10 Minute BidsHow to measure a roof
from blueprints.
The sheets to read, the scale, pitch factors, the lines and the waste, with a worked example. Then the shortcut: upload the plans and get the roof measured in 3D.
Six steps from plans to squares.
Get the right sheets
You need the roof plan (the outline, ridges, hips, valleys and the pitch arrows), the elevations (the pitch symbol on each roof: a small triangle with 12 and the rise), and a wall section (the overhang, fascia and the eave detail). The cover sheet and the general notes say what goes under the roofing: underlayment, ice barrier, flashing.
Check the scale
Read the scale off the sheet (1/4 in = 1 ft is common for plans), then check it against a dimension written on the drawing. A printed or re-sized PDF is often not at the scale it says, so trust written dimensions over a ruler.
Measure each roof plane in plan
On the roof plan, measure the horizontal (plan) area of every plane, overhangs included. A roof plan is a view from above, so every number you take off it is flat, not sloped.
Apply each plane's pitch factor
Multiply each plane's plan area by the pitch factor for its pitch (the table below): the factor is the square root of 1 + (rise ÷ 12)². Planes of different pitches get different factors, so do it plane by plane. Add the planes and divide by 100 for squares.
Measure the lines
Eaves and ridges are flat, so their plan length is their real length. A rake slopes: its length is its horizontal run times the pitch factor. Where two planes of the same pitch meet in a hip or a valley, the hip or valley is the common run times the hip / valley factor in the table.
Add the waste
Material is cut at every hip, valley, rake and penetration. A simple gable wastes the least, and a roof cut up with hips, valleys and dormers the most; your supplier or the maker's instructions set the percentage. Then count the parts: bundles or panels, starter, cap, drip edge, flashing, underlayment and vents.
Or skip the arithmetic with our free roof squares calculator and roof pitch calculator.
The multiplier for every common pitch.
| Pitch | Pitch factor | Hip / valley factor | Angle |
|---|---|---|---|
| 2/12 | 1.0138 | 1.4240 | 9.5° |
| 3/12 | 1.0308 | 1.4361 | 14.0° |
| 4/12 | 1.0541 | 1.4530 | 18.4° |
| 5/12 | 1.0833 | 1.4743 | 22.6° |
| 6/12 | 1.1180 | 1.5000 | 26.6° |
| 7/12 | 1.1577 | 1.5298 | 30.3° |
| 8/12 | 1.2019 | 1.5635 | 33.7° |
| 9/12 | 1.2500 | 1.6008 | 36.9° |
| 10/12 | 1.3017 | 1.6415 | 39.8° |
| 12/12 | 1.4142 | 1.7321 | 45.0° |
Pitch factor = √(1 + (rise ÷ 12)²). Hip / valley factor, for two planes of the same pitch = √(2 + (rise ÷ 12)²), per foot of common run.
A 40 by 30 foot gable house.
| Step | Number |
|---|---|
| Walls, from the floor plan | 40 ft x 30 ft |
| Overhang, from the wall section | 1.5 ft (18 in) on every side |
| Roof outline in plan | 43 ft x 33 ft = 1,419 sq ft |
| Pitch, from the elevations | 6/12, pitch factor 1.1180 |
| Roof area | 1,419 x 1.1180 = 1,586.5 sq ft |
| Squares | 1,586.5 / 100 = 15.86 |
| With 10% waste (a simple gable) | 17.45 squares |
| Shingle bundles (3 to a square) | 17.45 x 3 = 52.4, order 53 |
| Eaves (drip edge, starter) | 2 x 43 ft = 86 ft |
| Rakes | 4 x (16.5 ft run x 1.1180) = 73.8 ft |
| Ridge (ridge cap) | 43 ft |
A real roof is rarely one gable: do the same for every plane, each at its own pitch, then add them.
What the plans won’t hand you.
- What the plans leave out. Plans rarely give the panel profile, gauge or paint for metal, or the shingle line. Ask before you price it.
- Ice barrier. Where the local code requires it, the plans' notes or the jurisdiction's design criteria say so; it runs up from the eave.
- Flashing that isn't drawn. Step flashing where a roof meets a wall, kick-outs at the bottom of those runs, crickets behind chimneys: count them from the roof plan and the elevations.
- Different pitches on one roof. Porches, dormers and additions often have their own pitch. One pitch factor for the whole roof is the most common hand-takeoff mistake.
- Changes in the field. What gets built can differ from the drawings. A takeoff from plans is an estimate: check it on site before you order.
Or upload the plans.
Upload the plan set and we build the house in 3D, then measure every plane, pitch and edge off the model: squares by plane and pitch, eaves, rakes, ridges, hips, valleys, step and wall flashing, and the parts that go with them. You get the takeoff, a material list, a supplier order sheet and a draft bid.
You spend about 10 minutes checking it against your plans, and it's ready to send. Metal roofing: Shingle roofing:
Roof measurements from blueprints: questions
How do I find the pitch on a blueprint?
Look at the elevations: each roof has a small triangle with 12 on one side and the rise on the other (6 over 12 means 6 inches of rise for every 12 of run). Sections show it too. A roof plan may show it next to an arrow pointing downhill.
How do I convert plan area to roof area?
Multiply the plan (flat) area of each plane by its pitch factor: the square root of 1 + (rise / 12) squared. A 6/12 roof has a pitch factor of 1.1180, so 1,000 sq ft of plan is 1,118 sq ft of roof.
How many squares is a roof?
A square is 100 sq ft of roof. Divide the roof area by 100, then add the waste for the roof's shape.
Do overhangs count?
Yes. The roofing covers the overhangs, so measure the roof outline to the fascia, not the walls.
Can I measure a roof from blueprints if the house isn't built?
Yes. That is what plans are for: every number comes off the drawings, and nothing needs to be standing. It's an estimate, so check it on site before you order.
Have a plan set?
Send it in.
The whole roof, plane by plane, measured in 3D from your plans.