Can an old slate roof still have years of life left?

Sound slate rings when tapped, has clean edges and shows no delamination. Slate near the end of its life becomes soft at the edges, splits along its layers, or flakes; holes enlarge and tails break away.

Condition often differs by elevation, so a slope taking prevailing weather may be well behind a sheltered one on the same roof.

Corroded nails release slates one at a time over years. Where several have already been refitted with clips across a slope, that is a reliable indication of where the roof is in its life — regardless of how good the slate looks.

This is what makes stripping and relaying worth considering: sound slate is re-hung on new battens and underlay with new fixings, giving a long further life without buying a new covering.

Some slate simply will not take another century, and a proportion always breaks during stripping, so replacement slate is needed to make up the shortfall. Matching size, thickness and colour then becomes the practical question.

An honest assessment says what proportion is likely to be reusable and how the balance will be made up. Anyone certain of that figure before stripping begins is estimating.

A slope facing the prevailing wind and rain takes far more weathering over the same period than a sheltered one on the same building, so nail corrosion and slate wear are rarely uniform across a roof. This is why an assessment of only the most visible slope can understate how much life remains elsewhere.

Recording condition separately for each elevation, rather than describing the roof as a single average, gives a more honest basis for deciding whether the whole roof needs attention or only the exposed part of it.