Most Victorian slate roofs are still doing their job a century or more after they were laid. That is worth remembering before anyone reaches for the skip: Welsh slate quarried in the 1890s has frequently outlasted the nails and battens holding it up. The real question is not whether the roof looks tired, but which of its three main elements — slate, battens and fixings — has actually reached the end of its useful life. Get that judgement right and you can spend a fraction of the cost of a full re-slate.

Start in the loft, not on the scaffold

Before anyone goes up a ladder, spend half an hour in the attic with a torch. It is dry, safe and far more informative than a view from the garden path. On a wet day you can often trace exactly where water is getting in.

What to look for from inside

• Pinpricks of daylight through the covering. Note where they are and whether the slate above has slipped or the batten beneath it has failed.
• Orange-brown dust and rust streaks on rafters. That is corroding nail heads and batten fixings shedding material.
• Damp patches or white mineral staining on the underside of slates, which suggests water tracking along the laps.
• Lumps of torching — the lime mortar sometimes bedded under old slates — hanging loose or lying in the insulation.
• Battens that give when you press a bradawl into the side of them, or that crumble at the nail line.
• Rafters out of line, a sagging ridge, or timbers pulling away from the wall plate. That is structure, not covering, and it needs a chartered structural engineer.

Not every glint of daylight is a leak. At the eaves you may simply be seeing past the underlay, or through the gap that ventilation requires. Judge the roof as a whole rather than reacting to one crack of light.

Reading the roof from outside

A decent pair of binoculars from the ground will tell you a lot before anyone sets foot on the slates. Look for slipped and missing slates, especially at the eaves and along verges where wind gets underneath. Look for the patchwork of odd slates in different colours and thicknesses that betray decades of piecemeal repair. Check the valleys for moss and debris, the chimney stack for failed soakers and stepped flashings, and the ridging for cracked or missing mortar.

Up close, sound each slate lightly with your knuckles or the handle of a hammer. A clear ring means the slate is solid; a dull thud or a hollow note usually means it has begun to delaminate. Run a thumb along the edges — if thin layers are lifting, that slate has had it. Slate is brittle and unforgiving underfoot, so a proper roof ladder, crawl boards and edge protection are not optional, and anything more than a look from the ground is best left to someone who works at height every week.

Slates: what is worth keeping and what is not

Victorian roofs were usually laid with the best slate the budget allowed, often in diminishing courses, with the largest slates at the eaves and the smallest near the ridge. That pattern is part of the roof's character and worth preserving. Individual slates fall into three rough groups:

• Keep. Sound, evenly coloured slates with clean edges. They will outlast most modern replacements.
• Repair. Slates that have slipped because a nail has snapped, but are otherwise intact, can be rehung on slate hooks or copper tingles fixed to the batten. Nothing above them needs to come off.
• Replace. Delaminating, holed, badly chipped or frost-damaged slates. Match size, thickness and colour as closely as you can. Salvaged Welsh slate is still widely available, and grading your own stripped slates into sizes is time well spent.

Resist the temptation to pressure-wash a slate roof. It strips the weathered surface, opens the laps and drives water where it should not go. A soft brush and a bit of patience with moss is gentler and cheaper in the long run.

Battens, nails and the parts you cannot see

The slate is the visible half of the story. The half that decides whether your roof lives another thirty years is hidden behind it. Victorian battens were usually softwood, and the nails were iron or early galvanised wire. Both have a finite life, and once they go, the slates above them are simply resting in place.

Test the fixings rather than guessing. In the loft, grip a slipped slate's nail with pliers and pull gently. If it comes out easily, or snaps, the whole nail population is suspect. If the batten crumbles where a nail passes through it, no amount of new slating will hold. New fixings should be copper, silicon bronze or stainless steel, sized to suit the batten.

Many Victorian roofs had no underlay at all, or relied on torching and lime mortar. If the roof is being stripped anyway, a breathable membrane is worth discussing, but it changes how the roof dries, and on solid or torched historic roofs it can trap moisture rather than release it. Take advice on ventilation at the same time; new insulation pushed tight to the eaves without an air path is a reliable way to create condensation problems.

The items worth renewing while the roof is open

Scaffolding and access usually account for a large share of any roofing bill, so anything with a limited life should be dealt with while you are paying for it:

• Lead soakers, stepped flashings and cover flashings to chimneys and abutments.
• Valley linings, if they are near the end of their life.
• Ridge and hip bedding. On historic roofs, a lime-based mortar is more compatible and more forgiving of movement than a hard cement mix.
• Cracked or spalled ridge and hip tiles, replaced like for like.
• Fascias, soffits, gutter linings and hopper heads.

When patching stops making sense

There is no single number that tells you to re-slate, but the balance tips fairly clearly once you look at the right things. Scattered individual failures on a roof with sound battens can be managed with repairs for years. Failures that arrive in patches, winter after winter, with rust dust in the loft and soft battens underfoot, are a different proposition.

• The condition of the fixings. Sound slates on dead battens is a roof waiting to fail. This is usually the decisive factor.
• The rate of failure. Two slates a year is maintenance. Twenty a year is a signal.
• Accumulated bodges. Painted-on sealants, cement smears, over-sarking and layers of patch repairs all make future work harder and more expensive.
• The proportion involved. As a rough guide, once more than about a quarter of the slates need attention and the battens are suspect, much of the cost of a full re-slate has already been committed.
• Access costs. Paying for scaffold once is almost always cheaper than three separate visits.

A full strip does not mean throwing everything away. Salvage every sound slate, sort it by size and thickness, and reuse it where the pattern allows. Anything left over has value to someone restoring a similar roof.

Working through the job in the right order

1. Get a structural opinion first if the ridge line, rafters or walls show any sign of movement.
2. Have a small area opened up so you can see the battens, nails and underlay before agreeing a specification. Photos from inside the loft and from the roof are useful evidence later.
3. Agree the specification in writing: slate type and source, fixing material, batten size and spacing, membrane, mortar mix and ventilation.
4. Plan how salvaged slates will be sorted and stored — they are heavy, fragile and easily broken by careless handling.
5. Ask whether the work triggers Building Regulations requirements for insulation and ventilation. Re-roofing often does.
6. Keep a photographic record of the stripped roof. It is useful for insurance, for a future sale and for any heritage consent.

Photo: scholty1970 / Pixabay