How to calculate roof area from a plan, and why quotes come up short
A roof plan is drawn looking straight down. Every plane is shown flat, as if the roof had no pitch. The sheets don't lie flat. They run up the slope, and on a 25° roof every metre on the plan is more than a metre of steel.
Price the plan and you price a smaller roof than the one you'll fix. The drawings in this post are samples we made for illustration, not a real project.
Quick answer: to calculate roof area from a plan, measure the plan area of each roof plane, then multiply by the pitch factor, 1 ÷ cos(pitch). At 25° the factor is 1.1034, so 164.0 m² on plan is 181.0 m² of roof. Ridges and spouting stay at plan length. Barges, hips and valleys all get longer on the slope.
Sample: the roof plan as issued, dimensioned to the fascia, 25° pitch on every plane.
Plan area is not roof area
The sample is a hip roof, 16,000 mm long and 8,000 mm deep, with a 6,000 mm wide wing off the south side. The schedule calls for long-run corrugate, 0.40 BMT, 762 mm cover. Pitch is 25° on all planes. Eaves are 600 mm and gables 300 mm.
Take the main block (16,000 × 8,000 mm) and add the wing (6,000 × 6,000 mm). That's 164.0 m² on plan. It's the number most people stop at.
The factor that converts it is 1 ÷ cos(pitch). At 25°:
1 ÷ cos 25° = 1.1034
so 1 m measured on the plan is 1.1034 m measured up the roof
and 164.0 m² on plan × 1.1034 = 181.0 m² of roof
One check before you multiply. This plan says "DIMS mm TO FASCIA", so the 600 mm eaves are already inside the 164.0 m². If your plan is dimensioned to the wall frame, add the overhangs first.
How to calculate roof area from a plan, step by step
Split the roof into planes. The sample has 6: the north plane, the south plane, 2 hip ends and 2 wing planes.
Get the plan area of each plane. Triangles and trapezoids, kept separate.
Confirm the pitch of each plane. Check the elevations or section, not only the plan note. If planes differ, each gets its own factor.
Multiply each plane by its factor. Then add them up.
Worked on the sample, plan area to true area:
North plane: 48.0 m² to 53.0 m²
South plane: 39.0 m² to 43.0 m²
Hip ends: 16.0 m² to 17.7 m², 2 of them
Wing planes: 22.5 m² to 24.8 m², 2 of them
Total: 164.0 m² to 181.0 m²
At 762 mm cover, the sample tally counts 88 sheets.
Sample: each plane marked plan vs true. 164.0 m² on plan becomes 181.0 m² at 1.1034.
What pricing on plan area looks like on a quote
Here's the same sample with a quote line from "Subcontractor A". The roof is priced at 164 m², $72.00/m², long-run corrugate 0.40 BMT, supply and fix. That's $11,808.00 excl. GST, with flashings and spouting extra.
Nothing on that line looks wrong. The area matches the plan. That's the problem.
Sample: a quote priced on the plan area. Spot the error before you read on.
The quote is 17.0 m² short. There are two honest ways to say how short:
17.0 m² is 9.4% of the roof you'll actually sheet
17.0 m² is +10.4% on top of the 164 m² that was priced
at $72/m², that's $1,224 of sheeting nobody has been paid for
Both are correct. The first is how much roof is missing. The second is what to add to the quote when you reprice.
Sample: priced on plan area, not roof area. 17.0 m² short, $1,224 at $72/m².
The rate also moves. The $72.00/m² was really a rate on 164 m² of plan. Spread across 181.0 m² of steel, you've quoted less per square metre than you think.
We've written up a re-roof quote that was about to go out at a margin of roughly −5 to −10% (the per-m² sanity check that caught it). That's a different roof, driven by about 60 lineal metres of flashings. The rule is the same: divide by the measured roof area, not the plan area.
Flashings run along the slope too
The quote leaves flashings and spouting as "extra". Measured off the plan, some go short and some don't. On the sample:
Ridges are level, so plan length is true length: 8.0 m + 9.0 m = 17.0 m.
Spouting runs along the eaves, also level: 7 runs, 54.0 m, with 6 downpipes.
Barges run straight up the slope, so they take the full rafter factor: 2 × 3.31 m = 6.6 m.
Hips run diagonally: 5.66 m on plan, 5.96 m true. 4 × 5.96 m = 23.8 m.
Valleys also run diagonally: 4.24 m on plan, 4.47 m true. 2 × 4.47 m = 8.9 m.
A hip rises the same height as the rafters beside it, but over a longer run. So it grows by less than the area factor. Scale a hip off the plan and it's short. Multiply it by 1.1034 and it's long. Measure it as its own length.
Sample: Detail A. The wing ridge sits 466 mm lower, so the valleys stop at a valley head on the main roof.
Detail A shows one more catch. The 9.0 m wing ridge sits 466 mm below the 8.0 m main ridge, so the valleys stop at a valley head on the south plane. That junction is flashing labour no length list shows.
Checks before a roofing quote goes out
Dimension basis: fascia or frame. The sample is fascia, 600 mm eaves included.
Pitch: per plane, checked against the elevations.
Area: factor each plane, then add. 164.0 m² becomes 181.0 m².
Lengths: ridges and spouting at plan length, barges at the rafter factor, hips and valleys as their own lengths.
Rate: divide the roofing total by the true area and compare it with your floor.
Our guide to how to quote a roofing job in NZ covers the spec lines that sit on top of the area: wind zone, corrosion zone and exclusions. At the low end of pitch the question isn't area at all. It's whether the profile is allowed. We covered a 2° roof in the metal roof minimum pitch post.
Get the quantities checked before you commit
Unsure of the quantities? Takeoff Verification re-measures them off the drawings for $490 + GST per trade, in 48 hours. You get every quantity confirmed or flagged, in writing.
Got a roofing quote written, or one you've received? Start with the free quote check. We go through it line by line: area, pitch, flashing lengths and rate.
FAQ
What is the pitch factor for a roof? It's 1 ÷ cos(pitch). At 25° it's 1.1034. Our roofing quote guide puts an 8° roof at about 1% extra and a 30° roof at about 15% extra. The steeper the roof, the bigger the gap between plan and sheet.
Do I measure roof area to the fascia or the wall? To the fascia, because the sheets run out over the eaves. The sample includes 600 mm eaves. If your plan is dimensioned to the frame, add the overhangs first.
Does the pitch factor apply to hips and valleys? No. Hips and valleys run diagonally, so they grow by less than the area factor. On the sample a hip is 5.66 m on plan and 5.96 m true. Barges take the full factor. Ridges and spouting take none.
What if the roof has more than one pitch? Factor each plane on its own pitch, then add them up. An averaged factor is wrong on any plane that isn't at the average. On the sample all 6 planes are 25°, so 1.1034 covers the lot.
Got a re-roof or new roof quote on the desk? Send the drawings and the quote through our contact page for a free quote check. We'll tell you, in writing, whether the area, the flashings and the rate hold up before the price goes out.
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