The slates are the part of a slate roof you can see, and the layers beneath them decide whether the roof stays dry for the long haul. A sheet of underlay below the battens catches what gets past the slates. A route for air through the roof space carries moisture away before it condenses on cold timber. Neither is glamorous, and both are skipped on cheap jobs, which is why many roofs reach their first big repair earlier than they should. This guide explains what underlay is for, which types suit slate, how ventilation works on a slate roof, what the rules say, and the mistakes we see when roofs are opened up.
What underlay does on a slate roof
No slate roof is perfectly watertight on the surface. Wind-driven rain is blown up the laps between slates, snow can be forced under a course, and a cracked slate lets water through until it is spotted. The underlay is the second line of defence. It lies over the rafters, beneath the battens, and any water that gets through the slates runs down the membrane and out into the gutter. It also reduces draughts and keeps wind-blown dust and fine snow out of the loft.
On older houses, particularly in cities and in Scotland, slate was often laid over boards called sarking, which gave a flat, solid base for the slates. Many Basingstoke-area roofs of the Victorian and Edwardian years have no sarking and rely instead on battens over rafters, sometimes with no underlay at all, which is why the loft of a hundred-year-old house can be full of daylight and dust. Modern practice puts in a membrane, and a re-slate is the natural moment to add one. Our guide to re-slating a roof shows where it goes in the sequence.
Underlay types: felt and breathable membranes
Two families of underlay are in use.
Reinforced bitumen felt
This is the traditional choice: a bitumen-coated sheet reinforced with fibres. It does not let moisture vapour pass, so it needs ventilation above and below to stop condensation building up under it. It has a long record and is still used where the roof is designed as a cold, ventilated structure. Its weakness is age: it can go brittle and tear when disturbed.
Vapour-permeable (breathable) membranes
These are woven or non-woven synthetic sheets that block liquid water but let moisture vapour through. They allow the loft to dry out through the roof, and they reduce the need for ventilation openings. They are lighter and quicker to lay. Their weakness is that not all are equal: some are designed to be used with a ventilated air gap, and others with none, and the wrong pairing can trap moisture. The manufacturer's instructions state which, and they must be followed.
For slate, the choice depends on the roof. On a cold roof with insulation at ceiling level and a ventilated loft, either type can be used with the right detailing. Where the roof is insulated at rafter level, the choice interacts with the insulation, and that is where mistakes appear. Whichever you use, it should be tough enough to survive foot traffic while the slates are laid, with laps overlapped by the amount the maker specifies and running water into the gutter through a proper eaves detail.
Why a slate roof needs to breathe
A house makes moisture. Cooking, washing, drying clothes and breathing put water vapour into the air, and warm air carries it up into the loft. When that air meets cold timber and underlay, the vapour condenses. Left alone, condensation wets the insulation, lets mould grow on the rafters and sarking, and can rot the battens that hold the slates. The nails that fix the slates rust faster when they stay damp. A slate roof that cannot dry is a roof that is slowly rotting from the inside.
Ventilation works by moving air through the loft, from low openings at the eaves to high openings at the ridge or in the slope. Air enters at the eaves, rises as it warms, picks up moisture and leaves near the top. Even a gentle flow keeps the loft dry. The openings must be kept clear: insulation pushed hard into the eaves, a blocked soffit vent or a tray missing at the eaves will stop the flow. Our guide on what slate vents are describes the vents that sit in the slope, and our page on ridge options for slate roofs covers ridge ventilation.
Ways to ventilate
- Eaves vents or soffit vents: a continuous or spaced opening at the bottom of the slope, with a tray or carrier that stops the underlay sagging into it.
- Ridge vents: a vented ridge system that lets air leave at the top, often combined with a dry-fix ridge.
- Slate vents: a manufactured vent that looks like a slate and sits in the slope, used where eaves-to-ridge flow is not enough.
- Gable or hip vents: openings in the end walls for lofts where the geometry stops a normal flow.
The eaves detail that most roofs get wrong
The lowest course of slates is where underlay, fascia, gutter and ventilation all meet, and it is the detail we correct most often on older roofs. The underlay must run down over the fascia board or an eaves support tray and deliver water into the gutter, not behind it. If it stops short, rain that gets through the slates soaks the back of the fascia and the rafter ends, and the first sign is a rotten board from the inside out. If it droops into the gap between the rafters, it holds a pool of water against the wall plate.
At the same point, a ventilation gap has to stay open. Insulation laid across the top of the wall plate is pushed back with a baffle so that air can still enter at the eaves, and the vent in the soffit or fascia is kept clear of paint and debris. Birds will nest in an unprotected gap, so the vents are fitted with a mesh or comb that stops them without stopping the air. Nesting birds are protected, so a roof with an active nest may have to wait.
Cold roofs, warm roofs and slate
Most slate roofs in this area are cold roofs. The insulation lies on the loft floor, the roof space above it is cold and ventilated, and the underlay sits directly on the rafters. This is the layout the vents and the felt were designed for. It works well as long as the ceiling below is sealed, so that warm, damp air from the house does not leak up into the loft through downlighter holes, loft hatches and pipe penetrations.
Some owners convert the loft into a room, insulate between and below the rafters, and so create a warm or hybrid roof. The ventilation rules change, because the space between the insulation and the underlay is now the only route for air, and it must be sized and kept open. Choosing a breathable membrane that is rated for that arrangement matters, since a vapour-closed membrane over insulated rafters traps moisture against the timbers. Anyone planning this should have the whole build-up designed, not just the slates. If you are converting, the building control body will want to see the details.
The thing to avoid is half a solution: a new membrane laid over rafters with insulation stuffed to the underside and no air path. It looks fine on the day and rots the timbers in a few years.
Rules and standards that apply
When a roof is re-covered, the roof counts as a thermal element, so the energy rules for existing homes (Approved Document L) come into play if the job is big enough. Small repairs that touch under 25% of the whole shell of the house and under 50% of its roof sit outside the application rules, and bigger jobs do not. The insulation may then need to be improved, and that brings ventilation questions with it. The Planning Portal's re-roofing guidance explains the tests, and your building control body decides what applies. If it is notifiable, we arrange the application and inspection.
Ridge, hip and verge components are a related point. British Standard BS 5534:2014 requires mechanical fixing for them, and BS 8612:2018 covers dry-fix systems. Dry-fix ridge systems commonly include ventilation, so the choice of ridge and the ventilation plan should be made together.
If the house has been spray foamed on the underside of the roof, the usual ventilation logic changes, because the foam turns a cold roof into a warm roof and seals the rafters. That case needs separate assessment; see our guide on spray foam and breathable membranes.
Common mistakes we find when a roof is opened up
When we strip a roof, the same faults appear again and again. Loft insulation stuffed into the eaves so tightly that the ventilation gap is blocked. No eaves tray, so the underlay droops and holds a pool of water against the fascia. A breathable membrane used with an unvented warm roof it was never rated for. Felt laid with laps in the wrong direction, so that water runs in rather than out. Underlay that was cut short at the verge, leaving the batten ends exposed. Roof vents painted over, or covered by a bathroom extractor duct that blows steam straight into the loft.
Bathroom and kitchen extractors need ducting to the outside, not the loft. A fan that discharges into the roof space can put litres of water into the timbers over a winter. It is a small thing to fix, but it is often the cause of condensation that owners blame on a leaking roof.
What to ask your roofer
Ask which underlay is going on and why. Ask how the eaves will be ventilated and whether insulation will be kept clear of the vents. Ask how the ridge will be ventilated and fixed. Ask whether the roof has a cold or warm design and whether the membrane suits that. And ask for the answers in the quote. A roofer who cannot explain the layers cannot be trusted with the slates above them.
Our survey includes the loft: we photograph the underside of the covering, the insulation, the eaves and any signs of condensation, and the report sets out what the roof needs. You receive a written price that stays put, our ten-year workmanship guarantee covers the labour, and the waste leaves with us. See slate roofing in Basingstoke for the full service, or reach the same desk on 01256 213956 or WhatsApp.
