5 degrees for smooth, Ludlow Major 22.5 degrees for smooth, Ludlow Plus 22.5 degrees, Ashmore 22.5 degrees, Anglia 25 degrees for smooth, and Plain tile 35 degrees.
The spread shows why no single rule works. A plain tile needs a steep roof, and a pantile or a low-pitch concrete tile can serve a gentler one. These are Marley figures for Marley tiles, so they are not guaranteed to match another maker's product with a similar name. Our answer on types of roof tiles shows how the main profiles differ.
Natural slate follows a different reference. Within BS 5534 the floor for natural slate is 20 degrees, and Cupa, a slate producer, warns against slate hooks on anything shallower than 25 degrees.
Always the data sheet for the exact tile
Because the figure belongs to a product, the dependable source is the maker's own sheet for the tile in question, read for its lap, its fixing method and its exposure. Some figures apply only to certain laps, or only with an extra underlay detail or fixing at every tile. A roof that suits a tile at one headlap may not suit it at another.
Exposure also counts. The minimum is set for normal use, and a site that is high, open or driven by weather may call for a steeper slope or a different system. Your roofer should ask where the building stands and what the maker advises for it.
If you are planning an extension or a dormer, start with the slope. A low-pitch design may rule out the tile you wanted. The usual remedy is to choose a tile with a lower minimum, or to use another covering, such as a metal or a membrane, for the shallow part. Our page on a pitched roof on an extension covers the choice.
What happens below the minimum
A tile laid below its minimum may keep water out for a while, in light rain, and then let it in during a storm. The failure does not show on a dry day. The leaks tend to appear in driving rain, and they can be intermittent, which makes them hard to trace.
Fixing it is not a matter of sealant. The slope may need to be raised, the covering changed or, for a small area, a different material used. The time to check is before the tiles are ordered, especially for a new extension designed to a low pitch. Underlay can help but it is the second line, not the main one.
If you have a shallow tiled roof that leaks, we survey it, photograph the laps and measure the pitch. We compare the reading with the data sheet and tell you whether the roof is below its limit. We also look at the battens, the underlay and the eaves. Price follows the survey, and you receive a single written figure. Our pitched roofing hub links the other tile topics, and each is written the same way.
