Roof Pitch
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Roof Pitch
Roof pitch is the steepness of a roof. In North American practice it is stated as the number of units of vertical rise for every 12 units of horizontal run, written 4:12, 4/12 or "4 in 12". The same geometry can be expressed as an angle in degrees, as a percentage grade, or, in Canadian building codes, as a ratio such as "1 in 3". Pitch governs how quickly water and snow leave the roof, which coverings may legally be used, how snow load is calculated, how much material a roof of a given footprint requires, and how the Roof Structure is framed. It matters in both Residential Roofing and Commercial Roofing, where "low-slope" and "steep-slope" are the two fundamental categories of roof system.
Pitch, slope and angle
The words are used loosely in the trade, but they have distinct meanings:
- Slope is rise divided by run. A roof that rises 6 inches over 12 inches of run has a slope of 6:12, or 0.5, or 50 percent.
- Pitch, in the older carpentry sense, is rise divided by the total span of the roof, so a gable roof rising 8 feet over a 24-foot span has a pitch of 1/3 even though each side has a slope of 8:12. In everyday North American use "pitch" now means slope, and this article follows that usage.
- Angle is the inclination from horizontal in degrees, found from the slope by the arctangent: a 6:12 slope is arctan(6/12) = 26.6°.
Canadian codes state slopes as a ratio of rise to run in the form "1 in n", where a 1 in 3 slope rises one unit for every three of run and equals 4:12; a 1 in 6 slope equals 2:12; and a 1 in 50 slope equals about ¼:12.
Conversion and area factors
The pitch multiplier (also called the slope factor or area factor) converts the flat footprint of a roof to its true sloped surface area, and is the square root of 1 + (rise/run)². Multiplying the horizontal area of each roof plane by the factor gives the area to be covered, before waste allowances for cuts, hips, valleys and starter courses.
| Rise per 12 in. run | Canadian ratio | Angle (degrees) | Grade (percent) | Area multiplier |
|---|---|---|---|---|
| ¼:12 | 1 in 48 | 1.2° | 2.1% | 1.000 |
| ½:12 | 1 in 24 | 2.4° | 4.2% | 1.001 |
| 1:12 | 1 in 12 | 4.8° | 8.3% | 1.003 |
| 2:12 | 1 in 6 | 9.5° | 16.7% | 1.014 |
| 3:12 | 1 in 4 | 14.0° | 25% | 1.031 |
| 4:12 | 1 in 3 | 18.4° | 33.3% | 1.054 |
| 5:12 | 1 in 2.4 | 22.6° | 41.7% | 1.083 |
| 6:12 | 1 in 2 | 26.6° | 50% | 1.118 |
| 7:12 | 1 in 1.7 | 30.3° | 58.3% | 1.158 |
| 8:12 | 1 in 1.5 | 33.7° | 66.7% | 1.202 |
| 9:12 | 1 in 1.33 | 36.9° | 75% | 1.250 |
| 10:12 | 1 in 1.2 | 39.8° | 83.3% | 1.302 |
| 12:12 | 1 in 1 | 45.0° | 100% | 1.414 |
| 14:12 | 1 in 0.86 | 49.4° | 116.7% | 1.537 |
| 18:12 | 1 in 0.67 | 56.3° | 150% | 1.803 |
The multipliers are trigonometric identities and do not depend on any code. As a worked example, a simple gable house with a footprint of 1,200 sq ft and an 8:12 roof has a true roof area of 1,200 × 1.202 ≈ 1,442 sq ft, or about 14.4 roofing squares before waste.
Pitch categories
- Low-slope (often loosely called "flat"): slopes up to about 2:12 or 3:12. The U.S. Department of Energy's cool-roof guidance defines low slope as a pitch of 2:12 or less;[1] the National Roofing Contractors Association uses 3:12 as the boundary in its guidelines. These roofs rely on continuous waterproof membranes such as TPO Roofing, PVC Roofing, EPDM Roofing, Modified Bitumen Roofing and Built-Up Roofing, because water lingers on them, and enclosed low-slope assemblies need particular attention to Roof Ventilation and vapour control. See Flat Roofs.
- Conventional steep-slope: 4:12 to about 9:12, the range of most houses. Water-shedding coverings such as Asphalt Shingles, Metal Roofing, Tile Roofing and Wood Shingles and Shakes are designed for it.
- Steep: above about 9:12. Coverings such as Slate Roofing perform well, but staging, fall protection and fastening requirements increase installation cost, and the IRC requires shingle fastening to follow the manufacturer's steep-slope instructions where the slope exceeds 21:12.[2]
The transition between 2:12 and 4:12 is the most important in practice: shingles are permitted there by U.S. and Canadian codes only with special low-slope application methods, and many manufacturers and the NRCA advise against them.[3]
Minimum slopes by roof covering
Building codes set a minimum slope for each type of covering, and in some cases a maximum. The limits exist because water-shedding materials depend on gravity to move water past their laps, whereas membranes depend on sealed seams and positive drainage.
United States (International Residential Code)
The IRC, adopted with local amendments by most U.S. states, permits asphalt shingles only on slopes of 2:12 or greater and requires two layers of underlayment on slopes from 2:12 up to 4:12; single-layer underlayment is allowed at 4:12 and steeper.[2][4][5] Low-slope membranes and built-up roofs must be designed with a slope of at least ¼:12 (2 percent) for drainage; the reroofing section exempts existing roofs from that minimum only where they already provide positive drainage.[2] For other coverings, the IRC's published minimums, which should be confirmed against the edition adopted locally, are commonly cited as 2½:12 for clay and concrete tile (with double underlayment below 4:12), 3:12 for wood shingles and metal shingles, 4:12 for wood shakes and slate, and, for metal panels, 3:12 for lapped seams without sealant, ½:12 for lapped seams with sealant and ¼:12 for standing seam.
The NRCA's own guidance is more conservative than the code: it does not recommend asphalt shingles on slopes below 18 degrees (roughly 4:12), recommends a minimum of two layers of underlayment between 14 and 18 degrees, and notes that shingle roofs rely on gravity and slope to drain.[3]
Canada (National Building Code, Part 9)
The National Building Code and the provincial codes derived from it tabulate minimum and maximum slopes for each covering. The British Columbia Building Code 2018 table, which reproduces the NBC values, is summarized below; slopes are given as the code states them and converted to the x:12 form.[6][7]
| Type of roofing | Minimum slope | Equivalent | Maximum slope |
|---|---|---|---|
| Asphalt shingles, normal application | 1 in 3 | 4:12 | no limit |
| Asphalt shingles, low-slope application | 1 in 6 | 2:12 | no limit |
| Built-up roofing, asphalt base without gravel | 1 in 25 | ≈ ½:12 | 1 in 2 (6:12) |
| Built-up roofing, asphalt base gravelled | 1 in 50 | ≈ ¼:12 | 1 in 4 (3:12) |
| Modified bituminous membranes | 1 in 50 | ≈ ¼:12 | 1 in 4 (3:12) |
| Profiled metal roofing | 1 in 4 | 3:12 | no limit |
| Sheet metal shingles | 1 in 4 | 3:12 | no limit |
| Cedar shakes | 1 in 3 | 4:12 | no limit |
| Wood shingles | 1 in 4 | 3:12 | no limit |
| Clay tile | 1 in 2 | 6:12 | no limit |
| Slate shingles | 1 in 2 | 6:12 | no limit |
The same article allows asphalt-and-gravel and coal-tar roofs to be built flatter than the table where roof drains are placed at the low points, and allows profiled metal systems designed for low slope to be installed below 1 in 4 in accordance with the manufacturer's written instructions.[6] Asphalt shingles on slopes between 1 in 6 and 1 in 3 must follow the code's low-slope application rules, which correspond to the CSA standard for shingle application on slopes of 1:6 to less than 1:3.[6] Note the Canadian minimums for tile and slate (6:12) are steeper than the commonly cited IRC values.
Snow load and drainage
Slope affects both how much snow a roof retains and how the structure must be designed for it. Under Part 4 of the NBC, as adopted in the BC Building Code, the roof snow load is multiplied by a slope factor Cs that equals 1.0 for slopes of 30° or less, decreases linearly as (70° − α)/40° between 30° and 70°, and reaches zero above 70°; for unobstructed slippery roofs such as metal, where snow can slide completely off, the reduction begins at 15° and reaches zero at 60°.[8] In practical terms, a 7:12 roof (30°) is designed for the full snow load, a 12:12 roof (45°) for about 63 percent of it, and an 18:12 roof (56°) for about 35 percent. Steep roofs shed snow that would otherwise be carried, but shed it suddenly onto entrances, lower roofs and gas meters, which is why snow guards are common on steep metal roofs in Canada.
Low-slope roofs have the opposite problem: a ¼:12 design slope gives only about 2 percent fall, so deck deflection can create ponding, and codes require positive drainage with drains or scuppers at the low points.[2][6] Tapered insulation is frequently used to build slope into a structurally flat deck; see Roof Drainage and Flat Roofs.
Measuring roof pitch
- From the attic: hold a spirit level horizontally against the underside of a rafter, mark 12 inches from the point of contact, and measure the vertical distance from that mark up to the rafter. The result in inches is the rise per 12 inches of run.
- From the roof surface: place the level on the roof, raise the outer end until it reads level, and measure the gap at the 12-inch mark. Measure on a flat area of the covering, not over a hip or a course of shingles.
- From the gable end: measure the total rise from the top of the wall plate to the ridge and the horizontal run from the wall to a point under the ridge; divide, then scale to 12.
- With instruments: a pitch gauge or angle finder set on the rafter or a rake board reads degrees directly, and a smartphone inclinometer app does the same. Convert degrees to rise using rise = 12 × tan(angle).
Choosing a pitch
New roof pitch is usually fixed by architectural style and by the Roof Deck and framing already in place, but where there is a choice, three considerations dominate. Steeper roofs shed water and snow faster, tolerate a wider range of Roofing Materials and provide attic volume, at the cost of more surface area, more wind exposure and more difficult access. Lower slopes reduce material quantity and are simpler to walk on and to fit with rooftop equipment or solar arrays, but confine the choice of covering to membranes and require careful drainage design. In heavy-snow regions the structural saving of a steep slope must be weighed against sliding snow; in hurricane regions moderate slopes of roughly 4:12 to 6:12 are generally preferred. The framing options that produce each slope are described in Common Roof Structures and Sloped Roofs.
Frequently Asked Questions
What is roof pitch and how is it written?
Roof pitch is the steepness of a roof, written as inches of vertical rise per 12 inches of horizontal run: a 6:12 roof rises 6 inches for every 12 inches of run, which equals 26.6 degrees or a 50 percent grade. Canadian codes express the same slope as a ratio such as 1 in 2. The multiplier for converting footprint to roof area is the square root of 1 plus the slope squared.
What is the minimum roof pitch for asphalt shingles?
The IRC permits asphalt shingles on slopes of 2:12 or greater, with two layers of underlayment required from 2:12 up to 4:12. Canadian codes set the same limits as 1 in 6 (low-slope application) and 1 in 3 (normal application). The NRCA does not recommend shingles below about 18 degrees, roughly 4:12, because shingle roofs depend on slope to shed water.
What pitch does a flat roof need?
A "flat" roof still needs slope for drainage. The IRC requires a minimum design slope of ¼:12 (2 percent) for membrane and built-up roofs, and the NBC allows gravelled built-up and modified-bitumen membranes down to 1 in 50, which is about the same. Tapered insulation is commonly used to create that fall on a level deck, with drains or scuppers at the low points.
How does roof pitch affect snow load?
Under the National Building Code, the design snow load is multiplied by a slope factor of 1.0 for roofs up to 30 degrees (about 7:12), decreasing linearly to zero at 70 degrees. A 12:12 roof is designed for roughly 63 percent of the full load, and slippery metal roofs receive further reductions starting at 15 degrees. Steep roofs shed snow but may need snow guards to control where it lands.
How do you measure the pitch of an existing roof?
Hold a level horizontally against a rafter in the attic or on the roof surface, mark 12 inches along it, and measure the vertical distance from that mark to the rafter or roof: the reading in inches is the rise per foot. A pitch gauge, angle finder or smartphone inclinometer gives the angle directly, which converts to rise as 12 times the tangent of the angle.
Sources
- ↑ U.S. Department of Energy, Energy Saver — Consumer Guide to Cool Roofs (low-slope roofs defined as pitch of 2:12 or less; membranes and coatings for low slope versus shingles, tile and metal for steep roofs).
- ↑ 2.0 2.1 2.2 2.3 City of Grand Rapids, Michigan — Roof Assemblies (Chapter 9, Michigan Residential Code) (R905.1.1 double underlayment between 2:12 and 4:12; R905.2.2 asphalt shingle minimum slope 2:12; R905.2.6 fastening above 21:12; R908 minimum design slope of ¼:12).
- ↑ 3.0 3.1 National Roofing Contractors Association — Roofing Guidelines: Resources (consumer FAQ) (NRCA does not recommend asphalt shingles on slopes below 18 degrees and recommends two layers of underlayment between 14 and 18 degrees).
- ↑ City of Hopkins, Minnesota — Asphalt Shingles (R905.2) (asphalt shingles permitted only on slopes of 2:12 or greater; double underlayment from 2:12 to 4:12).
- ↑ City of Norman, Oklahoma — Common Questions for Roofing with Asphalt Shingles (Table R905.1.1(2): two underlayment layers from 2:12 up to 4:12, single layer at 4:12 and greater).
- ↑ 6.0 6.1 6.2 6.3 Government of British Columbia, BC Publications — British Columbia Building Code 2018, Division B, Section 9.26 Roofing (Table 9.26.3.1 Roofing Types and Slope Limits; Subsections 9.26.7 and 9.26.8 on asphalt shingles above and below 1 in 3).
- ↑ City of Vancouver, BC Publications — Vancouver Building By-law 2014, Section 9.26 Roofing (Table 9.26.3.1 slope limits by roofing type, HTML edition).
- ↑ Government of British Columbia, BC Publications — British Columbia Building Code 2018, Division B, Section 4.1 Structural Loads and Procedures (Article 4.1.6.2, slope factor Cs for roof snow load: 1.0 up to 30°, (70° − α)/40° from 30° to 70°, 0 above 70°; slippery-roof values of 15° and 60°).