What size slates does my roof need?

A slate size is two numbers in millimetres, length first and width second. A 500 by 300 slate is 500 mm from head to tail and 300 mm across. Thickness is a third measure and is usually described by grade rather than a figure, so two slates of the same length and width can feel very different in the hand.

Natural slate is sold in a range of lengths and widths. Some are cut to a regular rectangle, and others come in random widths so the roof looks less uniform. Because the slates are split from stone, sizes within a batch can vary slightly, and that is normal. What matters on the roof is not the exact figure on the label but how the slates suit the batten spacing the roof is laid to, and whether the width leaves a neat finish at the verge, where the slates stop against the gable wall. Slates that are too wide or too narrow for the run leave an awkward sliver at that edge, and it is the sort of detail that decides whether the finish looks square from the street.

Start with a loose or fallen slate if you have one: measure from top to bottom and across. If nothing has come down, the roof still tells the story. The distance between batten centres is the gauge, and the slate length equals twice the gauge plus the headlap. A roofer who can measure the batten spacing in the loft and the overlap at an exposed edge can work the length back out.

Please do not climb up to measure it, and do not trust a number taken from a photograph from the ground. Slates in the middle of a slope are hidden by the ones above them. We measure at the survey from a ladder, a scaffold or the loft, and put the result in the report. Our answer on headlap on a slate roof explains the overlap that goes into that sum.

For a repair, the new slate has to fit the batten spacing of the old roof, so the length is fixed. The width has a little give, and a slightly different width can be hidden in the course. Colour and surface need care. Fresh slate is darker and smoother than slate that has weathered for decades, and a mismatch shows from the pavement, especially on a terrace where the roofs sit side by side.

Reclaimed slate often gives the best match on an older roof, and the answer on reclaimed roof slates covers what is worth reusing. With a few cracked slates on an otherwise good roof, ask for the least work that stops the water coming in. Our slate roof repair guide shows how that is done, and the slate roofing page sets out the larger jobs.

When the whole slope is coming off, there is a real choice, but it is not unlimited. Size interacts with pitch, exposure and the fixings. A longer slate lets you keep a generous overlap with fewer courses, while a smaller slate means more joints and more nails. The sizes the maker lists for your pitch are the ones to use, which is why the data sheet is the starting point.

Appearance matters too. Some older roofs use slates that get smaller towards the ridge, which roofers call diminishing courses, and the look belongs to how the whole street reads. For a house in a conservation area, the covering is part of the consent discussion with the council, and getting the size and texture close to the original helps that conversation along.

Size affects the price in a way owners do not always expect. Smaller slates mean more of them to buy and fix across the same area, and the labour rises with the number. Larger ones cover more but must suit the frame and the pitch. After the survey, the quote arrives in writing, naming the slate, its size and its origin, with one total for the job.

Detail like this is what makes two quotes comparable at all, and it also tells you the roofer has measured the roof and not guessed at it. If one names a size and grade and another just says slates, you are not comparing the same job. Ask also whether scaffolding and VAT are included, since either can change the final figure. Any slate roofer should be happy to put the size on paper before you agree to anything.