Pitched roof calculator — area and PIR panel quantity

Calculate the exact pitched roof area (mono-pitch, gable, hip or half-hip) and PIR insulation panel demand. The calculator considers pitch angle, panel size (1200 × 600 mm standard for termPIR) and waste reserve. Instant result — no registration.

Building footprint dimensions
m
m
Roof pitch
Format:
Common angles:
203040506080100120150180200250
PIR panel format

Triangular slopes and dormers increase waste — set larger reserve

Panel layout on slope

0 Full panels 0 Cut panels 0% Waste 0% Efficiency

Exact layout available only for mono-pitch and gable roofs (rectangular slopes). For other types: estimate only.

How the pitched roof calculator works

We calculate pitched roof area with the formula:

A = (a × b) / cos(α)

where a and b are the horizontal footprint dimensions of the building, and α is the slope angle. The formula reflects that the roof plane is tilted — its actual area is larger than its horizontal projection.

Example

House 10 × 10 m, pitch 35°. Footprint = 100 m². Roof area = 100 / cos(35°) = 100 / 0.819 = 122.1 m². With PIR panel 1200 × 600 mm (= 0.72 m²) you need 122.1 / 0.72 = 170 pcs. + 10% reserve = 187 PIR panels.

What PIR thickness for rafters?

Per Polish WT 2021, the U-value for pitched roofs must not exceed 0.15 W/(m²·K). For PIR (λD = 0.022 W/(m·K)) that means a minimum of 150 mm. In practice 150–180 mm for new builds and 100–120 mm for retrofits. Verify precise thickness in the U-value calculator.

FAQ

How do I measure the roof pitch if I don't know it?
Measure ridge height (h) and half-building-width (b/2). Angle: tan(α) = h / (b/2). Or use a digital protractor against a rafter.
Can I calculate a roof with dormers in this tool?
Yes — add dormer area manually (triangular dormer = ½ × base × height + three small slopes). For multi-slope roofs, divide into sections and sum.
How many termPIR panels cover 100 m² of roof?
Panel 1200×600 mm = 0.72 m². 100 m² needs 139 pcs. + 10% reserve = 153 PIR panels. At 150 mm thickness = 16.5 m³ of insulation.
How do termPIR, insPIRe and izoGRASS differ?
termPIR® = bare PIR insulation board (glass-veil or AL facing), for over-/under-rafter use. insPIRe® = sandwich panel (steel facings) — finished roof of industrial halls. izoGRASS® = PIR tapered wedges for flat roofs.
Does the calculator account for overhangs?
Enter footprint dimensions WITHOUT overhangs (i.e. exterior wall length/width). To include overhangs, add 2 × overhang × building perimeter to the "reserve" field or compute overhang area manually.