Flat and low-slope roofs

Many Colorado homes have a flat or low-slope section: a modern addition, a porch roof, a garage or a mid-century design. These roofs work differently from shingle roofs, and they need a different kind of roofer.

Low-slope, not flat

Roof pitch is measured as rise per 12 inches of run. A 4:12 roof rises 4 inches every foot. Shingles depend on gravity to shed water between overlapping pieces, so they need a reasonable slope. Below that, water can back up under them. "Flat" roofs are actually built with a slight slope toward drains or scuppers, and they use a continuous waterproof membrane instead of overlapping pieces.

The main membrane types

This is general trade knowledge; product quality and installation vary.

• TPO (thermoplastic polyolefin) is a single-ply sheet, usually white. Seams are heat-welded. The white surface reflects sun, which helps keep the space below cooler.
• EPDM is a black synthetic rubber sheet, often called a rubber roof. Seams are glued or taped, and it's flexible in cold weather.
• Modified bitumen is asphalt modified with rubber or plastic, rolled out in sheets. It's often torch-applied or self-adhered and may have a granule surface like a shingle.
• Built-up roofing (BUR) is layers of asphalt and felt, usually topped with gravel. It's the traditional tar-and-gravel roof, heavy and common on older buildings.

Hail can puncture or bruise membranes, and damage is harder to spot than on shingles. After a storm, a roofer may need to probe the surface or check the insulation below. Check your area's hail history with the free Roof Report.

Ponding water

Ponding is water that sits on a roof long after rain stops. It speeds up membrane wear, finds its way into any weak seam, and adds weight. In Colorado, ponded water can freeze and thaw repeatedly. Common causes include clogged drains, sagging framing and compressed insulation.

• Keep drains, scuppers and gutters clear of leaves and debris.
• Note where water collects after a storm and how long it stays.
• Look for stains on the ceiling below, especially near drains and walls.
• Have a roofer check low spots. Tapered insulation or added drains can fix chronic ponding.

Snow sits longer on low-slope roofs too. See Colorado's climate for snow loads, and our seasonal checklist for upkeep.

Colorado rules for low-slope roofs

Licensing

Colorado has no statewide roofing license, so cities set the rules. Denver's Residential Roofing Guidelines, revised in 2019, listed separate license classes for shingle roofs and for roof coverings and waterproofing, with membrane roofs requiring the latter. That document is dated, and we couldn't confirm Denver's current license names, so ask the city or your contractor. Fort Collins has a "Roofing" specialty license for roofs steeper than 2:12, and a "Roofing(+)" license that also covers low slope.

Underlayment on low-slope shingle roofs

Some homes have shingles on a fairly shallow slope. Lakewood requires double felt or self-adhered underlayment under shingles on slopes below 4:12, as of its 2026 re-roof handout. Rules elsewhere may differ.

Inspections

Arvada requires a mid-roof inspection on low-slope re-roofs, in addition to the final. Requirements differ by city, so check your building department.

For contractor checks that apply to any job, see hiring a roofer. To compare low-slope systems with other roofs, return to the materials overview.

Common questions

Can you put shingles on a low-slope roof?

Only down to a point. Shingles need enough slope to shed water. Lakewood, for example, requires double felt or self-adhered underlayment under shingles on slopes below 4:12. Below the manufacturer's minimum slope, a membrane roof is the right choice.

How long should water sit on a flat roof?

As a general rule of thumb, it should drain within a couple of days of rain. Water that stays longer suggests a drainage or sagging problem worth having checked.

Does any roofer work on flat roofs?

Not always. Denver's 2019 guidelines required a different license class for membrane roofs, and Fort Collins has a separate Roofing(+) license that covers low slope. Ask whether the contractor is licensed for low-slope work in your city.