Use this page as an estimating workflow rather than a specification. Build the quantities in the order shown, use the linked calculators for the arithmetic, then verify every product-specific value against the material you intend to buy.
Measure the actual insulated footprint
Use the net loft area that will receive insulation rather than the external roof footprint.
Treat each layer as a separate coverage requirement
If multiple layers are being installed, calculate equivalent area using the product coverage for the chosen thickness and orientation.
Use current pack coverage
Roll and board coverage varies by dimensions and thickness. The manufacturer or merchant pack information should be the input.
Allow for awkward areas consciously
Tanks, hatches, eaves and obstructions can increase cutting. Keep the allowance separate from the base area.
Calculators for this job
Common mistakes
- Using roof area instead of loft floor area
- Assuming one roll coverage for every thickness
- Forgetting multiple layers
- Using a quantity estimator to choose the required thermal design
Pre-order checklist
- Re-measure the main dimensions or compare them with a current drawing.
- Replace generic coverage, density, spacing or pack figures with the actual product data.
- Keep the base quantity, waste allowance and pack rounding as separate steps.
- Check whether any part of the job requires professional design, approval or a manufacturer-specific installation method.
- Save the raw measurements so the estimate can be reproduced if the supplier queries it.
The project planners deliberately stop at quantity planning. They do not certify structural adequacy, regulatory compliance or installation suitability.