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Why the underlay is the unsung hero of a slate roof

Slate is the star of the show, but the underlay is the supporting act that keeps the performance going. On a traditional slate roof, the underlay is a secondary weather barrier. It catches wind-driven rain, snow and dust that finds its way past the slates, and it helps manage condensation inside the roof. In heritage work, the underlay choice matters even more. Many older roofs were built with sarking boards and no underlay at all, relying on the slate and the timber to dry naturally. Introduce the wrong membrane and you can trap moisture in historic timbers, causing rot and decay. Get it right, and you protect the building without sealing it up.

Two main families dominate: breathable membranes and bitumen felts. Both can work, but they suit different roofs, budgets and levels of heritage sensitivity. The key is to match the underlay to the roof's pitch, exposure, ventilation strategy and the age of the building.

Breathable membranes: letting the roof breathe without letting the weather in

Breathable membranes — also called vapour-permeable underlays or breather membranes — are engineered textiles. They usually have a low water vapour resistance and a high resistance to liquid water. In plain terms, moisture vapour from inside the roof can escape through them, but rain and snow cannot get in. Most are tested to BS EN 13859-1, and a good specification will state a low Sd value, a hydrostatic head and a tear strength.

For slate roofing, breathable membranes are popular on cold roofs and on heritage projects where sarking boards remain in place. Laid over the sarking, with counter-battens to create a drainage and ventilation gap, they allow the timber to dry inwards while keeping the weather out. They are also useful on complex roofs, low pitches and exposed sites where wind-driven rain is a real threat. Taped laps are essential; an untaped breathable membrane is just a windy sieve.

• Best for: heritage sarking, cold roofs, low pitches, exposed coastal or upland sites, and projects where ventilation is tight.
• Look for: an LR (low resistance) underlay to BS EN 13859-1, strong tear resistance, UV stability for the build stage, and a full taping system.
• Watch out: not all breathable membranes are equal. Some cheaper products have surprisingly high vapour resistance. Check the data sheet, not the label.

Bitumen felts: traditional, tough and budget-friendly

Bitumen felt is the old faithful. Type 1F reinforced bitumen felt has been used under slate for decades. It is cheap, robust and familiar to most roofing contractors. But bitumen felt has a high water vapour resistance, which means it does not let moisture escape easily. That is fine if the roof is well ventilated, but it is a problem in a heritage roof with sarking boards and limited airflow.

Bitumen felt also becomes brittle in cold weather and can tear if walked on carelessly. It is best suited to simple, well-ventilated cold roofs, outbuildings, garden rooms and budget-driven repairs. In a listed building or a historic roof, a high-resistance felt can trap moisture and accelerate decay in the very timbers you are trying to protect. If you do use bitumen felt, be generous with ventilation and make sure the eaves and ridge are clear.

• Best for: straightforward cold roofs, outbuildings, temporary weatherproofing, and projects where budget is the main driver.
• Look for: a reinforced felt with good tensile strength and a proven track record. Type 1F is the traditional choice.
• Watch out: high vapour resistance means more ventilation is needed. Do not skimp on eaves vents or ridge vents.

Ventilation, condensation and the heritage roof

Underlay and ventilation are a package. BS 5250 gives the framework for controlling condensation in roofs, and BS 5534 covers slate roofing practice. A cold roof with a high-resistance underlay needs a continuous path for air to move from eaves to eaves or eaves to ridge. A low-resistance breathable membrane may allow reduced ventilation, but it is not a licence to seal the roof completely. Even breathable underlays need some airflow, especially in heritage roofs where timber is old and damp-prone.

On a historic roof with sarking boards, the aim is to let the boards dry. A breathable membrane over the sarking, with counter-battens and a ventilated cavity, is often the best compromise. It keeps wind-driven rain out while allowing vapour to escape. In Scotland, where sarking is common and the weather is unforgiving, this approach is well proven. Always check with the conservation officer before making changes to a listed building, and avoid any detail that cannot be reversed.

Budget, specification and making the call

Compare costs honestly. A breathable LR membrane might cost roughly two to three times more per square metre than a basic bitumen felt, but you may claw some of that back through simpler ventilation detailing and faster installation. Bitumen felt is cheaper on the roll, but it can demand more labour for vents, and it may not satisfy modern building control or heritage expectations. For a small outbuilding, the saving might be worth it. For a principal elevation on a listed house, it usually is not.

Use this quick guide:

• Heritage or listed roof: choose a vapour-open breathable membrane, lay it over sarking with counter-battens, and keep original timber breathing.
• New-build cold roof: specify an LR breathable underlay with taped laps and follow BS 5250 ventilation requirements.
• Budget outbuilding: a reinforced bitumen felt can work if you ventilate generously and accept a shorter service life.
• Low pitch or severe exposure: invest in a high-performance breathable membrane with sealed laps and a robust batten system.
• Repair or partial re-slate: match the existing underlay where possible, but upgrade to breathable if the roof is being opened up and ventilation is poor.

Take the time to match the underlay to the slate, the pitch, the exposure, the timber and the budget. A well-chosen underlay will let your slate roof do its job for generations — quietly, reliably and without trapping the moisture that heritage roofs need to shed.