What Size Slates Does Your Roof Need?

A slate has three measurements. Length runs from the head, the top edge, to the tail, the bottom edge. Width runs across the face. Thickness is the depth of the stone, and it changes how a slate lies against its neighbours. Merchants list natural slate by length and width in millimetres or inches, and the two sets of numbers do not always line up.

Why does length matter most?

Because every slate is covered by the course two rows above it. The slate's length, less the overlap and the part left showing, decides how the courses fall down the roof. A slate that is too short cannot make its overlap at the pitch you have. Width is more forgiving, though each joint between slates still has to be covered by the slate below. Thickness, the third measurement, is checked on every sample we take, because a slate of the right length can still sit badly if it is too thick.

Sizes also affect the look. Long, narrow slates give strong horizontal lines, while squarer ones make a calmer, more even surface, and a roof that has lost its original size can read differently from the street even when the colour is right.

The headlap is the overlap between a slate and the one two courses below, and the exposure is the part of each slate left showing. On a shallow slope water travels slowly and wind can drive it upward, so the headlap needs to be deeper. A deeper headlap leaves less of each slate on show, so a longer slate is needed to cover the same roof.

What pitch does a slate need?

According to Cupa, BS 5534 sets natural slate's floor at 20 degrees, and hooks are not for slopes under 25 degrees. So the pitch comes first, then the headlap, then the slate length. We work the sums slope by slope and write them down. See the minimum pitch for slate for the angle limits, and slate headlap and fixing for the overlap itself.

A steeper roof lets water run away quickly, so it can use a smaller headlap and a shorter slate. A roof close to the pitch limit is where size choices become critical and where the maker's data sheet should be read closely before anything is ordered.

Many repair questions are really matching questions. If the roof is not being replaced in full, new slates must come out the same size as the old ones, or the courses will not line up and the joins will show. A slate from another merchant, sold under the same nominal size, can be a few millimetres adrift, and that is enough to leave a gap or a step in the line.

Can I order slates from a photo?

Not safely. A photograph shows colour but not size, and a few millimetres matter. We take a real sample from your roof, measure it with callipers and a rule, and give the merchant exact figures, so the delivery matches the order. If the merchant's version of the size differs from ours, the sample settles the argument. Old roofs may also have been laid in sizes that are no longer sold, so a close match might be a slightly larger slate cut down. Our answer on matching replacement slates goes through colour and thickness as well.

At the survey we work through your roof in a set order. First the pitch of every slope, since an extension or porch is often a different angle. Then a sample slate from each slope, measured for length, width and thickness. Then the exposure, which we read off the roof by measuring the visible part of a run of slates, and the headlap that follows from it.

Will slates from a different quarry fit?

Only if the sizes agree, and sometimes they do not, because quarries cut to their own sizes and old stock may have been cut by hand. Dressing a slate down on site is skilled work and wastes stone, so we avoid it where a matching size can be bought. The survey tells us whether your size is easy to find, which is worth knowing early because a scarce size can mean waiting for stock to be cut, scarce or cut down from a larger stock, and we explain what that means for the work. You see photos of your own slopes beside the figures. A slate roofing job begins with that visit, so send a photo of the roof and a few details of the house, and we will arrange a time to measure up.