How-to guide

How to measure roof area from a plan with the pitch

Name: How to measure roof area from a plan with the pitch
Uploaded: 2026-10-31
Duration: 32 s
Description: How to measure the true area of a pitched roof from a PDF roof plan: draw the slope on plan, enter the pitch in degrees or as a ratio, get the on-slope area. Free.

A roof plan shows each slope squashed flat, so the area you measure on plan is less than the roof you'll actually cover. Takeoff Studio, free on Windows, corrects for it: measure the slope on plan, type the pitch, and it gives you the true area on the slope. Your drawing never leaves your PC.

4 stepsFreeVideo 32 seconds

Step by step

Open the roof plan and set the scale

Open the drawing with Load PDF. In the PDF tab choose Set Scale from Title Block; if a sheet has more than one scale, pick the one for the plan. Or Calibrate (R) from a dimension you trust.

Draw round the slope on plan

Choose Area (A) and click each corner of one roof slope - eaves, verge, ridge and any valleys - then press Enter or double-click. Measure slopes that share the same pitch together, or one at a time if they differ. This slope is 536.58 m² on plan.

Enter the pitch

Select the area and, in the Selection tab under Quantity, type the Slope as degrees (18.4), a rise to run ratio (4:12 or 1:3) or a percentage (33.3%), and press Enter. If the other side of the roof is the same, set Times to 2 instead of drawing it again.

Read the true roof area

The Selection tab shows the plan area, the slope and the area on the slope - 565.61 m² for this slope, and 1,131.21 m² for both with Times 2. The on-slope figure is what goes into the label, the Totals and any export.

Tips

The true area is the plan area divided by the cosine of the pitch, so a 30° roof is about 15% bigger than its plan and a 45° roof about 41% bigger.
Slope works on lengths too: put it on a rafter or verge line drawn on plan to get its true length. Only use it on lines that run up the slope - not on the eaves or ridge.
Perimeters aren't slope-corrected, so measure eaves, verges and ridges as separate lengths.
Hips and valleys run at a different angle from the rafters - measure them separately rather than relying on the roof slope.
Name each slope by its covering (for example Main roof - standing seam) so the totals add up by material.

Frequently asked questions

Is this free?

Yes. Slope, Times and every measuring tool are free in Takeoff Studio, with no account. Takeoff Studio Pro (£24.99 a month or £199.99 a year, first month free) adds exports, costed schedules, assemblies and more.

What formats can I type the pitch in?

Degrees (30), a rise:run ratio (1:2 or 6/12) or a percentage (15%). The Selection tab shows the pitch both ways, for example 18.4° (pitch 1:3).

Does it work for ramps?

Yes. Set the slope on a ramp's area or length measured on plan in the same way.

Are my drawings uploaded anywhere?

No. Takeoff Studio is a Windows app and works offline - the PDF and your measurements stay on your PC.