How roof slope is measured and written as rise over run

Every roof has a slope, even the ones that look flat from the parking lot. The slope decides which roof systems can go on the building, how water leaves the roof, and what the building code requires when the roof is replaced. If you own or manage a commercial building, it helps to know how that number is written and what it tells you.

This post covers how roofers measure slope, how to read it on a drawing or a proposal, and how the common numbers convert to degrees and percent.

Slope is rise over run

Roof slope compares two distances. The first is the rise, which is how far the roof surface goes up. The second is the run, which is how far it travels level across the building. In the United States, roofers and the building code keep the run fixed at 12 inches and state how many inches the roof rises over that distance.

So a roof written as 4:12 rises 4 inches for every 12 inches it runs. A roof written as 1/4:12 rises one quarter of an inch for every 12 inches. You will also see the same thing written as "4 in 12" or "1/4 inch per foot." They all mean the same slope.

Some people use the word pitch for the same idea. Industry writers often put the two words side by side, as in "pitch (slope)."

Converting slope to degrees and percent

The rise over run number is the one you will see most, but engineers and product sheets sometimes use degrees or percent. The conversion is simple math. Percent is the rise divided by the run. Degrees come from the angle that rise and run make together.

Here are the numbers that come up most often on commercial and residential buildings:

• 1/4:12 is about 2.1 percent, or about 1.2 degrees.
• 1/2:12 is about 4.2 percent, or about 2.4 degrees.
• 2:12 is about 16.7 percent, or about 9.5 degrees.
• 3:12 is 25 percent, or about 14 degrees.
• 4:12 is about 33.3 percent, or about 18.4 degrees.
• 12:12 is 100 percent, or 45 degrees.

A 1/4:12 roof drops only 1 inch over 4 feet of run. That is hard to see by eye, which is why so many commercial roofs are called flat even though they are sloped.

Why 3:12 is the number to remember

The National Roofing Contractors Association (NRCA) sorts roof systems into two groups. Low-slope roof systems are weatherproof membranes installed at 3:12 (about 14 degrees) or less. Steep-slope roof systems are water-shedding coverings, such as shingles, installed on slopes greater than 3:12.

That split matters because the two groups keep water out in different ways. A membrane forms a continuous layer that water cannot pass through. A shingle or tile roof relies on gravity and overlapping pieces to move water down and off the roof. Roofs that people call flat belong to the low-slope group.

The slopes the building code asks for

The building code sets minimum slopes for each kind of roof covering. For new construction, the 2018 International Building Code calls for at least 1/4:12 for built-up, modified bitumen, single-ply and liquid-applied membranes, and the same minimum for spray polyurethane foam roofs.

Steep-slope coverings need more. The same code sets 2:12 as the minimum 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 asphalt shingle, tile, metal shingle, slate and wood roofs. NRCA describes its numbers as best practice guidelines, and they are generally steeper than the code minimums.

Metal panel roofs vary by type. The code allows standing-seam metal panels at 1/4:12. NRCA recommends 1/2:12 or more for structural metal panels and 3:12 or more for architectural metal panels.

Reroofing an existing low-slope roof

Many older commercial buildings were built with less slope than a new building would get today. When an existing low-slope roof is replaced or recovered, the code does not require the building to be rebuilt to 1/4:12. It uses a performance standard called positive roof drainage instead.

The code defines positive roof drainage as a roof where the deck deflection under load has been accounted for and enough slope has been added so the roof drains within 48 hours after rain. NRCA recommends that membrane, liquid-applied and spray foam roof systems be sloped to provide positive drainage. In practice this can mean tapered insulation, added crickets or new drains during a replacement.

Slope is lower where two roof planes meet

The slope printed on a drawing is the slope of the main roof surface. Where two roof planes meet in a valley, the water runs along the line where they meet, and that line is flatter than either plane. NRCA gives the example that where two 4:12 roofs meet at a valley, the valley itself has a slope of only about 3:12.

The same thing happens along crickets and saddles on low-slope roofs. NRCA notes that some ponding water along cricket and saddle valleys typically will occur and should be anticipated. Knowing this helps an owner tell the difference between a small expected puddle and a real drainage problem.

How to find the slope of your own roof

You can often find the slope on the original building drawings, where it appears as a small triangle with the rise and run marked, or as a note such as "1/4 in/ft." A roof survey or a replacement proposal should also list it.

On site, a roofer can measure slope with a level and a tape, or with a digital level that reads the angle directly. On a low-slope roof, the slope may change from area to area because of tapered insulation, crickets and drains, so one reading does not describe the whole roof.

What to take away

Roof slope is written as inches of rise per 12 inches of run. A roof at 3:12 or less belongs to the low-slope group and uses a membrane. A roof steeper than 3:12 belongs to the steep-slope group and uses a water-shedding covering. The building code and NRCA each set minimum slopes for every kind of roof, and on reroofing projects the goal is a roof that drains within 48 hours. When you know your roof's slope, you can read a proposal with a clearer idea of why a contractor is recommending one system over another.