Are Slate Roofs Fire Resistant?

What slate does well

Natural slate is a metamorphic rock, formed under heat and pressure over a very long time. It has no fuel in it, so it does not ignite, does not feed a fire and does not drip burning material. When a spark or ember lands on a slate roof, nothing happens. That is a clear advantage over thatch, which burns readily, and over some synthetic coverings that melt or char. For a house close to a boundary, or in an area with outbuildings, woodland or other roofs nearby, a non-combustible covering helps limit how fire spreads from one building to another.

Slate also keeps its shape in heat better than many people expect, though a very intense fire can crack it. The slates are individually nailed or hooked, so a fire that damages a few leaves the rest of the roof intact.

What the rest of the roof is made of

Under the slates sit battens, the thin timber strips that hold each course, and below those a breathable or bitumen underlay and then the rafters. All of these can burn if a fire starts below the covering, in the loft or a chimney, or if flames get through a broken area. In a house fire, the likely path is from the inside out: fire moves up through the ceiling or around a faulty flue, takes hold in the roof timbers and only then reaches the slates. A slate covering does not stop that, though it does slow the spread outwards.

So the fire performance of a roof is a matter of the whole build-up. Modern underlays are available with improved fire resistance, and fire stopping at the eaves, around chimneys and where roofs meet walls matters. Our page on slate roof underlay and ventilation explains the layers under the slates.

Man-made slate and other coverings

Fibre-cement slates and other man-made alternatives are made to imitate natural slate, and their fire performance depends on the product. Some are rated as highly as natural slate, others are not, and the rating can change with the type of fixing or underlay. Always check the manufacturer's literature and the classification given there, rather than assuming they match natural slate. Our guide to natural versus man-made slate compares them more fully.

Clay and concrete tiles are also non-combustible, so they sit alongside slate on this point. Plastic and some bitumen-based roofs do not, which is one reason that choice of material near a boundary can be subject to Building Regulations. If you are unsure about a specific boundary distance, your building control body is the right place to ask, and we do not give legal advice on it.

Practical points for owners

Slate does not remove the need for sensible precautions. Keep the chimney swept and the flue lined so that sparks stay inside. Fit working smoke alarms on every floor, including the landing below a loft. Store nothing flammable in the loft against the chimney breast. If you have solar panels, ask that the cabling is fixed and protected correctly. A roof that has been repaired in the past with mismatched materials can have gaps at the abutments where fire could pass, so a survey is worth having after any major work.

How long the covering lasts matters too, since failed slates expose the underlay to the elements and make the roof less safe. Our guide to how long a slate roof lasts covers ageing.

Fire and the boundary rules

Part of the reason roofing materials are tested at all is the risk of fire spreading between buildings. The Building Regulations look at how close a roof is to the boundary of the plot, and how the covering performs, because a roof near a boundary is more likely to be exposed to a neighbour's fire. A non-combustible covering such as natural slate is usually easy to justify under those rules, while a lightweight synthetic covering may be restricted close to a boundary. When we are changing a covering, we check the proposed material against the notifiable work rules and tell you if building control will want to be involved.

Signs the roof needs a look after a fire scare

After a chimney fire, an electrical fault in the loft or a lightning strike, check the roof space with a torch for scorched timber, blackened battens and bent or cracked slates. Look at the lead around the chimney for melting or distortion. Discoloured slates near the stack are worth recording with a photograph. Any sign of charring in the rafters means the structure should be assessed before the house is used as normal.

We survey slate roofs from the ground and, where safe, from the roof, and we take photographs of the covering, the underlay and the junctions so you can see their condition. You then receive a fixed written quote. See our slate roofing in Basingstoke page for the full range of work.