Should a stone roof have an underlay?

An underlay is a sheet fixed across the rafters, beneath the battens, the narrow strips of timber from which each course of slabs hangs. Its job is modest. Rain driven sideways by a gale, or fine snow blown up between the courses, gets past the covering from time to time. The underlay catches that water and sends it down to the gutter instead of into the loft.

On a roof of modern tiles, a sheet like this is standard. Stone is a different case. The slabs are thick, rough and uneven, so they do not sit as tightly as a machine-made tile, and the gaps between them are part of how the roof dries out. A layer beneath changes that drying, which is why the question deserves more thought than a yes or no.

Think of the underlay as a second line of defence, not the roof itself. The slabs, hung in graded courses, carry the weather. Our guide to diminishing courses shows how that covering is built up, from the heavy slabs low down to the small ones near the ridge.

A roof framed and covered a very long time ago may have been finished with nothing under the slabs. Later owners sometimes added a sheet during a repair, so one slope can hold several generations of work. From the loft hatch you can often tell. You may see the undersides of the stone and bare battens, or a patchwork of different sheets over different areas.

That is why we do not assume. Finding no underlay is not a fault in itself. Many stone roofs with nothing beneath have kept the weather out for a very long time, because the slabs lap well, the pitch is steep and the roof can breathe. What matters is whether this roof is dry, whether the timbers are sound and whether the covering still hangs properly.

The honest verdict is sometimes that nothing needs adding. If the loft is dry, the timbers are healthy and no stain runs down a wall, a layer nobody asked the roof to have may only bring new risks. Photographs of your own loft, taken at the survey, show which case you are in.

The reason to be careful is moisture. Warm, damp air from the house rises into the roof space, and if it has nowhere to go it condenses on cold timber. BS 5250 covers moisture management in buildings; BSI lists the 2021 edition as current but under review.

Marley, the tile maker, explains how underlays differ. A high-resistance type lets no vapour through, so the eaves must be ventilated: Marley gives 25 mm along the eaves for pitches of 15 degrees or less, and 10 mm above that. A low-resistance type passes vapour, and Marley allows normal eaves construction to supply the airflow in a well-sealed ceiling, under independent test conditions.

Those figures are for tile and slate systems, and an old stone roof has no data sheet. They still show the principle. A layer under stone can shut off the airflow that kept the timbers dry for generations. Choosing the type, and keeping a path for air, is part of the decision, and it is why the person who has seen the loft should make the call.

An underlay sits beneath the battens and slabs, so it cannot be slipped in from outside. The covering comes off course by course and goes back in the same order. On Horsham stone that is skilled, slow work, because every slab was chosen for its place and thickness. Any that crack on the way off need replacing from reclaimed stock, a cost in its own right.

It is why the work and the underlay decision go together. If a stone roof is being stripped anyway, because the battens have failed, then adding a layer costs little extra effort. If the roof is sound, stripping it only to fit a sheet is hard to justify. Our repair or relay answer explains when a stone roof should come off.

Weight matters while the covering is off, and the timbers are checked before a single slab goes back. Our guide to stone roof weight and timbers covers what the frame carries.

Many stone roofs sit on listed buildings or inside conservation areas. The South Downs National Park Authority says small repairs that match the existing roof, made in place, need no listed building consent. A new layer beneath the covering is not obviously like for like, so ask the council's conservation officer before work starts.

Building Regulations come into it too. The Planning Portal's usual test is under 25 per cent of the building envelope and under 50 per cent of the roof, and a much heavier covering, or any material with a different fire rating, brings approval back in. Insulation and ventilation can add requirements, so put your own roof to your building control body.

The survey comes first. We photograph the loft and the slope, give a plain verdict on whether an underlay belongs, and then post you a written price with one figure at the bottom. The Horsham stone roofing page covers the wider work.