Roof Weight Calculator: How Much Does a Roof Weigh?

Roof coverings are not interchangeable, because they do not weigh the same. Concrete plain tiles weigh close to three times what natural slate does, so re-covering a 100 m² slate roof in concrete plain tile adds roughly 4.8 tonnes to the same rafters. This tool works out what your finished roof weighs, compares it with what is up there now, and tells you when the change is big enough to need a structural check before anyone orders materials.

Typical roof covering weights

These are the figures the calculator uses. They are laid weights per square metre of roof slope, dry, for the covering alone. Battens, underlay and fixings add roughly another 3 kg/m² on a tiled or slated roof.

Why the weight decides what you can put on

A roof structure was sized for the covering it was built with. A Victorian roof built for natural slate has lighter rafters at wider centres than a 1970s roof built for concrete interlocking tiles, because it never had to carry that load. Swap the slate for concrete plain tile and the timbers pick up roughly 48 kg on every square metre, which on a modest semi is several tonnes that nobody designed for. The damage shows up slowly: a dipped ridge line, rafters bowing between purlins, or the tops of the walls pushed outwards, which is roof spread.

Going the other way is usually welcome, but it is not free of rules either. A much lighter covering resists wind uplift less, so it has to be mechanically fixed for the wind loading at your site rather than held down by its own mass. That is why clip and nail specifications matter far more on metal and lightweight tile systems than they did on the heavy roof being taken off.

What the Building Regulations say about re-covering

In England and Wales, re-covering a roof is controlled work if you change the covering to one that is significantly heavier or significantly lighter, if you change the roof's shape or pitch, or if the work covers more than a quarter of the roof area. Two separate parts apply:

• Structure. The roof has to be shown capable of carrying the new covering. In practice that means a structural engineer's check, and sometimes sistering or replacing rafters while the roof is stripped.
• Energy efficiency. Where more than 25% of the roof surface is renovated, the insulation normally has to be brought up to current standards for the whole of that element where it is technically and economically feasible. Budget for it on any full re-roof.

Your roofer should be notifying building control, either directly or through a competent person scheme. If a quote for a full re-roof with a change of material says nothing about building control, ask why before you sign it. Our guide to planning permission and building regulations for a new roof goes through the notification routes.

Three things this figure does not include

• Water. Clay and concrete absorb it. A soaked roof is heavier than the dry figures above, which is one reason design loads are set higher than the covering weight.
• Snow. Snow load is added separately by an engineer and varies with your location, site altitude and roof shape. A shallow pitch holds more of it than a steep one.
• Mortar and extras. Bedded ridges, hips and verges, dormer cheeks, lead work and anything mounted on the roof all add weight the per-square-metre figure misses. Dry fix systems at the ridge and verge cut some of that back, and they are now the normal specification on a new roof.