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.

Enter footprint dimensions (without overhangs).

Building footprint dimensions
m
m
Roof pitch
Format:
Common angles:
203040506080100120150180200250
PIR panel format
Edge profile (rebate)

FIT = flat butt joint (full coverage). LAP/TAG (rebate) reduce coverage area by 2–4% — the calculator adds more panels. Offcuts can be partly reused, minding the rebate direction.

Triangular slopes and dormers increase waste — set larger reserve

Panel layout on slope

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

Full and cut panel layout per slope. For hip and half-hip roofs standard slope proportions are assumed.

Full panels Cut panels

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 a standard 1200 × 2400 mm PIR panel (= 2.88 m²) that is ≈ 122.1 / 2.88 = 43 panels (before reserve). The calculator above computes the exact layout with cutting and offcut reuse, plus the number of packages.

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

Does the calculator handle gable and hip (four-slope) roofs?
Yes. Pick the roof type — mono-pitch, gable, hip/pyramid (4 slopes) or half-hip. The calculator computes each slope area, the PIR panel layout with cutting and the number of packages separately for each type.
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?
Standard panel 1200×2400 mm = 2.88 m². For 100 m² of roof that is ≈ 35 pcs. + 10% reserve ≈ 39 PIR panels. At 150 mm thickness = 15 m³ of insulation. The smaller 1200×600 mm format (0.72 m²) gives ≈ 139 pcs.
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.