The short answer
A slate roof is usually ventilated at the eaves, sometimes with a high-level vent at the ridge or a slate vent set into the slope. Marley's summary of BS 5250 asks for a 25 mm eaves gap running the full eaves line when the pitch is 15 degrees or less, and 10 mm above that, plus 5 mm at high level in some cases. Breather underlay changes the numbers.
Why a slate roof needs airflow at all
Warm, damp air rises from kitchens, bathrooms and living rooms and finds its way into the roof void. Where it reaches cold timber and underlay, it turns to water. Damp battens, rafters and insulation follow, and the fixings holding the slates can corrode. Airflow carries that moisture out before it settles.
The guidance behind this is BS 5250, whose subject is moisture control in buildings. We have read it through Marley's page on ventilating a cold roof, not the standard itself, so what follows is Marley's summary of it. In a cold roof the warmth stays below: insulation lies across the ceiling joists, and the void above is left unheated, as in most lofts.
Natural slate itself is dense and does not absorb much water, but the way it is laid leaves small gaps at the laps. That helps airflow slightly, and it should not be relied on as ventilation. Planned vents do the work.
Eaves, ridge and slate vents
There are three common routes for air. Eaves vents sit at the bottom of the slope, either as a strip at the soffit or as a tray fitted over the fascia, letting air in below the slates. Ridge vents sit at the top and let moist air out. Slate vents are individual units that replace a slate in the slope and connect to the roof void, used where eaves or ridge ventilation is hard to fit.
Air moves best when it can enter low and leave high, because warm air rises. Eaves-only ventilation works for many roofs, and a high-level route improves it on steeper, longer or lean-to roofs. On a slate roof the ridge detail has to be mechanically fixed, as BS 5534:2014 requires of ridge and hip tiles, so any ridge vent must be installed in a way that satisfies it.
Soffit gaps, which are the spaces under the overhanging eaves, are easy to choke when quilt is stuffed hard into the corners. We check the eaves gap from inside the loft early in a survey.
What Marley's summary of BS 5250 asks for
The figures depend on the underlay. For a non-breathable (HR) underlay, Marley's page asks for 25 mm of eaves opening on pitches of 15 degrees or less, and 10 mm above 15 degrees. Roofs steeper than 35 degrees, wider than 10 m in span, or built as a lean-to or mono-pitch also get a continuous 5 mm gap near the top.
Marley treats a breather (LR) underlay differently. Where the ceiling below is airtight and the slates let air pass, the page says the 3 mm that normal eaves construction leaves is enough, or else a 5 mm ridge or high-level vent. With an ordinary ceiling instead, the void wants eaves openings, and the batten space needs them as well if the covering lets no air through.
| Set-up | Marley's summary of BS 5250 |
|---|---|
| HR underlay, pitch 15 degrees or less | 25 mm eaves ventilation along the eaves |
| HR underlay, pitch above 15 degrees | 10 mm at the eaves |
| Pitch over 35 degrees, span over 10 m, or lean-to | Add 5 mm continuous at high level |
| LR underlay, sealed ceiling | 3 mm at the eaves, or 5 mm at the ridge |
Check the figures against the manufacturer's own data for the product being fitted, as Marley's page covers Marley products.
How the underlay changes the answer
The underlay under the slates is the layer between battens and rafters. Older roofs often have none, or traditional bitumen felt, which does not let vapour through. Newer breather membranes pass moisture vapour up and out while keeping rain off. That is why the table above gives different numbers for each type.
Changing the underlay in a re-slate therefore changes the ventilation need. A breather membrane laid over a roof built around felt may lower the requirement, but that is no licence to seal the existing vents. Our answer on what underlay goes under a slate roof covers the membrane itself, and this one stays with the air.
If you have converted or insulated the loft, the picture changes again. Insulation fitted between the rafters can close off the gap that air needs. Putting insulation into a home's loft is zero-rated for VAT up to 31 March 2027, which makes timing a fair question, and your quote shows VAT on the roofing itself.
Signs a slate roof is not breathing properly
Go into the loft on a cold morning. Beads of water under the underlay, damp or darkened rafters, rusty nail points showing through, and black mould on timber all point to trapped moisture. Outside, there are fewer signs, though slipped slates caused by corroded fixings can be one. See our guide to keeping a slate roof waterproof for the weather-side of the picture, and how long a slate roof lasts for why the timbers tend to matter as much as the slate.
Our survey photographs the roof void as well as the slopes, checks the eaves for blocked gaps and notes the underlay in place. Should vents be needed, they are covered in the quote, and our written 10-year guarantee applies to the workmanship. For more on slate work in general, see our slate roofing page.