The short answer
There is no single figure, because the minimum pitch belongs to the tile or system, and the product data sheet gives it. Marley lists 35 degrees for plain concrete tiles, 22.5 for Double Roman and Ludlow Plus, and 15 for Wessex or smooth Mendip. Natural slate needs about 20 degrees by BS 5534. Many conservatory roofs are shallower than that, so a lightweight system is often chosen.
Why tiles have a minimum slope
A tile sheds water by gravity, and the overlap between courses stops it blowing or creeping under. On a steep roof the water runs off quickly. On a shallow roof it slows down, and wind or capillary action can push it sideways and up under the tile.
Each tile is designed and tested for a range of slopes, and the maker publishes the lowest pitch at which it works. Below that figure, the manufacturer's conditions are not met, and a leak is likely sooner or later. That is why pitch is checked before a tiled roof is quoted.
Pitch is measured in degrees from the horizontal. A conservatory lean-to may sit at anything from a shallow slope to a steep one, and Victorian and Edwardian styles with a ridge are often steeper than a lean-to. A pitch gauge or a digital level on a rafter gives a reading, and a surveyor checks it against the product data sheet, not against a general rule.
Treat the figures that follow as published minimums for particular tiles, and ask for the data sheet for the tile you are offered. Tile ranges change over time, so an old brochure is not a safe guide.
Published minimums for common tiles
Marley's published figures show how wide the range is. Among its clay tiles, the Canterbury needs 40 degrees, the Ashdowne 35, the Hawkins and the Acme Single Camber 30, and the Lincoln pantile 15.
Among concrete tiles, the Plain tile needs 35 degrees. The Edgemere and the Modern-Duo, the latter in its smooth finish, need 17.5. The Double Roman and the Ludlow Major, in smooth finish, and the Ludlow Plus and Ashmore need 22.5. The Anglia, smooth, needs 25. The Mendip, smooth, and the Wessex need 15, and the Mendip 12.5 needs 12.5.
For natural slate, BS 5534 indicates a minimum roof pitch of 20 degrees, and Cupa, a slate producer, advises keeping hooks off any slope under 25 degrees.
These are published minimums for specific products. They are not rules for every tile of a type, and the data sheet for the tile you are offered, with its fixing and headlap requirements, is the document that counts. Headlap is the overlap between one course and the next, and on a shallow roof it usually has to be larger.
Lightweight systems and low-pitch options
Many conservatory roofs are shallow, and some are well below 20 degrees. That is the reason lightweight systems exist. They use large panels, a tile-effect surface or a slate-effect surface bonded to an insulated panel, designed for lower pitches and for the loads a conservatory frame can carry.
Each system has its own minimum, set by its maker, and some are tested for pitches that no conventional tile would accept. Request the maker's data sheet and its conditions, including the required underlay, fixings and flashings. A system installed outside its stated range is not covered by the maker's terms.
If the existing pitch is too low for the tile you like, there are choices. A different tile with a lower minimum may suit, a lightweight panel system may fit, or the roof structure can be altered to raise the pitch, which means a structural design and Building Regulations approval. Our guides to the lightweight tiled conservatory roof and conservatory roof options compare the routes.
Remember that a tiled roof is heavier than glazing, so the wall and frame need checking too. A heavier covering on a shallow pitch also puts more load on the lower rafters, which is another reason the structure is looked at before the tile is chosen.
How we check on your roof
We measure the existing pitch on the survey, at more than one rafter, because older frames are not always true. We photograph the gauge reading so you can see the number, and we compare it with the data sheets of the products that could suit the room.
If a tile you have seen and liked is below its minimum on your slope, we tell you so, and we say what the alternatives are. Selling a tile onto a slope that is too flat for it is how leaks begin a year or two later, and no later repair is cheap.
The measurement also feeds the other decisions: how much insulation fits in the rafters, what happens at the wall junction, and whether a lantern or roof window is practical. Pitch influences more than the tile. It also decides how quickly rain leaves the roof, how a gutter copes at the eaves and how the flashing against the wall has to be formed.
Bring the data sheet question to any quote you receive. Ask for the product name, its minimum pitch and the fixing specification, and check the pitch figure against your own roof. The conservatory roofs hub sets out how a survey runs, and a written total follows the visit.
