How Much to Add for Roof Pitch

A footprint measured from the ground or a plan is flat. The roof is not. Multiply by the pitch factor to get the real surface you have to cover.

Add for pitch calculator

The pitch multiplier

Sloped area = flat area × multiplier Multiplier = √(pitch² + 144) ÷ 12 Add for pitch (%) = (multiplier − 1) × 100

The multiplier is the length of a rafter for each foot of level run. Because every plane of a roof with one pitch tilts by the same amount, it works for gable, hip and most combination roofs alike: measure the outside footprint including overhangs, then multiply once. Roofs with mixed pitches need each section handled separately, which the measure a roof guide covers.

Pitch factor table

How much area each pitch adds
Pitch	Multiplier	Add for pitch	1,000 sq ft footprint becomes
1/12	1.0035	0.3%	1,003 sq ft
2/12	1.0138	1.4%	1,014 sq ft
3/12	1.0308	3.1%	1,031 sq ft
4/12	1.0541	5.4%	1,054 sq ft
5/12	1.0833	8.3%	1,083 sq ft
6/12	1.1180	11.8%	1,118 sq ft
7/12	1.1577	15.8%	1,158 sq ft
8/12	1.2019	20.2%	1,202 sq ft
9/12	1.2500	25.0%	1,250 sq ft
10/12	1.3017	30.2%	1,302 sq ft
11/12	1.3566	35.7%	1,357 sq ft
12/12	1.4142	41.4%	1,414 sq ft
13/12	1.4743	47.4%	1,474 sq ft
14/12	1.5366	53.7%	1,537 sq ft
15/12	1.6008	60.1%	1,601 sq ft
16/12	1.6667	66.7%	1,667 sq ft
17/12	1.7341	73.4%	1,734 sq ft
18/12	1.8028	80.3%	1,803 sq ft

Worked examples

Ranch house, 4/12. Footprint 60 × 30 ft with 1 ft overhangs: 62 × 32 = 1,984 sq ft. × 1.0541 = 2,091 sq ft, or 20.9 squares. The pitch adds only 107 sq ft.

Two-story colonial, 9/12. Footprint 40 × 28 with overhangs: 42 × 30 = 1,260 sq ft. × 1.25 = 1,575 sq ft. The pitch adds a full quarter.

Rafter length. The same multiplier converts horizontal run to sloped length. A 13 ft run at 8/12 is 13 × 1.2019 = 15.62 ft along the slope, before the overhang.

The multiplier gives the area to cover, not the amount to buy. Add waste on top: about 10% for simple gable roofs and 15% or more for hips and valleys. The waste factor calculator walks through it.

Frequently asked questions

What is the pitch factor for a 6/12 roof?

The pitch factor (multiplier) for 6/12 is 1.118. Multiply the flat area by 1.118, which adds about 11.8%.

Do I use the pitch multiplier on a hip roof?

Yes. If every plane has the same pitch, the sloped area of a hip roof is the footprint times the same multiplier as a gable roof. Hips change the waste and the ridge-cap footage, not the area formula.

Does the multiplier include waste?

No. It gives the true surface area. Add a waste allowance separately when ordering material.

How do I find the footprint?

Measure the outside of the walls and add the overhang on every side, or measure the drip edge-to-drip edge dimensions from the ground. Building plans list the roof outline too.

Last reviewed 2026-10-10. Results are estimates for planning; confirm quantities with your supplier and requirements with your local building code and the manufacturer's instructions.