Loads
Roof covering weight (dead load) in kg/m² and kN/m²
- where
- covering weight per m² of slope (kg/m²)
- tiles or slates per m²
- mass of one tile or slate (kg)
- battens per m² (m/m²)
- batten mass per metre (kg/m)
- underlay mass (kg/m²)
- dead load per m² of slope (kN/m²)
- total covering weight (kg)
- sloping roof area (m²)
Explain this equation
Works out how heavy a roof covering is, per square metre and in total. Use it to compare a new covering with the old one, or to give an engineer the dead load for checking the structure.
Each square metre carries n tiles of mass m, B metres of batten at m_b per metre, and the underlay. Adding them gives the mass per square metre of slope w_c. Multiplying by g = 9.81 m/s² turns mass into force in newtons, and dividing by 1000 gives kN/m². Multiplying by the roof area gives the total.
Load is mass times gravitational acceleration, summed over the layers.
Sense check: concrete interlocking tiles are around 45 to 55 kg/m² laid; plain clay tiles 60 to 80; natural slate 25 to 40. 50 kg/m² is about 0.49 kN/m².
Watch out: this is per m² of slope. Engineers often want load per m² on plan, which is larger by 1/cos of the pitch. Swapping slate for concrete tiles can overload old rafters.
Weight of the roof covering per square metre of slope and in total: useful when re-roofing with a heavier tile, checking a roof structure or planning loading-out.
Enter your values to see the result.
Good to know
Manufacturers publish the weight as laid, for example Marley Modern 50 kg/m² (0.49 kN/m²) at 75 mm headlap and concrete plain tiles 73.8 kg/m²; fibre cement slates are about 20 kg/m². Battens add a little: 25 × 50 mm softwood at about 370 kg/m³ is roughly 0.46 kg/m. Changing from slate to concrete tiles can more than double the dead load: Approved Document A expects the structure to carry the new load, and a heavier covering usually needs a structural check.
Common mistake
Comparing per m² of plan with per m² of slope. Manufacturer weights are per m² of roof slope; convert using the pitch factor if a structural engineer wants plan loads.
Limits
Dry weight. Wet tiles and slates absorb some water. Imposed (snow, maintenance) and wind loads are separate and need a structural designer.
Sources
- Marley Modern concrete interlocking tile: technical data (420 × 330 mm, 292 mm cover width, 345 mm max gauge, 9.9 tiles/m², 2.9 m battens/m², 50 kg/m²)
- Marley concrete plain tile: technical data (267 × 168 mm, 65 mm headlap, 100 mm gauge, 60 tiles/m², 10 m battens/m², 73.8 kg/m²)
- Marley Rivendale fibre cement slate fitting instructions (gauge formula, 5 mm gap, 2 nails + 1 disc rivet per slate, coverage and weight table)
- Approved Document A: Structure (GOV.UK)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.