Can you walk on a slate roof?

Each slate is a thin plate of split stone, hung from a nail or hook and supported only along its head. It was never meant as a stepping surface. A boot placed in the wrong spot, or a heel that catches an edge, can crack the slate across, and the damage stays out of sight until water finds it. An owner who goes up to look at one slipped slate can come down having broken three.

Walking also loosens what holds the slates on. The fixings are sized to resist wind, not body weight, and pressing down at an angle can slide a slate on its nails. We meet roofs where a well-meant inspection has turned a small fault into a bigger one, which is a poor return for the cost of a ladder and ten minutes. The kinder option for the roof, and for you, is to look without touching. Slates that are already cracked underneath, which you cannot see from above, are the ones that give way first, and they give no warning.

Slate has a smooth face. Once it is damp, or coated with moss or algae, it turns very slippery underfoot. Slopes facing away from the sun and those under overhanging trees stay wet longest, so they are the worst. A roof that looks dry at breakfast may still carry a slick film where the light has not reached, and you cannot tell by looking from the top.

A fall from a roof is serious, so the risk is not worth taking for a quick look. If there is moss, our answer on moss on a slate roof explains how it is cleared from safe access and what the process involves. Never use a pressure washer on slates yourself, because it can strip the edges and force water underneath them, leaving you with a bigger problem than moss.

There is plenty to see from the ground. Walk round the house and study each slope through binoculars, or use the camera zoom. You are looking for slates that have dropped below their neighbours, bare patches, gaps along the top, and fragments of slate in the flower bed or the gutter.

Then take a torch into the loft. Check the timbers under the slope for chinks of daylight, damp marks and drips after rain, and look for pale patches where water has run and dried. Keep to the joists, never step between them, and do not lean on the ceiling. Photograph anything you find. Those pictures help us as much as a visit does, and our guide to the signs of a failing slate roof lists what to look for. The slate roofing page explains what follows.

The usual answer is not to stand on the slates at all. A roof ladder lies on the slope with a hook over the ridge, spreading the load so no single slate takes a boot, and a crawling board, which is a plank with cleats, serves a similar purpose across a narrower area. For anything bigger than a spot repair, scaffolding provides a platform at the eaves and a way up. A ladder from the ground is fine for a quick look at a low roof.

Scaffold that takes up part of the footway has to be licensed by the highway authority, so ask who is applying. Ask who is arranging it and how long it takes before a start date is promised. Our guide to slate roof repair covers how access is chosen for each kind of job, and why a small repair does not always need a tower.

Slate cracks, but it is not what the Health and Safety Executive calls a fragile roof. That label covers materials such as fibre cement sheets and rooflights, which can give way under a person and drop them through. Extensions and garages beside a slate roof often have exactly those surfaces, and they are hard to spot when weathered and mossy.

The safe rule is to treat every part of an unfamiliar roof as unsafe until it has been checked. The roof is assessed from a safe position before anyone steps onto it, and the findings go into the survey report. If you find loose or broken slates after a gale, stay out from under that part of the roof, and read our guide to slate roof storm damage for the next steps.