Calculate roof area

What does this calculator do?

The Roof-Area-Calculator returns the gross + net roof area, rafter length, ridge height, material need, and optionally a PV quick estimate — all in one step, all in the browser, all local with no server round-trip. It handles the seven dominant residential roof forms: gable, hip, shed, flat, mansard, half-hipped, and pyramidal. Each form gets its own live cross-section sketch with labelled width, pitch, ridge, and (for mansard) knee-line.

Five differentiating features no other online calculator combines:

1. Seven roof forms instead of four. Most competitor tools stop at gable + hip + shed + flat. Mansard, half-hipped, and pyramidal are missing — and mansard is a DACH-region heritage standard, especially relevant since 2024 for energy-upgrade applications.
2. Live SVG cross-section per form with labelled width, pitch, and ridge. You see what you’re calculating instead of mentally piecing together the geometry.
3. Optional PV rooftop quick estimate. Module count, kWp, and annual yield — east/west/south factor included, the standard module footprint 1.7 × 1.1 m kept conservative. No “request a solar installer” lead capture.
4. DIN 18338 / VOB/C measurement-mode toggle. Gross DIY (+5 % overhang flat surcharge) vs. net trade billing (gable +30 cm, eaves +20 cm, opening deduction > 0.5 m²). DIY estimate and professional billing are two technically different things — the calculator makes that explicit.
5. Print-ready A4 measurement PDF with input echo, cross-section sketch, and result table. Directly suitable as preparation for the trade-firm meeting.

The calculator follows the current measurement rules of DIN 18338 (roofing works, part C of the German VOB). For Austria the parallel ÖNORM B 3521 applies.

How is the roof area computed?

The base formula depends on the roof form. For a gable roof with length L, width B, and pitch α:

Roof area    = 2 × L × (B/2) / cos(α)
Rafter length = (B/2) / cos(α)
Ridge height = (B/2) × tan(α)

Worked example: gable roof 10 × 8 m at 30° pitch:

• Rafter length = 4 m / cos(30°) ≈ 4.62 m
• Roof area = 2 × 10 × 4.62 ≈ 92.4 m²
• Ridge height = 4 × tan(30°) ≈ 2.31 m

With a form-recommended wastage of 7.5 %, the order area is ≈ 99.3 m².

For a shed roof the formula is simpler because there is only one inclined surface: Roof area = L × B / cos(α). For a flat roof (≤ 7° pitch) the roof area is set ≈ the footprint area — the cosine is close enough to 1 that the difference disappears in the wastage band.

Mansard roof has a two-knick profile per side. The lower steep section (typically 60°) covers 60 % of the half-width, the upper shallow section (typically 30°) the remaining 40 %. The total area is the sum of the four slopes:

Mansard area = 2 × L × (B/2 × 0.6) / cos(α_lower)
            + 2 × L × (B/2 × 0.4) / cos(α_upper)

Hip and half-hipped roofs get additional triangles at the gable ends. The calculator approximates the exact geometry with an empirical factor (hip × 0.97, half-hipped × 0.95 against the gable base) — for order quantities the difference to the exact measurement is typically below 3 %.

Which seven roof forms are supported?

Roof form	Typical use	Default pitch	Wastage
Gable	Single-family house, terraced house	30°	5–10 %
Hip	Townhouse, bungalow	30°	10–15 %
Shed	Garage, modern home, carport	15°	5–10 %
Flat	Bungalow, Bauhaus-style	3°	5–12 %
Mansard	Heritage, energy upgrade	60°/30°	15–20 %
Half-hipped	Northern European farmhouse	35°	10–15 %
Pyramidal	Pavilion, small extension	35°	10–15 %

Switching forms loads sensible default pitches and wastage bands. Every value remains user-editable — defaults are a starting point, not a constraint.

Gross measurement or net (VOB/C) measurement?

The VOB Part C / DIN 18338 (roofing works) governs the measurement between owner and roofing trade. It distinguishes two modes that mainly differ in how openings and overhangs are handled:

Aspect	Gross DIY	Net trade (VOB/C)
Gable overhang	+5 % flat surcharge	+30 cm additive
Eaves overhang	(included in the flat surcharge)	+20 cm additive
Openings > 0.5 m²	not deducted	deducted per VOB/C
Openings ≤ 0.5 m²	not deducted	not deducted (“gross-for-net”)
Audience	Supply-yard order	Roofing-trade billing

Important: The +5 % gross surcharge is NOT the same as the wastage surcharge. The +5 % covers gable/eaves overhangs that count toward the roof area; the wastage surcharge covers cut-edge waste. Both are additive — the calculator shows the breakdown in the “Measurement details” box.

Rooftop photovoltaic — how big could the system be?

The optional PV block estimates three quantities:

1. Module count based on 1.7 × 1.1 m = 1.87 m² standard module and an 80 % roof usability factor (spacings, maintenance gap, wind reserves, gable clearance).
2. Peak power (kWp) at 0.42 kWp per module — matching a typical 400–420 W rooftop module for residential buildings.
3. Annual yield (kWh) at 950 kWh/kWp as the central-European south-30° reference. East/west orientation × 0.85, south-east/south-west × 0.95, flat with south-tilt frames × 0.9.

Worked example: gable roof 92 m² south 30°:

• Usable area = 92 × 0.8 = 73.6 m²
• Modules = 73.6 / 1.87 ≈ 39 pcs
• kWp = 39 × 0.42 ≈ 16.4 kWp
• Annual yield = 16.4 × 950 × 1.0 ≈ 15,580 kWh/year

The estimate is conservative and does not replace a professional economic analysis. It covers the pre-design question: does the system fit on the roof? How big could it be? What does orientation cost in yield? The calculator collects no contact data and does not refer to a solar installer — pure-client, no lead generation.

Material need — tiles, shingles, or roofing felt?

The calculator supports three generic material classes, each vendor-neutral:

• Roof tiles (clay or concrete pantiles, ~12.5 pcs/m², ~45 kg/m²) — classic covering for gable, hip, and mansard roofs.
• Roofing shingles (bituminous shingles, ~8 kg/m²) — lighter, good for garages, carports, shed and pyramidal roofs in secondary buildings.
• Roofing felt / bituminous membrane (V13 / G200 or self-adhesive, ~4 kg/m²) — standard for flat-roof sealing and low-pitch shed roofs.

Material quantities are calculated against the order area (incl. wastage), not the net roof area. For tiles the calculator rounds up to whole pieces — you cannot buy 0.3 of a tile.

Which mistakes happen most often when calculating roof area?

Three recurring pain points from DIY forums and trade reports:

1. “I just took footprint × pitch surcharge — way too little.” The footprint is NOT the roof area. At 30° gable pitch the roof area is about 15 % larger than the footprint; at 45° already 41 %; at 60° about 100 % larger. The calculator makes the cosine projection visible in the cross-section sketch.
2. “My calculator doesn’t handle mansard.” That’s exactly why this form is here. A mansard roof has two slopes per side with different pitches; the calculator splits the upper and lower slopes cleanly.
3. “Measurement difference roofer vs. order — who is right?” Both. The roofer measures net per VOB/C with additive overhangs and opening deduction. The order quantity contains gross area + wastage + overhang surcharge. The gross mode of this tool covers the order use case, the net mode covers the billing use case — side-by-side instead of mixed up.”

What does this tool NOT compute?

Deliberately scoped out to keep liability and complexity clear:

• No structural design. Load-bearing capacity of rafters, purlins, posts — that is the job of a structural engineer per DIN 1055 (load assumptions) and DIN 1052 (timber construction). We calculate geometry and material need, not load assumptions.
• No snow or wind load calculation. Both require the regional snow-load zone or wind zone and are structural-design domain.
• No € price per m² for tiles, shingles, or felt. Prices are regionally and temporally highly volatile; a stale value would be worse than no value. Phase-2 candidate if real search queries confirm the demand.
• No maps/satellite measurement of the specific roof. Pure-client + privacy: no external map APIs, no polygon fetching, no location tracking.
• No 3D/WebGL visualisation. Refined minimalism — a 2D cross-section is enough for 99 % of use cases and stays print-friendly in the PDF export.
• No insulation thickness calculation. That lives in our Insulation Calculator with U-value logic per GEG 2024.
• No trade-firm referral. The calculator shows results — you take them to your local roofer or to the supply yard. We don’t collect contact data and don’t broker assignments.

Which construction tools are related?

For complementary tasks around the roof and energy-upgrade project: Insulation Calculator for U-values and layer thicknesses per GEG 2024, Window U-Value Calculator for KfW/passive-house classification of roof windows, Heating Cost Calculator for fuel-cost analysis after renovation, Screed Calculator for the floor slab under a mansard build-out, and Masonry Calculator for adjoining exterior-wall construction.

Specialist modules for roof-renovation costs, KfW subsidy calculators, and snow-zone lookup follow in phase B once real search-query data confirms demand.

Frequently Asked Questions

The frontmatter of this page lists eight FAQ entries with schema.org/FAQPage markup, sourced from the top-Google “people also ask” patterns for roof-area topics. Each answer addresses the question in the first 10 words — voice-search-optimised for smart speakers and AI search assistants.