What Is the Minimum Pitch for Natural Slate?

The pitch of a roof is the angle between its slope and a level line. A garage roof may sit at a low angle, and a Victorian gable is usually much steeper. Angle matters for drainage. Rain races off a steep slope and dawdles on a flat one, and where it dawdles, a gust can shove it back up between the slates. That is the whole reason the standards set a floor.

How can I tell what my roof pitch is?

From the lawn you can only guess, and a roof nearly always looks steeper than it is. We measure on the survey with a level and an angle finder, slope by slope, because a house with an extension or a porch often has more than one pitch. A spirit level and a protractor laid along a rafter in the loft give a rough reading, and the survey confirms it from outside.

Write the figure down for every slope you plan to cover in slate. It is the first question a slate supplier will ask, and a vague answer leads to the wrong product being delivered.

On its fixing guidance page, the slate producer Cupa reports that BS 5534 sets 20 degrees as the lowest slope for natural slate. That standard is the British code for laying slates and tiles. Roofers treat it as the yardstick, though it is guidance and not an Act of Parliament, and Marley has said that building to it is not a legal duty. Even so, a surveyor looking into a bad leak will check the work against it first.

Can slate ever go on a roof below 20 degrees?

The standard supplies the starting point. The product's own data sheet decides the final figure, so we read the sheet for the exact slate before quoting. Some other coverings go lower. Marley lists its Lincoln clay pantile at 15 degrees and its smooth Mendip concrete tile at 15 degrees, which means an awkward low extension roof could suit a different covering better than slate. Our answer on slate headlap and fixing explains how the overlap grows as the slope flattens.

Slate can be held with nails near the head or with hooks at the tail. Cupa tells fixers not to use hooks on slopes below 25 degrees. A slope somewhere between 20 and 25 degrees can therefore carry slate, but only with nails.

Why do hooks need more pitch?

A hook grips the tail of the slate and presses it against the one below. On a shallower slope the slate lies looser against its neighbour and the hook has less to work with, which is why the maker sets a limit. Put plainly, the fixing relies on gravity and friction as well as the metal, and a shallow roof gives it less of both. Nails fix the slate from its head and do not depend on the slope in the same way. Our answer on slate hooks versus nails weighs the two methods against each other.

The same rule reaches repairs. A tingle or hook that holds a single replacement slate follows the rules for the pitch it sits on, so if your roof has a shallow section we choose the repair method to suit it, and we tell you why.

A roof close to the limit is a judgement call, and we do not make it from the ground. If the survey finds a slope under 20 degrees, there are three options. One is a different covering for that part of the roof. Another is to rebuild the timbers so the slope is steeper. The third is to ask the slate maker, and get the answer in writing, whether their product is rated for that angle. Each has a cost in time and money, which is why we show the choices before a price is fixed.

Do I need permission to change the covering?

Under permitted development, Class C usually covers re-roofing a house with what it had before. A new material changes how the building reads from the road, so ask the Somerset Council planning team first if the home is listed or sits within a conservation area, and expect to describe the product you have in mind. Per the Planning Portal, a covering that is 15 per cent or more heavier needs Building Regulations approval. Our slate roofing page describes how we work, and a few photos of the slopes are enough to start a survey.