What pitch does a stone roof need?

Every pitched covering works by overlap. Rain runs down one slab, reaches the lower edge and drops onto the course beneath, and the overlap stops it finding the gaps. The lower the slope, the slower the water moves, and the more chance wind has to push it sideways and upwards under the laps. A steep roof clears rain fast and keeps its joints dry.

Stone is a special case for two reasons. The slabs are thick, so there is a real gap beneath each edge for water to creep along, and the surface is rough, so water does not run off in a tidy line. That is why the old stone roofs of the Weald look so steep from the lane. The builders knew what angle their stone would shed on, and they framed the roof to give it. The covering and the frame were designed together, which is why changing one without the other so often goes wrong. A gentle pitch asks stone to do something it was never cut to do, and the leaks show up at the laps first.

For natural slate, Cupa Pizarras, a slate supplier, reports that BS 5534:2014+A2:2018 indicates a minimum pitch of 20 degrees for a 500 mm slate in moderate exposure, while hook fixings are not recommended on a pitch under 25 degrees. Marley lists a different minimum for each of its tiles, from 15 degrees for a clay pantile to 35 degrees for a concrete plain tile, and says to check each product's data sheet.

None of those figures can be lent to stone. Slate is thin, factory-regular and tested. Stone slabs are thick, hand-sorted and different from course to course, so there is no standard number to quote. What can be said comes from Marley again: a slope under 30 degrees can see strong suction on the side facing the wind. A low-pitched stone roof carries more weight than slate on a slope that gives the covering less help. The pitch an old roof already has is a fact to respect, and the survey records it before anything else is decided. It also helps to know what the original builders had in mind: an old farm building and a grand house were framed differently, and each frame tells its own story about the stone it was meant to carry.

If your stone roof is steep, that is the design doing its work, and altering it is a big step. Raising or lowering the pitch changes how the roof sheds water and how the slabs lap, and on a listed building it is no longer like-for-like repair. Our guide to like-for-like consent explains why altering the structure is treated differently from repair.

Ask any roofer to keep the original line unless a structural fault gives a reason to open it up. Over the decades a slope sometimes sags under the weight of the stone. The right answer then is usually to correct the timbers while the covering is off and lay the stone back to its original line. How much that weight amounts to is a separate matter, covered in our page on how heavy a Horsham stone roof is. Anyone suggesting a different pitch should explain what the new frame will carry and who has agreed to it.

The trouble spots are rarely the main roof. A lean-to, a porch or a later extension sometimes carries stone at a lower angle than the stone would originally have been given. Wind-driven rain finds those laps, and the symptom indoors is a stain that appears after certain storms and fades in dry spells, which makes the source tricky to trace.

Before anything is cut out, we study the underside. On a shallow slope the fixings and the layer beneath do more of the work, as our guide to underlay below a stone roof explains. A very shallow area may need a different covering altogether, such as lead laid to its own falls, with the stone kept for the steeper slopes. That is a decision for the survey, and we show you the roof in pictures before suggesting it. If the building is protected, the council's conservation officer should hear about it first.

At the survey we measure each slope instead of assuming. Pitch is rarely identical across a farmhouse or a barn conversion, because additions were built to suit the money and timber of their day. We note where the roof is steep, where it eases and where a slope has dropped, then compare that with the places where leaks and slipped slabs have appeared.

Findings go into the written quote, which gives one total price and carries pictures of the slopes concerned. We also say plainly if the existing pitch means a part of the roof is best left alone, patched lightly or covered in a different material. If the angle is the reason a particular slab keeps moving, you will read that in the report. Our answer on why stone slates slip covers the other usual causes. For the full range of stone work, go to the Horsham stone roofing page.