The short answer
Modern slate roofs usually go on over a breathable membrane, a vapour-permeable underlay that lets moisture from the house escape while shedding any water that gets past the slates. Older roofs carry bitumen felt, which does not breathe and so needs more ventilation. Underlay is renewed on re-slating because the slates come off anyway, and it is far cheaper to replace it then than later.
What the layer under the slates is for
Slates overlap, but they do not seal. Wind can push rain or fine snow through the joints, and moisture condenses on the cold face of the covering. Underlay is the back-up layer for both. It sits over the rafters, the sloping timbers, and beneath the battens, the thin timber strips the slates are nailed to. Any water that gets in should run down the sheet and out at the eaves, never drop into the house.
Under slate that job is heavier than it sounds. Slate is a thin, hard covering, and when the roof is shallow, wind-blown rain is pushed further up the slope. At low pitches the underlay is doing a share of the weatherproofing, not just catching the odd drip. See our answer on the minimum pitch for slate for how pitch and overlap trade off.
A tear in underlay will not show from the street. It shows up as a damp patch on a ceiling after a storm, and by then the timber above may already have been wet for a while.
Breathable membrane against old felt
Two families of underlay are in common use. Bitumen felt, once the standard, is a sheet of bitumen-coated material that blocks water and, with it, nearly all passage of water vapour. A breathable membrane, also known as a breather or vapour-permeable underlay, is a woven or laminated sheet that holds back liquid water but passes vapour.
Why does that matter? Steam from cooking and washing drifts upward into the roof void. Trapped there, it turns to droplets on cold surfaces such as the rafters and the back of the sheet. A breather membrane lets some of that moisture pass through the underlay and out between the slates, while a felt does not.
The ventilation advice below traces back to BS 5250, a British Standard on moisture in buildings. Marley's guide to cold roofs describes the two types as HR, high resistance, for non-breathable products, and LR, low resistance, for breathers. Which one a roof has changes how much ventilation it needs.
What the ventilation rules say
These figures are from Marley's page on cold roof ventilation, citing BS 5250, and apply to a conventional insulated-floor loft. Check the product's own data sheet for the job.
| Underlay type | Eaves ventilation | Extra high-level ventilation |
|---|---|---|
| Non-breathable (HR), pitch 15 degrees or less | 25 mm along the eaves | Added as the product guidance requires |
| Non-breathable (HR), pitch above 15 degrees | 10 mm along the eaves | 5 mm continuous where pitch exceeds 35 degrees, span exceeds 10 m, or the roof is lean-to |
| Breather (LR), well-sealed ceiling | 3 mm, from normal eaves construction | Or 5 mm at ridge level |
A slate roof must pitch at 20 degrees or more under the usual rules, so the first row will rarely apply to slate. The rows beneath do, and they show how a breather can reduce the number of vents needed, provided the ceiling below is well sealed.
Why underlay is renewed when a roof is re-slated
On a re-slate the slates and battens come off and the underlay lies bare, which makes it the natural time to renew it. The old sheet has been under the slates for decades, taking condensation, heat in the loft and the knocks of nails and boots. On a Victorian or inter-war roof it may be bitumen felt that has hardened and cracked, or the roof may never have had one, with the slates bedded straight onto battens.
Cutting new underlay in after the slates are back on is not realistic, so the choice is made once, at the start. Set against the scaffold and labour already in the job, the extra cost is small. Our re-slating guide sets out the whole process; this page is just about the one layer.
Roofs with no underlay are common on older houses and we do not treat that as a fault in itself. Fitting a membrane as part of a re-slate is an improvement, and it needs to be detailed carefully at the eaves, so water drains into the gutter rather than behind the fascia.
Rules to check before you strip
Stripping and rebuilding a roof can bring in Part L of the Building Regulations, which covers fuel and power. Approved Document L counts a strip back to the rafters as renovating a thermal element, and if over 50% of that surface is renovated, the entire roof is brought to 0.16 W/(m²·K). If the work affects less than 50% of the roof and less than 25% of the building envelope, approval is not normally needed.
Notifiable work is our paperwork to sort out: we tell you which consent applies and make the Building Regulations notification. For a listed house or a conservation area, check with Stroud District Council's planning team on materials before ordering.
What the survey checks in your loft
At survey stage we photograph the underside of the slates from the loft where we can reach it, noting daylight, stains and the state of the existing underlay and battens. Damaged underlay turns up often, and it is high on our list whenever slates have been slipping or the roof leaks. It does not always mean the roof has to come off.
If the underlay is sound but the slates are failing in places, the better course may be repair. Read why slates delaminate and when patching stops making sense to see where the line falls. The rest of the slate service is on our slate roofing page, and the 10-year workmanship guarantee we give comes in writing.