Minimum Roof Pitch by Roofing Material

The International Residential Code (IRC) is the International Code Council's model code for one- and two-family homes, adopted (often with amendments) by states and cities across the US. Chapter 9, Section R905, sets the requirements for each roof covering, including the lowest slope it can be installed on. Lapped coverings such as shingles and tile depend on slope to shed water over each lap, so the lower the roof, the fewer coverings are allowed.

The values on this page are read from the 2021 IRC as adopted in the 2021 Seattle Residential Code. Your jurisdiction may use a different edition or add amendments. Use this as a guide, then confirm with your building department.

Minimum slope for each roof covering

Ordered from steepest minimum to lowest. Percent and degrees are calculated from the pitch.

To check your own roof, the roof pitch calculator lists which of these coverings are allowed at the pitch you enter.

Asphalt shingles: 2/12, double underlayment to 4/12

Section R905.2.2 allows asphalt shingles on slopes of 2 units vertical in 12 horizontal (17%) or more. Between 2/12 and 4/12 the code requires double underlayment: two layers, applied as Table R905.1.1(2) describes. From 4/12 up, one layer applied shingle fashion is enough. So a 3/12 porch roof can take asphalt shingles, but it needs twice the underlayment of a 6/12 main roof. Price that in.

There is a steep limit too. Where the slope exceeds 21/12, R905.2.6 says shingles must be installed according to the manufacturer's approved instructions, which usually means extra fastening.

Tile, slate and wood: 2½/12 to 4/12

Clay and concrete tile (R905.3.2) need 2½/12 or more, with double underlayment from 2½/12 to 4/12.Wood shingles (R905.7.2) and wood shakes (R905.8.2) need 3/12. Slate(R905.6.2) needs 4/12, the steepest minimum in the section. These are heavy or thick, lapped products that rely on the slope to shed water over each lap.

Metal: from 3/12 down to ¼/12

Metal is where the type of product matters most. Metal roof shingles (R905.4.2) need 3/12. Formetal roof panels, R905.10.2 gives three minimums:

• Lapped, non-soldered seams without lap sealant: 3/12.
• Lapped, non-soldered seams with applied lap sealant: ½/12.
• Standing-seam roof systems: ¼/12.

So "metal roof" alone doesn't tell you the minimum. An exposed-fastener panel with plain laps is limited to the same 3/12 as wood shakes, while a standing-seam system can go almost flat.

Roll roofing and low-slope membranes

Mineral-surfaced roll roofing (R905.5.2) can go down to 1/12. Below that you are into membrane roofs, which the code treats as low-slope systems. Built-up roofing (R905.9.1), modified bitumen (R905.11.1), thermoset and thermoplastic single-ply (R905.12.1, R905.13.1), sprayed polyurethane foam (R905.14.1) and liquid-applied roofing (R905.15.1) all need a design slope of at least ¼/12 "for drainage". The one exception is coal-tar built-up roofing, which can go to 1/8/12.

"Design slope" is the point. These minimums are about water draining off the roof, not about laps shedding water. A nominally flat roof is framed or tapered to at least ¼ inch per foot so water doesn't pond.

Photovoltaic shingles

Solar shingles that replace roofing, as opposed to panels mounted above it, are covered by R905.16. They are allowed only on slopes of 2/12 or more. With rack-mounted panels, the roof covering underneath still has to meet its own minimum slope from the table.

Reading the code's slope notation

The IRC writes slopes as "units vertical in 12 units horizontal", with a percentage in brackets. "2 units vertical in 12 units horizontal (17-percent slope)" is a 2/12 pitch. The percentages are rounded. 2/12 is really 16.7%, and the code describes both 3/12 (25.0%) and 2½/12 (20.8%) as "25-percent slope". When a roof is close to a limit, compare the pitch in x/12, not the rounded percentage. The table above gives the exact percentage and angle for each minimum.

What "double underlayment" means

Between 2/12 and 4/12 for asphalt shingles, and between 2½/12 and 4/12 for tile, the code requires two layers of underlayment instead of one. For asphalt shingles, Table R905.1.1(2) says that from 2/12 up to 4/12 "underlayment shall be two layers", while from 4/12 and steeper it "shall be one layer applied shingle fashion". The table sets out how the layers lap. In practice, the low-slope section needs roughly twice the underlayment per square of the same area at 4/12 or above. Include that in the estimate when a porch or addition is shallower than the main roof.

Worked example: roofing a 3/12 addition

A single-storey addition has a measured pitch of 3/12 (14.0°, 25% slope). Going through the table:

• Slate (4/12) is out.
• Clay or concrete tile (2½/12) is allowed, with double underlayment.
• Wood shingles and shakes (3/12) and metal shingles (3/12) are right at the minimum, so allowed.
• Asphalt shingles are allowed with double underlayment.
• Lapped metal panels without sealant (3/12) are at the minimum. With lap sealant they could go down to ½/12.
• Standing-seam metal, roll roofing and membranes are all well within their minimums.

If you want the addition to match an asphalt-shingled main roof, the code allows it, with two layers of underlayment. Being exactly on the 3/12 minimum for wood or metal shingles leaves no room for a slightly low reading, so measure carefully. The roof pitch calculator shows this same check for any pitch you enter.

Low-slope choices compared

Below 2/12 the shingle options are gone. In falling order of minimum slope: lapped metal panels with sealant (½/12), mineral-surfaced roll roofing (1/12), and standing-seam metal or membranes (¼/12). Coal-tar built-up roofing can go to 1/8/12. The ¼/12 membrane figure is a design slope for drainage, so the structure or tapered insulation has to provide it.

Working near the minimum

• Measure, don't assume. A roof you think is 4/12 might be 3¾/12, and that puts asphalt shingles into the double-underlayment range. See how to measure roof pitch.
• Read the product's instructions. The code sets a floor. A particular shingle, tile or panel may carry its own higher minimum or extra requirements at low slopes.
• Check each roof section. Porch roofs, dormers and additions are often lower than the main roof and may need a different covering.
• Mind the steep end too. Above 21/12, asphalt shingles must follow the maker's steep-slope instructions.

How this list was checked

Each minimum was read from the text of Section R905 in the 2021 IRC, in the City of Seattle's published adoption, which reproduces the model code's wording. The section number is shown next to each value. The ICC's own online copy and the 2024 edition couldn't be checked automatically, so this page says 2021 throughout. The check date at the top and in the sources below is the last time the values were compared with the code text. They are re-checked at least every quarter. If your jurisdiction uses a different edition and you find a difference, please tell us.

Using the numbers

Find your pitch with the roof pitch calculator, which also shows which coverings it allows. Look up an angle or slope with the roof pitch chart. Then estimate the material with the roofing squares calculator. If you are adding double underlayment for a 2/12–4/12 shingle roof, double the underlayment quantity for that section.