Slate roof underlay
Do slate roofs need felt underneath?
The short answer
Yes, in practice. A slate roof is normally laid over an underlay, the sheet beneath the battens that felt used to provide, to catch wind-blown rain and dust that gets past the slates. Many older slate roofs were laid without one, so a re-slate is the moment to add it. The type you choose also decides how the roof space must be ventilated.
What the layer under the slates does, which types exist, and how the choice ties to ventilation.
What the sheet under the slates is for
Slates overlap and shed water, but they are not watertight by themselves. In a gale, rain is driven sideways and upward and can pass between the slates. Fine snow and dust get through the same way. The underlay hangs beneath the battens, draped over the rafters, and it intercepts that water and leads it to the eaves, where it drains into the gutter. Without it, every drop that slips past a slate lands on the timbers or the ceiling below. It also stops wind blowing straight through the roof space, which makes the loft calmer and the house less draughty.
Roofers used to call it felt because the early versions were bitumen-based sheets. Modern ones are synthetic. The name has stuck, which is why people still ask about felt. Underlay and membrane are two more names for the same layer, so a quote using any of the three is talking about the sheet under the battens.
High-resistance and low-resistance types
Underlays divide into two broad kinds. A high-resistance one does not let water vapour pass. A low-resistance, or breathable, one does. Marley's guidance is that a sheet of the high-resistance sort will not pass vapour, and that matters for how much air has to move through the space beneath it, because the moisture has no other way out. A breathable sheet lets some escape through itself, which is why its air requirements are lighter.
You do not need to pick between them from a catalogue. The right choice depends on the ceiling below, the insulation and the layout of the roof. The underlay is specified at the survey and named in the paperwork, so you know exactly what is going on your roof and can check it afterwards. Our guide to re-slating a roof shows where the underlay sits in the order of work, and the slate roofing page covers the larger jobs.
Ventilation follows the choice of underlay
The two types need different airflow. For a high-resistance underlay, Marley gives an eaves opening of 25 mm for pitches up to 15 degrees, and 10 mm for anything steeper than 15 degrees. Natural slate has a minimum pitch of 20 degrees, so for a slate roof the 10 mm figure applies.
With a low-resistance underlay over a well-sealed ceiling, Marley allows the 3 mm gap that ordinary eaves construction leaves to count as ventilation, subject to the underlay's third-party conditions being met. Mixing the two approaches is a common mistake, and it is how a roof ends up with the wrong airflow for its sheet. Our answer on how a slate roof is ventilated covers the air paths, and is useful reading before any specification is agreed. It also explains why a sheet that suited one house can be the wrong one for the house next door.
Finding out what is under the slates on your roof now
You can often tell from the loft. Shine a torch up at the underside. If you can see the backs of the slates and the battens with daylight at the joints, there is probably no underlay at all, which is common in older houses. If there is dark sheeting, look for tears, sagging between the rafters, or brittleness where it meets the eaves.
Some old roofs have boards under the slates, called sarking. Boarding is a different build-up and needs its own advice. Do not poke at damp or damaged sheeting yourself, and do not step off the joists. Photograph what you see and send it over. We can tell you whether the layer is still doing its job. Our guide to Victorian slate roofs describes how roofs of that age were put together.
Adding or replacing the underlay
The underlay sits under the battens, so the only sound way to renew it is to lift the slates and battens off the slope. That is why it is added during a re-slate and not as a stand-alone repair. The slates are sorted as they come off, the new sheet is run from the eaves upward with proper laps, and the battens go back at the set-out gauge before the slates are refixed.
Where the slate is sound and the sheet below is the weak point, relaying the existing slates can be a sensible choice. Where the slates are tired as well, a new covering makes more sense. You get that verdict with photographs, and the written quote sets out one total for the work. The aim is the least work that leaves the roof sound, and not a bigger job than the fault deserves. Photographs of the exposed rafters can be sent to you while the slope is open, so you see what the new sheet is laid over.
Related questions
Underlay questions
Can underlay be fitted without taking the slates off?
Not in the normal way. The sheet runs beneath the battens, so the slates and battens have to come off the slope first. For a small area there are repair methods that slide a membrane under the slates, but they patch a local fault. They are not a replacement for the full layer.
Does torn old felt mean my roof is leaking?
Not necessarily. Torn sheeting can be dry if the slates above are sound and well lapped. It does mean the second line of defence is weak, so the roof depends on the slates alone. We look for stains, rot and damp timber to see whether water has already got through.