The short answer
For most houses the answer is a 500 mm by 250 mm slate or a 600 mm by 300 mm slate, with 400 mm by 200 mm used on small or steep roofs. Longer slates suit lower pitches and exposed sites because they allow a deeper head lap. On a repair, though, you match the slates already on the roof, and a measurement of one loose slate usually settles it.
The sizes in common use
Slate is sold by length and width in millimetres, and a few sizes cover most work in this country. The 600 by 300 is the large format, often used on bigger roofs and lower pitches. The 500 by 250 is the all-round choice for houses, and the 500 by 300 gives a wider face with fewer joints. The 400 by 200 is the small format, found on older cottages, garages and steep porches. Natural slate also comes in random-width and diminishing-course forms for period buildings, where the slates get smaller towards the ridge, and those are specified by the slater rather than picked from a catalogue.
Man-made slates, made from fibre cement or composite, are usually sold in the same nominal sizes, though their thickness and weight differ. If you are weighing the two materials, our page on natural versus man-made slate sets them side by side.
What decides the size
Three things steer it. The first is pitch. The lower the slope, the larger the lap the slates need, and a longer slate can supply that lap without crowding the gauge. The second is exposure: a roof that takes driving rain from the west needs a generous lap, so again a longer slate. The third is the roof itself. Rafter spacing, the span between ridge and eaves and the existing batten layout all feed in, and a house with a very steep gable end may suit a smaller slate that looks right in proportion.
Looks count as well. A large slate on a small cottage looks flat and heavy, and a small slate on a long, low roof looks fussy. Many conservation areas expect the original format to be kept, which is a good reason to measure what is on your roof before ordering. The head lap guide explains how the size feeds into the overlap.
Weight and fixing come into it as well. A larger slate is heavier per piece and presents a bigger sail to the wind, so it needs sound nailing or hooks and battens of the right section. Smaller slates spread the load over more fixings, which some owners of older roofs prefer where the rafters are slender. None of this is a reason to avoid the bigger sizes on a sound structure, but it is why we look at the timbers before agreeing a format, and why a roof that carried heavy clay tiles may take slate without strengthening while a light timber outbuilding needs a check first.
Matching a repair to existing slates
If you are replacing a few broken slates, the job is to match, not to choose. Old slates are not always the size they were sold as. Hand-split Welsh slates in particular vary a little in width, and a slate that was a nominal 500 by 250 two generations ago may have been trimmed down since. A slater checks the length and width of the slates on both sides of the gap, the thickness, the colour and the hole position, and picks a replacement that sits in the course without rocking.
Colour is the hardest part. Slate weathers, so a brand-new slate beside seventy-year-old neighbours will show for a few seasons. Where matching is poor, reclaimed slate often does better, and our page on reclaimed slate covers when to use it. For a full re-roof there is no matching issue at all, because the whole roof is new and the size is chosen to suit the pitch and the site.
Thickness is the other measurement people overlook. Natural slate is sold in grades, and a thin slate beside a thick one leaves a visible step in the course and lets water track sideways. Hand-split slate has an irregular surface and gets laid differently from a machine-cut one. If you only have a small area to patch, a fair rule is to bring the old slate to the supplier, or give us a clear photograph beside a tape measure, and let the match be made against the real thing.
Checking before you order
Do not order slates from a measurement of the roof alone. The number needed depends on the gauge, which depends on the head lap, which depends on the pitch and the slate length. A new batch ordered in the wrong size cannot be returned if it has been cut or fixed. We measure the roof on the free survey, record the existing size where there is one, and write the slate size and quantity into your fixed quote, so the order is correct the first time.
If you are only thinking about prices at this stage, the size you choose affects the cost mainly through the quantity and the labour to fix each slate, and our slate roofing in Basingstoke page explains how a quote is built. Typical UK ranges for slate work are in the cost guides, and your fixed quote follows the survey.
Every roof is different. A free survey with photos settles it for yours.
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