Roof calculator
Roofs
Dedicated page for flat and pitched roofs, with common build-ups and design guidance.
Common roof build-ups and practical decision points.
Inverted flat roof
Common in modern buildings, with insulation above the waterproofing layer.
Warm flat roof
A standard layout with insulation below the waterproof membrane.
Pitched roof
Requires insulation placement between rafters and/or above them.
Green roof
Vegetation, substrate, and drainage increase the demands on the layers below.
1. Choose the roof type
Pick a flat, pitched, inverted, or green roof as the starting point.
2. Build the layer stack
Use the main calculator to add layers, set thickness, and edit lambda values.
3. Check the U-value
Compare the result against the target and export the report if needed.
How roof insulation should be arranged to control heat loss, moisture, and detailing risks.
Roof insulation should be continuous, well supported, and matched to the roof type. The thermal layer must work together with the waterproofing, ventilation path, and structural layout so the final assembly behaves as intended.
1. Keep the thermal layer continuous
Avoid breaks in the insulation line around rafters, supports, upstands, and service penetrations. Small gaps can create disproportionate losses.
2. Match insulation to moisture strategy
The roof should be designed so condensation risk is controlled and drying can happen in the intended direction. Vapour control and ventilation must fit the build-up.
3. Protect all junctions
Roof edges, parapets, dormers, and wall connections need careful detailing so the design U-value is not lost at perimeter zones.
4. Verify the full build-up
Check the actual layer sequence in the calculator, including any extra boards, membranes, or finishing layers that affect performance.
Roof problems usually appear at edges, penetrations, and changes in geometry.
Roof penetrations
Vent pipes, skylights, and mechanical penetrations should be detailed so insulation and airtightness remain continuous around them.
Edges and parapets
Perimeter zones often have the highest heat loss risk. Extend the insulation cleanly to the roof edge and avoid thin uncovered strips.
Ventilation and drying
For pitched roofs especially, ventilation and vapour control should be designed as one system so the roof can dry safely over time.
A good roof build-up is one where insulation, waterproofing, ventilation, and structure are designed together instead of treated as separate layers.
Typical reasons roof assemblies underperform in practice.
1. Leaving gaps at rafters or supports
Even small breaks in the insulation line can weaken the whole roof because heat will always follow the easiest path out.
2. Ignoring condensation risk
A roof that is thermally good but not moisture-safe can fail over time. The layer sequence and vapour strategy must be checked together.
3. Underinsulating edges
Roof edges, parapets, and junctions with walls often need extra care. If they are thin or poorly covered, the real performance drops fast.
4. Not checking the final build-up
Always verify the exact assembly in the calculator before concluding that the roof meets the target U-value.
A quick check before you finalize the roof design.
- Insulation is continuous across the roof area.
- Edges, penetrations, and junctions are fully detailed.
- Moisture control and drying paths are considered.
- The target U-value was checked in the calculator.
- Material lambda values were verified against technical data.
Common roof questions
Can I calculate flat roofs?
Yes. Add layers from the inside out and check the U-value of the full build-up.
Can I calculate pitched roofs?
Yes. Model layers between rafters and any additional layers above or below them.
Can roofs be compared with wall insulation?
Yes. The calculator uses the same thermal model for all layered assemblies.