Slate is one of the most durable roof coverings ever used in Britain. A well-laid Welsh slate roof can last a century or more, and plenty are still going strong at 150 years old. But that longevity comes with a catch: slate roofs fail slowly and quietly, and by the time water is dripping into a bedroom ceiling, the damage has usually been developing for years. Regular, careful inspection is how you stay ahead of it.

The single most important rule is this: do not climb onto your roof. Slate is brittle, fixings corrode, and a roof that looks solid from the pavement can be treacherous underfoot. Everything below can be done safely from the ground, from a loft, or from an upstairs window. Anything else is a job for someone with the right training, equipment and insurance.

Ground-level checks: your first line of defence

Give yourself fifteen minutes with a pair of binoculars and a notepad, ideally on a dry, overcast day when harsh shadows won't hide problems.

• Check the slope lines. Stand back and look along the roof. Slates should run in neat, even courses. A dip, a bulge or a wavering line often points to a slipped slate or a failing batten beneath.
• Count the casualties. Look for gaps, cracked slates, and slates that have slid out of position or are sitting proud of their neighbours.
• Examine the ridge and hips. These are usually bedded in mortar, which cracks with frost. Crumbling pointing or missing sections of ridge are common and comparatively cheap to fix if caught early.
• Look at the flashings. The lead or mortar where the roof meets a chimney, dormer or party wall is a classic weak point. Lifting, cracked or missing flashing lets water in sideways.
• Check the edges. Look at the verge along the gable and at the eaves. Missing mortar or slipped edge slates allow wind-driven rain straight in.
• Inspect gutters and downpipes. Fragments of slate in a gutter, or a downpipe staining the wall below, tell you something above is moving.
• Note the moss and lichen. A little is harmless and often a sign of age rather than trouble. Heavy moss, though, holds moisture against the slate and can lift edges. It should be removed gently by hand or with a soft brush, never pressure-washed.

Also note anything fixed to the roof: aerials, satellite dishes, solar panels. Every fixing is a potential leak point, particularly if it was installed without proper sealing.

What to look for in the loft

The loft is where a slate roof tells the truth. Take a torch, wear old clothes, and step only on joists or boarded areas.

• Daylight through the roof. Pinpricks of light mean gaps. Any visible daylight between slates is worth investigating, even if it isn't currently leaking.
• Water stains on rafters and battens. Brown tidemarks, dark patches or white mineral deposits all indicate historic or ongoing water ingress. Note where they sit, because a stain's position tells you roughly which part of the roof is responsible.
• Damp or rotting timbers. Prod suspect timbers with a screwdriver. Soft, crumbly wood means prolonged moisture and a genuine problem.
• Debris on the loft floor. Small fragments of slate or mortar underneath a particular section of roof point to movement above.
• The condition of the underlay. Older roofs may have nothing more than torching, a mortar skim brushed onto the underside of the slates, while newer ones have a felt or membrane. Torn or sagging underlay still sheds some water, but it is a sign of age.
• Ventilation and condensation. Not every damp patch is a leak. Poor ventilation causes condensation that mimics water ingress, though it tends to appear on cold surfaces in still, humid weather rather than during driving rain.

Know your slates and their age

Not all slate is equal, and it helps to know what you have. Thick, heavy West Country slates behave differently from the thinner Welsh variety, and Scottish slate is often laid in diminishing courses. Ask a roofer to identify what you have and to estimate the roof's age. If your property is listed, or sits in a conservation area, repairs may need consent and matching materials, so it is worth knowing that before any work begins.

When to call a qualified roofer

Call a professional straight away if you spot any of the following:

• Water actually getting inside, or damp patches that spread during rain.
• Slates missing, cracked or slipped across a significant area.
• Storm damage, or anything that looks like a potential fall risk.
• Anything that would require access above ground level.

When you do call someone, ask about their insurance, whether they will provide before-and-after photographs, and whether scaffolding will be needed. Get the scope of work in writing, obtain more than one quote, and never pay large sums in cash up front. A good roofer will happily talk you through the repair in plain English and tell you honestly what can safely wait until next year.

Make inspection a seasonal habit

Two short inspections a year, one in spring and one in autumn, plus a look after any major storm, will keep you well informed. Keep dated photographs of the roof from the same viewpoints so you can compare year on year, and note any repairs in a simple log. Clear gutters and downpipes each autumn. Small, timely repairs are what keep a slate roof doing its job for another generation.