The short answer
Walking on a slate roof is not advised. Slates are thin and brittle and snap under a boot, often unseen, and a pitched roof is slippery and unforgiving to fall from. Roofers share their load with roof ladders and crawling boards, work from scaffold where they can and step only on sound areas. If something on your roof needs checking, photographs or a survey are safer than a climb.
Why slate does not take a footstep
A natural slate is a thin sheet of split rock. It is hard and long-lasting under rain, but it is not built to bend. Load it at a point between two battens and it flexes slightly, then cracks along the grain. The crack may be hairline, invisible from above and entirely hidden by the slate that overlaps it.
On a slate that has aged, nails have rusted, edges have weathered and the stone itself may be starting to split in layers. Our guide to delaminating slates describes what that looks like. A step on a slate in that state can break it in two, and the pieces will slide away down the roof.
The damage often stays invisible for a season or two, because slate hides its own faults so well and the ceiling below only stains once the timber has soaked through. Water finds the crack in the next heavy shower, runs along the batten and turns up as a brown mark on a ceiling, often well away from where the boot landed. By then the slate has been letting water in for weeks, and the timber beneath it has been wet for just as long.
The risk to you
Slate roofs are generally steep, and the surface is smooth. Add morning dew, moss, a light frost or a film of algae and it behaves like a playground slide. People fall off roofs they believed were safe. Roof falls rank with the worst injuries in the building trade, and a homeowner has no harness, no edge protection and often nobody nearby to call for help.
If a slate breaks under you, the next thing to go is your footing. Gutters are not designed to hold a person, and rotten fascia boards give way. Please keep both feet on the ground, whatever the neighbours or a passing caller may suggest, and if you want to look at the roof, photograph it from the garden or from an upstairs window.
How roofers work on slate
Professionals do not stroll over slate either, and they do not hurry. Even a short repair is planned around where the roofer will stand and where the load will go. The usual tools are a roof ladder, a long ladder with a hook that rests over the ridge and shares the load across several courses, and crawling boards, which are planks laid on the slates so no single slate takes a boot. For anything beyond a small job, scaffold with an eaves platform gives a flat surface to work from and somewhere to put tools and slates.
Where a roofer must stand on the slates, they choose sound ones close to a supporting batten, step on the lower part of a slate where the course below backs it up and carry as little as possible. Our slate sizes and headlaps guide explains the overlap that makes this possible: the deeper the cover, the more support each slate gets from those beneath.
What if a slate has already broken?
A broken slate often shows as a sliver on the ground, a darker patch on the roof or a ceiling stain after rain. If you know someone has been on the roof, a new leak is a fair thing to check for. Water follows the crack, then creeps along a batten before it finally drips through.
The repair is usually small, since a roofer can lift the slate above, slide in a replacement and secure it, and where the broken one has left a hole, the fix may take only a few minutes once the access is in place. The more useful job is a survey that finds every cracked slate, not only the one you noticed. Our slate repair or reslate guide explains how the count of broken slates points towards a repair or a larger job.
A cracked slate can often be told from a sound one by its dull, flat sound when tapped, but that test belongs to someone working safely at the eaves, not to a person on the ridge.
Getting the roof looked at safely
Our survey begins at ground level, with binoculars and a camera, and our people step onto the slope only when the access is safe. Where a slope needs a closer look, we use the equipment described above. Your roof's photographs travel with the written quote, which makes any cracked or slipped slates easy for you to see too. Seeing them on a screen is better than guessing from the lawn. If a broken slate has come to rest on a porch or bay roof underneath, send a photograph of that as well, since the position of the fall helps us work out which slope it came from.
Whoever quotes, ask whether the total they give includes VAT and the scaffold. Cost comparisons between slate and tile are in our slate against tile cost guide. A low figure that leaves out proper access probably relies on a ladder, and ladders and slate do not mix well. Workmanship is guaranteed for 10 years, in writing, and the garden and drive are left clear when the roof is done.
Once you know the condition of the roof, you can choose to repair it, maintain it or plan a larger job, and that choice is far easier made from photographs than from a climb. For the wider service, see slate roofing.