Why the underlay deserves your attention

When the slates come off a period roof, the conversation usually turns to slate size, colour, and whether the originals can be salvaged. The underlay — that unglamorous sheet between slate and rafter — often gets decided in a hurry. Yet on a heritage building it is one of the most consequential choices you will make. Get it right and the roof breathes, sheds wind-driven rain, and lasts another century. Get it wrong and you can trap moisture in timbers that have stayed dry since Victoria was on the throne.

Modern breathable membranes are genuinely excellent products. The trick is understanding that "breathable" is not a synonym for "suitable", and that a listed or traditionally built roof behaves very differently from a new-build one.

How heritage roofs were built — and why it matters

Traditional slate roofs vary enormously across the UK, and the detailing under the slates is where the differences show.

• Scottish sarking — horizontal timber boards laid over the rafters, with slates nailed to them. Sometimes a hessian-reinforced bitumen felt was laid over the boards, sometimes not.
• English and Welsh battened roofs — most commonly slates fixed to battens over rafters, with no underlay at all. Wind-driven rain and snow powder simply blew through the gaps and drained away below.
• Torching — a lime mortar bedding applied to the underside of slates in some regions to reduce draughts and dust. Crude, but it worked, and it was vapour-permeable.

The crucial point is that these roofs were designed to get wet and dry out again. They relied on ventilation and evaporation, not on a waterproof layer. Dropping in an impermeable membrane changes the physics of the whole assembly.

Breathable underlays: LR, HR and choosing wisely

Underlays tested to BS EN 13859-1 are classified by their resistance to water vapour. The LR (low resistance) types are the ones generally appropriate for heritage work: they allow vapour to pass relatively freely while still resisting wind-driven rain and providing a secondary barrier if a slate slips.

High-resistance, near-impermeable membranes belong in well-ventilated modern roof spaces, not draped over historic sarking where moisture from below has nowhere to go. If you are retaining original sarking boards, an LR underlay laid over the boards, with a ventilated cavity created by counter-battens above it, is usually the most sympathetic approach. Leave the sarking intact wherever possible — it is a moisture buffer and part of the building's structure.

Also resist the temptation to solve a leak with a new membrane alone. Underlays are a secondary line of defence. If slates are under-sized, under-lapped, or laid at too shallow a pitch, no membrane will compensate for long.

Ventilation and condensation: getting the air paths right

This is where most heritage roof problems begin. Vapour rising from below — from kitchens, bathrooms, and the general occupancy of an old, often unsealed house — will condense on the first cold surface it meets. If that surface is now an impermeable membrane, you have created a problem that did not exist before.

• Aim for a continuous 25mm ventilation gap at the eaves and at least 10mm at the ridge, or the equivalent continuous airflow path.
• Where insulation sits at ceiling level, make sure the loft space above it is ventilated and that no insulation blocks the eaves path.
• Where insulation follows the rafters, specify a vapour-permeable underlay and a separate air and vapour control layer on the warm side. Do not rely on the underlay to act as one.
• Never leave sarking boards sealed on both faces with impermeable materials — trapped moisture will rot the boards and the rafters behind them.

Battens, counter-battens and drainage details

Underlay should be laid with a gentle drape between rafters, not stretched tight like a drum skin. A taut membrane can pond and tear, and it eliminates the small drainage path that sag provides. Allow roughly 100mm vertical laps and 150mm horizontal laps, and keep fixings to the minimum needed to hold it until battens go on.

Battens should be graded, treated, and sized to suit the slate. Large or heavy heritage slates need more than the minimum, and the roof's headlap must be correct for its pitch — underlay is not a substitute for proper double-lap detailing. Where counter-battens are used, they must run vertically over the underlay to maintain the drainage and ventilation path, with horizontal battens fixed on top to carry the slates. Pay attention to verges, valleys, and abutments: these are where wind-driven rain actually gets in, so flashings and secret gutters need to be in good order regardless of what membrane you choose.

Consent, compliance and looking after the roof afterwards

If the building is listed, the roof covering and its substrate are almost always covered by the listing. Replacing an underlay as part of a slate re-roof is usually acceptable, but listed building consent is required for alterations that affect the building's character — and conservation officers may reasonably ask you to retain sarking, reuse sound slates, and repoint with lime. Discuss the specification before work starts, not after the scaffold goes up.

Once the roof is complete, look after it. Twice a year, clear gutters, hoppers and valleys, and check flashings, ridge mortar and any slipped slates. From inside the loft, note sagging or torn underlay at the eaves and any sign of damp timbers. Keep a photographic record of the roof during the works — it will be invaluable for future maintenance, insurance, and any future consent applications. A heritage roof rewards patient, well-informed care, and the underlay you choose is a quiet but permanent part of that promise.