Find a tool

Roof Pitch Calculator

The factor that turns a footprint into roof area.

It does not measure the pitch for you.

40' · 480 in · 40 ft · 12.192 m

30' · 360 in · 30 ft · 9.144 m

6/12 · 26.6° · 50%

1' · 12 in · 1 ft · 0.3048 m

1' · 12 in · 1 ft · 0.3048 m

1,502.64 ft²

Cutting waste on the covering — the shingles, or the metal panels. The makers allow 10% on a gable roof and 15% on a hip, where every course runs into a hip at an angle; CertainTeed's own manual says 10% is generous on a large simple roof. It moves the covering only — the starter, the cap, the drip edge and the underlayment are cut to length, not off a square.

This page recalculates as you type. There is no Calculate button.

Answer

Save imageExcel

Slope factor

1.118

1,502.64 ft² of roof, 15.03 squares, 52 bundles

Roof area, up the slope 1,502.64 ft²
Bundles of shingles 52 bundles
Roofing squares 15.03 squares
What the list comes to $3,034.00
More numbers
Footprint, on the ground 1,344 ft²
Bundles of starter 2 bundles
Bundles of ridge cap 4 bundles
Sticks of drip edge 16 sticks
Rolls of underlayment 2 rolls
Nails 3,847 nails
DrawingThe project drawn to scale. Roof 1: 40' × 30'.40'30'Roof 1

Buying noteNo ice barrier is on this list. The code requires one where your town's table says so — look up Table R301.2 for where you are, and switch it on if it says yes.

Breakdown

Plane Measurements Qty Area on the slope Share
Roof 1 40' × 30' × 6/12 × 1' × 1' Add ×1 1,502.64 ft²

What to buy

Buying noteNo ice barrier is on this list. The code requires one where your town's table says so — look up Table R301.2 for where you are, and switch it on if it says yes.

Ways to buy the shingles

Every bundle is worked out, each rounded up to whole bundles; pick the one you will buy. The three are different shingles, not three sizes of one, so a mix means two products on one roof — buy a mix only if you meant to.

52 × bundles of HDZ Bundles 52 Left over 5.13 ft² Weight 4,046 lb Cost $2,184.00 Reference price $42.00. Type your store's.
53 × bundles of Oakridge Bundles 53 Left over 10.37 ft² Weight 3,816 lb Cost $2,014.00 Reference price $38.00. Type your store's.
52 × bundles of Landmark Bundles 52 Left over 5.13 ft² Weight 3,900 lb Cost $2,288.00 Reference price $44.00. Type your store's.

Shopping list

52 52 × bundles of HDZ $2,184.00
2 2 × bundles of OC starter $108.00 Reference price $62.00. Type your store's. Reference price $54.00. Type your store's.
4 1 × bundles of ProEdge + 3 × bundles of Seal-A-Ridge $200.00 Reference price $48.00. Type your store's. Reference price $56.00. Type your store's.
16 16 × sticks of drip edge $216.00 Reference price $13.50. Type your store's.
2 2 × rolls of synthetic $256.00 Reference price $128.00. Type your store's.
5 5 × 5 lb boxes of nails $70.00 Reference price $78.00. Type your store's. Reference price $14.00. Type your store's. Reference price $5.00. Type your store's.
Ticked items $3,034.00

Untick what you already have. Each row has its price beside it — type what your store charges.

Save imageExcel

Standards

We hold the criteria and the clause, not the wording of the standard.

Meets IRC 2021 · R905.2.2

At 6/12 this roof is steep enough for asphalt shingles.

Meets IRC 2021 · R905.2.2

At 6/12 a single layer of underlayment is enough.

Meets IRC 2021 · R905.2.8.5

160' of drip edge covers the eaves and the rakes.

Not checked IRC 2021 · R905.1.2

The code requires an ice barrier only where your local Table R301.2 says so, and we hold no table by town. Look yours up and switch it on.

How this was worked out

Every number below is yours. Each constant links to the row where you can change it.

Roof 1 42' × 32' × 1.118 (6/12) 1,502.64 ft²
Squares of roof 1,502.64 ft² ÷ 100 ft² 15.03 squares

Change:A roofing square

With cutting waste 1,502.64 ft² × (1 + 15%) 1,728.03 ft²
Bundles to buy 1,728.03 ft² ÷ 33.33 ft² 52 × bundles of HDZ · 5.13 ft² over

Change:GAF HDZ bundle coversOC Oakridge bundle coversCT Landmark bundle covers

Starter course 148' ÷ 105 lf 2 × bundles of OC starter

Change:GAF Pro-Start bundle runsOC starter bundle runs

Ridge cap 106' ÷ 33 lf 1 × bundles of ProEdge + 3 × bundles of Seal-A-Ridge

Change:Seal-A-Ridge bundle runsOC ProEdge bundle runs

Drip edge 148' ÷ (10' − 2") 16 sticks

Change:A stick of drip edgeDrip edge lap

Underlayment 1,502.64 ft² ÷ (1,001 ft² × (1 − 10%)) 2 rolls

Change:Synthetic roll coversLaps take off a roll

Nails 15.03 squares × 256 3,847 nails → 5 × 5 lb boxes of nails

Change:Nails a square

What it comes to 52 × $42.00 + 2 × $54.00 + 1 × $56.00 + 3 × $48.00 + 16 × $13.50 + 2 × $128.00 + 5 × $14.00 $3,034.00

Assumptions

Every one of these is editable. Change one and the answer changes with it.

Assumption Value Confidence
A roofing squareA roofing square is 100 ft² by the trade's definition; every shingle wrapper and every estimate is quoted in them. ≈ ft²/square Trade convention
GAF HDZ bundle coversGAF Timberline HDZ: 33.33 ft² a bundle, three bundles to the square, from the Home Depot listing. ft²/bundle From a data sheet or an agency document
OC Oakridge bundle coversOwens Corning Oakridge and Duration: 32.8 ft² a bundle. Three of them cover 98.4 ft², not 100, which is why this page never counts three bundles to the square. ft²/bundle From a data sheet or an agency document
CT Landmark bundle coversInferred, not printed: the CertainTeed Landmark data sheet gives 66 shingles to the square, and three bundles to the square is the trade's convention, so a bundle covers 100/3 ft². The sheet does not state a coverage per bundle. ~ ft²/bundle Derived or single-supplier
GAF Pro-Start bundle runsGAF Pro-Start starter strip: 120.33 linear feet a bundle, from GAF's own product page. lf/bundle From a data sheet or an agency document
OC starter bundle runsOwens Corning Starter Strip Plus: 105 linear feet a bundle, data sheet dms/10011118. lf/bundle From a data sheet or an agency document
Seal-A-Ridge bundle runsGAF Seal-A-Ridge ridge cap: 25 linear feet a bundle, from the Home Depot listing. lf/bundle From a data sheet or an agency document
OC ProEdge bundle runsOwens Corning ProEdge ridge cap: 33 linear feet a bundle, from the Home Depot combination listing. lf/bundle From a data sheet or an agency document
A stick of drip edgeDrip edge is sold in 10 ft sticks in every profile. Gibraltar Building Products catalog 2026 (AWC020926B) page 5. ft From a data sheet or an agency document
Drip edge lapDrip edge sticks lap at least 2 in, so a 10 ft stick covers 9 ft 10 in. JLC on IRC R905.2.8.5, which requires drip edge at eaves and rakes but gives the lap as a minimum rather than a number. ≈ in Trade convention
Synthetic roll coversOwens Corning ProArmor synthetic underlayment: 1,001 ft² a roll, from the Home Depot combination listing, read 14/10/2026. ft²/roll From a data sheet or an agency document
Laps take off a rollWhat the head and side laps take off a roll's printed area. A 36 in felt lapped 2 in at the head over a 36 in exposure loses about a tenth; synthetic sheets print a lap line at much the same place. An editable single number because the exact loss depends on the exposure the installer works to. ~ ratio Derived or single-supplier
Nails a squareGAF: 256 nails a square. The figure is theirs; how it divides over shingles and courses is not stated and is not inferred here. nails/square From a data sheet or an agency document
Shallowest pitch for shinglesThe floor for asphalt shingles: at this pitch and above they may be used, and from here up to the next limit the underlayment doubles. IRC R905.2.2 states both. CertainTeed's Landmark data sheet is one step stricter - "greater than 2:12" - so a roof at exactly 2:12 passes here and not on their sheet; this box is where you put their figure. rise per 12 From a data sheet or an agency document
Double underlayment belowBelow this pitch, and at or above the shallowest pitch for shingles, the underlayment has to be doubled. IRC R905.2.2; CertainTeed's Landmark data sheet states the same range. rise per 12 From a data sheet or an agency document
Hire a roofer aboveWhere a roof stops being walkable without staging. Not a code limit: it is the slope at which this page says to get a quote rather than climb up. 6:12 itself is walkable, so the warning starts above it. ≈ rise per 12 Trade convention
Cutting waste, hip15% on a hip roof: four planes means a cut at every hip. Same sources as the gable figure. A hip roof needs no second 'hip factor' on its area - the slope factor already covers the whole surface - only more cutting. ≈ ratio Trade convention

The number a pitch is for

A roof pitch is a rise over a run of twelve. What you do with it is turn it into the slope factor: the number that multiplies a footprint into roof area. It is √(rise² + 144) ÷ 12 — the hypotenuse of a right triangle 12 across and rise up, over the 12.

At 6:12 the factor is 1.118. A house whose footprint is 1,200 square feet has 1,341.6 square feet of roof over it. At 12:12 the factor is 1.414 and the same footprint carries 1,697 square feet.

That factor is the number printed largest above, and the roof area it makes is the line right under it. Type a bigger house and the area grows while the factor stays put: it belongs to the pitch, not to the size. Only on a roof of several planes at different pitches does it move with their sizes, because then it is the sloped area over the footprint of all of them together.

Pitch Slope factor Degrees Percent
1:121.00354.8°8.3%
2:121.01389.5°16.7%
3:121.030814.0°25.0%
4:121.054118.4°33.3%
5:121.083322.6°41.7%
6:121.118026.6°50.0%
7:121.157730.3°58.3%
8:121.201933.7°66.7%
9:121.250036.9°75.0%
10:121.301739.8°83.3%
12:121.414245.0°100.0%
18:121.802856.3°150.0%
24:122.236163.4°200.0%

Measuring it

From the attic, against the underside of a rafter, or on the roof itself if the pitch is gentle enough to stand on: hold a spirit level horizontal, mark 12 inches along it from where it touches, and measure straight down from that mark to the roof. The drop is the rise. Six inches of drop is 6:12.

From the ground you can do it on a gable end: the rise is the height from the top of the wall to the peak, the run is half the width of the house, and the pitch is rise ÷ run × 12.

Three ways of saying one slope

A roofer says 6:12. A speed square or a phone says 26.6 degrees. A surveyor says 50 per cent. All three are the same roof, and mixing them up is the one mistake worth guarding against: 50 typed into a box that means a percent is a gentle roof, and 50 typed into a box that means a rise over twelve is almost vertical.

Here a bare number is the rise over twelve, because that is what the trade says. Degrees and percent have to say so — 26.6°, 50% — and the line under the box reads your pitch back in all three so you can see which one it took.

Where the pitch stops being a detail

Asphalt shingles need a slope steeper than 2:12, and from 2:12 up to 4:12 they need the underlayment doubled. At 8:12 and steeper the ice barrier at the eaves has to be at least 36 inches up the slope. Past about 6:12 a roof is not safe to walk without staging. The code panel on the roofing pages checks the first three against the edition you pick.