The short answer
You should not walk on a slate roof. Slate is a thin, brittle stone that cracks under a boot, often without any sound, and a cracked slate lets water in later. It is also slippery when damp or mossy, and the pitch is steep. Roofers work from roof ladders, crawling boards or scaffold that spread their weight and give them something to hold.
Why slate cracks underfoot
A roofing slate is a thin sheet of stone, split from a block along its natural layers. It sheds water very well, but it has little strength across its thickness, and it is supported only along its top edge by a nail and at its tail by the slate below. Press down in the middle of its length and it can crack in the unsupported span.
The danger is that the damage is often invisible. A hairline split may not show from above, because the slate above overlaps it, and it may not leak until a hard frost opens it up. The result is a leak in the following weeks, with no obvious cause on the outside. That is why a roof walked on by a gutter cleaner, an aerial fitter or an enthusiastic visitor can start leaking some time later.
Thinner and softer slates are worse. Ageing slate that is starting to delaminate, which means peel in layers, breaks even more easily.
The safety problem
Slate is smooth, and its surface holds water, algae and moss. A damp or green slope is very slippery, even in good boots. Shade makes it worse: a north-facing slope, or one under trees, dries slowly and can stay damp for days, and the green film on it behaves rather like ice. Slate roofs are usually steep, and once footing is lost, there is little to stop a slide towards the eaves.
Falls from roofs are among the most serious risks in building work, which is why roofers use edge protection and access equipment. Guidance on working at height sits on the HSE website, and the principles apply to anyone on a roof, paid or not.
A householder going up to inspect a displaced slate takes on a risk that a trained roofer manages with equipment. The sensible answer is to inspect from the garden or an upstairs window, perhaps with binoculars, and leave the climbing to someone with the right gear.
How roofers work on slate
Roofers avoid putting weight directly on the slates. The usual tools are listed here.
- Roof ladders. A ladder with wheels and a hook that fits over the ridge, laid flat on the slope so the roofer's weight is spread across several courses and anchored at the top.
- Crawling boards. Boards that span the battens or sit on the slope, giving a wider footprint than a boot.
- Scaffold with a working platform. For anything beyond a quick repair, scaffold gives edge protection and a stable base, and work can be done from the eaves.
- Cherry pickers and towers. Used where the roof is high or hard to reach.
Ladders at the eaves need firm, level ground and ideally a stand-off bracket, so the ladder does not bear on the gutter or the bottom slates. A harness helps the person, though it does nothing for the slate. Never lean a ladder on the slates themselves, or on the gutter without a stand-off, because the load can crack the bottom course.
Roofers also learn where to step. Weight is placed over the batten line and rafters, not in the middle of a slate's span. Even so, a careful slater who cracks a slate swaps it at once, which costs far less than finding the leak later. That habit is part of working on slate rather than a sign of poor work.
What this means for inspections and repairs
It is why a proper inspection does not rely on walking the roof. Much of the information comes from the loft: wet or stained battens, light through the slope, and the state of the underlay. From outside, a roofer can use a ladder at the eaves, a zoom lens or a drone, and only lifts slates where there is a reason to.
It also affects the cost of repair. Getting safely to the slope is a genuine part of the cost, because scaffold or a roof ladder takes time to set up. MyBuilder's guides list scaffold as a cost driver on repairs and re-roofs alike. A job that looks tiny from the ground may involve more set-up than you expect.
If you suspect damage from someone walking on the roof, say so when you call. A roofer will look for cracked slates in the line the person took. Our slate roof repairs page covers the usual fixes.
Roofs where access matters even more
A listed house, or one within a conservation area, has a roof that counts as part of its historic fabric. A cracked slate that is hard to replace like for like is a bigger problem there than on a modern estate house. Thinking of changing slate? Replacing Welsh slate with Spanish explains why the source can matter to the council.
Slipped slates are a related issue, because a roof that has lost slates is more fragile still. The causes are on our slipped slates page.
The same goes for solar installers, aerial fitters and chimney sweeps. If anyone needs to go on the roof, ask how they will protect the slate. If the answer is that they will just step carefully, think again. Our survey is free, and if you think a visitor has trodden on the slope we can check the slate afterwards and photograph any cracks. See also our slate roofing service.
