Illinois Roofing License IDFPR #19HC00108700 · GAF Certified Contractor
Not every roofing material works on every roof. The slope (or pitch) of your roof determines which products are viable, which warranties apply, and which installation techniques are required. This guide explains how pitch is measured, the minimum pitch requirements for each common roofing material, and what your options are if your roof falls into the low-slope category.
How roof pitch is measured
Roof pitch in the US is expressed as rise over run – how many inches the roof rises vertically for every 12 inches of horizontal run. Common pitches:
- 2/12 – very low slope (2 inches of rise per 12 inches of horizontal). Almost flat to the eye.
- 4/12 – low to moderate. Typical of ranch homes and additions.
- 6/12 – standard residential. Most Illinois homes are 5/12 to 7/12.
- 9/12 – steep. Many Victorian and historic Illinois homes.
- 12/12 – very steep. 45-degree angle.
- Beyond 12/12 – extreme. Requires roof jacks, scaffolding, and specialized installation.
You can measure your roof’s pitch using a level and a tape measure: hold the level horizontal against the roof for a 12-inch run, then measure the vertical gap at the end. Or measure from inside the attic against the rafter.
Asphalt shingle minimum pitch
Asphalt shingles – the material on most residential roofs – have a minimum pitch requirement:
- Below 2/12 – NOT allowed for asphalt shingles. Manufacturer warranties void; the shingles will leak.
- 2/12 to 4/12 – asphalt shingles allowed only with full coverage of ice and water shield underlayment across the entire roof (not just at eaves and valleys). Special installation, additional cost.
- 4/12 and above – standard asphalt shingle installation with synthetic underlayment + ice barrier at eaves only.
If your home has a section below 2/12 (porch roof, dormer roof, low-slope addition), asphalt shingles aren’t the right material for that section even if they’re fine for the rest of the roof.
Low-slope material options (below 4/12)
For low-slope or near-flat sections, three real options:
1. Rolled roofing (mineral surface roll roofing) – large rolls of asphalt-coated material installed in long strips. Lower-cost, shorter lifespan (10-15 years), works down to 2/12. Common on porch roofs and outbuildings.
2. Single-ply membrane (TPO, EPDM, PVC) – what most commercial flat roofs use. Works on residential low-slope sections too. Longer lifespan (20-30 years), more expensive than rolled roofing, requires specialized installation.
3. Standing seam metal – works at any pitch down to about 1/2 inch per foot (very low). Long lifespan (50+ years). Higher upfront cost. Premium option.
Don’t try to install asphalt shingles on truly low-slope sections – the shingle won’t shed water properly and will leak regardless of how well the installation goes.
Steep-slope considerations
At the other end of the spectrum, very steep roofs (over 8/12) come with their own considerations:
- Labor cost increases – slow, careful work with roof jacks and safety harnesses. A 12/12 roof can cost 30-50% more in labor than a 6/12 of the same square footage.
- Material choices expand – slate, tile, and metal all work well on steep roofs. Asphalt shingles are still the most common.
- Walk-on damage – steep roofs are vulnerable to shingle damage from foot traffic. Avoid HVAC techs, antenna installers, and others walking on steep roofs.
- Snow shed – steep roofs shed snow faster than low slopes; that’s mostly good (less ice dam risk) but the falling snow load can damage landscaping or hit people below
Mixed-pitch roofs
Many Illinois homes have multiple roof slopes – a main 6/12 gable section plus a 3/12 addition or porch roof. The right approach is to install different materials on different sections:
- Main roof (4/12 and above): standard architectural asphalt shingles
- Low-slope porch (2/12-3/12): asphalt shingles with full ice and water shield underlay, OR rolled roofing, OR single-ply membrane
- Below 2/12 flat sections: single-ply membrane or metal
The cosmetic appearance is a question to think through. Standard asphalt shingles look continuous across the roof. Mixed materials can look intentional (rolled roofing matches a porch roof aesthetically) or jarring (commercial TPO on a residential roof). A roof inspection can identify the right approach for your specific home.
Schedule an evaluation
If you’re not sure what your roof’s pitch is or what material options you have, schedule a free Trill Roofing inspection at /free-inspection/. We measure pitch, identify any low-slope sections that need different treatment, and recommend materials that work with the geometry.
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