Does Slate Thickness Matter?

Slate is split from blocks of rock into thin sheets, and the depth of each sheet affects how much load it can take. A thicker slate stands up better when a gust lifts its tail or a ladder rests against the eaves. A very thin slate flexes more, and tends to snap at a nail hole or along a hidden fault.

Is thicker always better?

Not quite. A thick slate is heavier, and an old roof built for fine slates may not welcome the extra load. Strength also depends on the quality of the stone, so a thick slate of poor stone can still peel or split. Our answer on why slates flake shows what weak stone does to a roof.

No one thickness suits every house. The covering should suit the pitch, the exposure and the way the building was designed. On an exposed Somerset hillside that judgement matters more than a catalogue range, and we make it on the day of the survey, with the old slates in our hands.

Where the original slates are visibly thicker or thinner than modern stock, you hear about it up front, because it changes both the look and the lap.

Because slate is stone, a change in thickness is a change in weight across the whole roof. A tiny difference per slate adds up over hundreds of slates, and the total lands on the rafters, purlins and walls below.

When does the weight question become a legal one?

According to the Planning Portal, a new covering that makes the roof 15 per cent or more heavier needs Building Regulations approval, and it urges a structural check before the work starts. Putting thicker slates on a roof that had thinner ones is one way to cross that line, so put the question to your roofer before ordering anything.

On a repair of a few slates the change is negligible and the question never comes up. On a full re-slate, thickness becomes part of the specification, and building control can confirm what applies to your house. Our guide to re-slating a roof describes the point where repairs stop being enough.

If sound slates are being reused, the weight stays the same, which is one argument for salvaging them. It also keeps the old, weathered colour on the front of the house, where it counts.

The reverse can apply on an old roof that has been patched with heavy modern slate: the load is uneven, and the survey is the place to spot it. A roofer who knows the property will see how the rafters have settled over the years, and whether any run of slates is sagging between supports.

Slate laps over the course below, so each one has to sit snugly on the one beneath it. A replacement thicker than its neighbours lifts at the tail and leaves a gap for wind and rain. One that is thinner sags into a hollow and may rattle in a gale.

How much difference is too much?

Even a small change can show as a line of shadow along a course, and the eye picks that out from the pavement. Old slates also vary from one to the next, which is part of their appeal, so the aim is a close match and not a perfect one.

For that reason a roofer holds a new slate against the old before fixing it, and checks that the headlap, the overlap between courses, still works. The fixing side of the same question is covered under headlap, nails and hooks in our slate guides.

A slate that does not lie flat can also work loose faster, because wind gets under it and tugs at the nails every time it blows. Over a few winters that repeated pull can turn a neat repair into the next slipped slate, and the neighbours may begin to shift as well.

When one slate breaks, the replacement has to match on three counts: size, thickness and colour. Size and thickness decide whether it fits, and colour decides whether it shows. A new slate is often a shade darker than weathered stone beside it, and reclaimed slate can be the better answer.

Where does a matching slate come from?

Sources include salvage from a roof being stripped, the back slope of your own house, or a new slate chosen to suit. A roofer should measure the width, length and thickness of the originals instead of guessing. Our answer on matching replacement slates goes further.

If you have a spare slate in the shed, keep it. A broken piece from the lawn is just as useful, as long as one edge is clean, because it gives the roofer a true sample of thickness and colour to match against. Our slate roofing service starts with a look at your roof, then pictures of the slates in question, then a single written price before any slate is lifted.