Why pitch decides everything
Every tile and slate has a manufacturer-tested minimum pitch — the shallowest angle at which it reliably sheds water. Lay a tile below its minimum and capillary action pulls rain uphill between the laps, straight into your felt. It might survive a dry summer; it won't survive a Sussex February.
The quick reference
- Under 10° — flat roof territory: GRP fibreglass, EPDM rubber or high-performance felt.
- 10–15° — specialist low-pitch tiles or metal systems only; most tiles are ruled out.
- 15–22.5° — many modern interlocking concrete tiles work; check each product's certificate.
- 22.5–35° — comfortable range for most interlocking tiles and many slates.
- 35° and up — classic plain clay tile and natural slate country, the look of the old South East.
Extending a house and matching the new roof to the old? Pitch is the first thing to check — plenty of extensions end up with a different covering than the main roof purely because the new pitch is shallower. We can usually design around it.