Minimum Roof Pitch Calculator

Which Roof Coverings the Building Code Allows at Your Pitch

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Asphalt shingles need a pitch of at least 2/12, and between 2/12 and 4/12 the International Residential Code requires two layers of underlayment (R905.2.2). Clay and concrete tile need 2 1/2:12, slate 4/12, metal roof shingles, wood shingles and lapped metal panels without sealant 3/12, lapped metal panels with lap sealant 1/2:12, and standing seam metal, built-up roofing, modified bitumen and EPDM, TPO or PVC single-ply membranes 1/4:12. Enter your roof's pitch, its angle in degrees, or a measured rise and run, and the Minimum Roof Pitch Calculator converts it and checks every covering against the 2021 IRC section R905 minimums, flagging where a double layer of underlayment is required. A 3/12 porch roof is 14.04°: shingles and tile are allowed with two layers of underlayment, slate is not.

The International Residential Code sets a minimum slope for every roof covering in section R905. Below it, a water-shedding covering such as shingles, tile or slate lets wind-driven water back under the laps, so low-slope roofs need a sealed membrane or standing seam metal instead. The product installation instructions can be stricter than the code and govern the warranty.

Minimum roof pitch by roofing material (2021 IRC)

Minimum slope and double-underlayment threshold for each roof covering, 2021 IRC R905
Roof covering	Minimum pitch	Angle	Double underlayment below	Code section
Asphalt shingles	2:12	9.46°	4:12	IRC R905.2.2
Clay or concrete tile	2 1/2:12	11.77°	4:12	IRC R905.3.6 / R905.3.3.1
Metal roof shingles	3:12	14.04°	—	IRC R905.4.2
Mineral-surfaced roll roofing	1:12	4.76°	—	IRC R905.5.2
Slate shingles	4:12	18.43°	—	IRC R905.6.2
Wood shingles	3:12	14.04°	—	IRC R905.7.2
Standing seam metal panels	1/4:12	1.19°	—	IRC R905.10.2 (3)
Lapped metal panels with lap sealant	1/2:12	2.39°	—	IRC R905.10.2 (2)
Lapped metal panels, no lap sealant	3:12	14.04°	—	IRC R905.10.2 (1)
Built-up roofing (asphalt)	1/4:12	1.19°	—	IRC R905.9.1
Modified bitumen	1/4:12	1.19°	—	IRC R905.11.1
EPDM / TPO / PVC single-ply	1/4:12	1.19°	—	IRC R905.12.1 / R905.13.1

Which roofing works at your pitch

Roof coverings the IRC allows at common low pitches
Pitch	Angle	Allowed coverings
1/4:12	1.19°	Standing seam metal panels, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply
1/2:12	2.39°	Standing seam metal panels, Lapped metal panels with lap sealant, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply
1:12	4.76°	Mineral-surfaced roll roofing, Standing seam metal panels, Lapped metal panels with lap sealant, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply
2:12	9.46°	Asphalt shingles (2 layers underlayment), Mineral-surfaced roll roofing, Standing seam metal panels, Lapped metal panels with lap sealant, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply
2 1/2:12	11.77°	Asphalt shingles (2 layers underlayment), Clay or concrete tile (2 layers underlayment), Mineral-surfaced roll roofing, Standing seam metal panels, Lapped metal panels with lap sealant, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply
3:12	14.04°	Asphalt shingles (2 layers underlayment), Clay or concrete tile (2 layers underlayment), Metal roof shingles, Mineral-surfaced roll roofing, Wood shingles, Standing seam metal panels, Lapped metal panels with lap sealant, Lapped metal panels, no lap sealant, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply
4:12	18.43°	Asphalt shingles, Clay or concrete tile, Metal roof shingles, Mineral-surfaced roll roofing, Slate shingles, Wood shingles, Standing seam metal panels, Lapped metal panels with lap sealant, Lapped metal panels, no lap sealant, Built-up roofing (asphalt), Modified bitumen, EPDM / TPO / PVC single-ply

Worked example: a 3/12 porch roof

Angle = arctan(3/12) = 14.04°; slope = 25%.
Asphalt shingles: allowed (minimum 2/12) with two layers of underlayment, because 3/12 is below 4/12.
Clay or concrete tile: allowed (minimum 2 1/2:12) with two layers of underlayment.
Slate: not allowed — it needs 4/12.
Standing seam metal, lapped metal panels and metal shingles: allowed.

Frequently asked questions

What is the minimum roof pitch for shingles?

2/12. The International Residential Code (R905.2.2) allows asphalt shingles on slopes of two units of rise in 12 of run or steeper, and from 2/12 up to 4/12 it requires two layers of underlayment. Below 2/12 shingles are not permitted — use a membrane such as modified bitumen, EPDM or TPO.

What is the minimum slope for a metal roof?

It depends on the seam. Under IRC R905.10.2, standing seam panels can go down to 1/4:12, lapped (non-soldered) panels with lap sealant to 1/2:12, and lapped panels without sealant need 3:12. Metal roof shingles need 3:12 (R905.4.2). The panel manufacturer's listing can be stricter than the code, and it governs the warranty.

What is the lowest pitch for a tile roof?

2 1/2:12 for clay or concrete tile (IRC R905.3.6), with two layers of underlayment required from 2 1/2:12 up to 4:12 (R905.3.3.1). Slate needs 4:12 (R905.6.2).

What roofing can go on a 1/12 pitch?

Membranes and standing seam. Built-up roofing, modified bitumen and EPDM, TPO or PVC single-ply are allowed from 1/4:12, standing seam metal from 1/4:12, lapped metal with sealant from 1/2:12, and mineral-surfaced roll roofing from 1:12. Shingles, tile and slate are all too steep a requirement for 1/12.

What is a 2/12 pitch in degrees?

9.46 degrees, which is a 16.7% slope. 3/12 is 14.04°, 4/12 is 18.43° and 1/4:12 is only 1.19° — barely more than flat, which is why low-slope membranes are designed for it.

Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.