Where the line between low-slope and steep-slope roofs falls

Roofers talk about two families of roofs: low-slope and steep-slope. The two families use different materials, keep water out in different ways, and follow different rules in the building code. Knowing which family your roof belongs to makes it easier to understand a repair quote, a replacement proposal or a maintenance plan.

This post explains where the dividing line sits, what kinds of roofs fall on each side, and what the difference means for a building owner.

The dividing line is 3:12

Roof slope is written as inches of rise for every 12 inches of level run. A 3:12 roof rises 3 inches for every 12 inches across, which is an angle of about 14 degrees.

The National Roofing Contractors Association (NRCA) uses 3:12 as the line between the two families. Roof systems installed at 3:12 or less are low-slope. Roof systems installed on slopes greater than 3:12 are steep-slope. Other industry writers describe the same split, and they note that low-slope roofs are often called flat roofs.

Low-slope roofs seal water out

A low-slope roof does not rely on gravity alone. Water moves slowly on a nearly flat surface, and it can sit for a while after a storm. So the roof covering has to be a continuous, weatherproof membrane that water cannot get through, even when it stands on it.

A low-slope roof assembly is usually described as three parts that work together: the roof deck, the insulation and the membrane on top. The common membrane types are:

• Built-up roofing (BUR), made of layers of bitumen and reinforcing felts. Industry writers describe it as the traditional flat roof system in the United States.
• Modified bitumen, a bitumen membrane with added polymers, often called mod-bit.
• Single-ply membranes, which include EPDM (a synthetic rubber), TPO and PVC.
• Liquid-applied membranes and spray polyurethane foam, which the building code groups with the other low-slope systems.

Steep-slope roofs shed water

A steep-slope roof works in a different way. The pieces overlap, and gravity pulls water down across them and off the edge before it has time to work its way underneath. NRCA describes asphalt shingle roofs as water-shedding systems that rely on gravity and roof slope to drain water off the roof.

Common steep-slope coverings include asphalt shingles, clay and concrete tile, slate, wood shingles and shakes, and metal shingles. Because these coverings are not sealed like a membrane, they need enough slope to keep water moving. The building code sets minimums for each one, such as 2:12 for asphalt shingles, 2 1/2:12 for clay and concrete tile, and 4:12 for slate and wood shakes. NRCA recommends 4:12 or more for these coverings as a best practice.

Where metal roofs fit

Metal panel roofs show up on both sides of the line. Standing-seam metal panels can be installed on slopes as low as 1/4:12 under the building code. NRCA separates metal panels into structural panels, which it recommends at 1/2:12 or more, and architectural panels, which it recommends at 3:12 or more.

This is why a metal roof on a warehouse and a metal roof on a house can look alike from the ground but be built in different ways. The seams, the panel type and the slope all have to match.

Some systems also have a maximum slope

Most people think only about how flat a roof can be. Some low-slope systems also have an upper limit. NRCA notes that some EPDM membrane assemblies are limited to slopes as low as 1/2:12. Steep-slope assemblies generally do not have maximum slope limits.

This matters on buildings where a low-slope roof meets a sloped section, such as a mansard or a canopy. The system chosen for each area has to be rated for the slope of that area.

Why the difference matters to an owner

The family your roof belongs to shapes almost every decision about it:

• It decides which materials a contractor can offer you.
• It decides how the roof is inspected. A low-slope roof is checked for seams, flashings, drains and standing water. A steep-slope roof is checked for missing, lifted or worn pieces.
• It decides which code rules apply when the roof is replaced.

Many commercial buildings have both kinds of roof. An office may have a flat main roof with a shingled entry canopy, or a church may have a steep sanctuary roof next to a flat classroom wing. Each area needs a system that suits its own slope.

NRCA does not recommend one family over the other. The right choice depends on the building's slope, its use and its design.

What to take away

The line between low-slope and steep-slope roofs sits at 3:12, or about 14 degrees. Low-slope roofs use a sealed membrane such as built-up, modified bitumen, EPDM, TPO or PVC. Steep-slope roofs use overlapping pieces such as shingles, tile or slate that shed water with the help of gravity. Metal panels can be used in either family, depending on the panel type. When you know which family each part of your roof belongs to, it is easier to follow a contractor's recommendation and to ask the right questions.