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Minimum Roof Pitch for Asphalt Shingles

The 2021 IRC allows asphalt shingles on slopes of 2/12 or more, and requires double underlayment from 2/12 up to 4/12. Here's what that means for your roof.

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Key takeaways

  • The 2021 IRC allows asphalt shingles only on slopes of 2/12 or greater.
  • From 2/12 up to 4/12, it requires double underlayment.
  • Your local code and the shingle's instructions can differ, so check both.

Under the 2021 International Residential Code (IRC), asphalt shingles can be used only on roof slopes of 2/12 or greater. From 2/12 up to 4/12, the code requires double underlayment.

Roof pitch What the 2021 IRC says about asphalt shingles
Below 2/12 Not allowed
2/12 up to 4/12 Allowed, with double underlayment
4/12 and above Allowed, with standard underlayment rules

These come from IRC Section R905.2.2.

Check what applies to you

The IRC is a model code. States and cities adopt it at different times, and can change it. Before you start:

  • Ask your local building department which code edition applies and whether a permit is needed.
  • Read the shingle's installation instructions, which can set their own requirements.

How to find your pitch

Measure it with these methods, then confirm it with the roof pitch calculator. Measure each section, because a porch or addition can be flatter than the main roof.

A pitch of 2/12 is an angle of about 9.5°, and 4/12 is about 18.4°.

If your roof is below 2/12

Asphalt shingles aren't an option under this code. The IRC covers other roof coverings in Section R905, each with its own slope rules. Ask a licensed roofer which systems suit your roof and local rules.

Decide what to do

  1. Measure the flattest section, which is often a porch or addition rather than the main roof.
  2. Below 2/12: asphalt shingles aren't allowed under this code, so ask about other systems.
  3. From 2/12 up to 4/12: shingles are allowed with double underlayment, so make sure it's in your materials list and quote.
  4. At 4/12 and above: the double underlayment rule doesn't apply under this code.
  5. In every case: check your local code and the shingle's instructions, as either can be stricter.

Frequently asked questions

Can you put shingles on a 1/12 roof?

Not under the 2021 IRC, which sets a 2/12 minimum for asphalt shingles.

Can you put shingles on a 3/12 roof?

Yes, under the 2021 IRC, but it requires double underlayment for slopes from 2/12 up to 4/12.

Does a 4/12 roof need double underlayment?

The double underlayment rule applies "up to" 4/12, so standard underlayment rules apply at 4/12 and above under this code. Check your local code and the shingle instructions.

Sources

We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.