Tips For How To Measure The Slope of a Commercial Flat Roof

by | Apr 16, 2024 | Featured, Flat Roof Management

Flat roofs are not flat. They have a slight angle or slope built into them so water can drain properly. This helps avoid leaks, mold, and structural damage.

What Is Slope?

The components that go into a flat roof slope calculation |

Slope (C) is a measure of the roof’s rise over a set distance. For flat roofs, it is expressed as a ratio of the number of inches a roof rises (A) over every 12 inches of the roof’s width across to its midpoint or run (B).

Because slope affects your choice of flat roofing materials, how they’re installed, and how well they perform over time, understanding how to measure and calculate the slope of a commercial flat roof can be a valuable tool for property managers and building owners.

Measuring Slope

Measuring the slope of a commercial flat roof can be tricky because of the size of the roof. We will show you three different ways to simplify the task below. You can use a tape measure, digital level, or laser level, depending on your roof type and tool preference.

Roofs With Parapet Walls

Parapet walls are walls that extend upward, above, and around the edge of a flat roof.
Example of a Commercial Flat Roof With Parapet Walls |

If a roof has parapet walls, then you can use a regular tape measure to determine its slope. Note that parapets are equal in height relative to the foundation of the building, but vary in height when you’re measuring from the rooftop up.

To measure the roof’s slope when you have parapet walls:

1. Measure the height of the parapet wall at the tallest point on the roof. Measure from the point where the roof surface meets the parapet.

2. Do the same thing at the lowest point of the roof.

3. Subtract the lowest parapet height from the highest. This is the “drop”.

4. Measure the distance between the two points. This is the “run” (B).

5. You now know how much drop, or slope (C), you have over a set distance or run.

6. Next, convert the drop number into inches and divide it by the number of feet in the run. This will tell you how many inches per foot of slope you have.

For example, if you have a roof drop of 36 inches over a 144-foot distance or run, divide 36 by 144 to yield a slope of 0.25 inches for every 1 foot of horizontal run on the rooftop.

Inside the Building With a Laser Level

Commercial Flat Roof Joists |

Another way to measure the slope of a commercial flat roof is to use a laser level inside the building.

1. Set your laser level up close to the roof joists, making sure it is shining on the high and low points.

2. Mark on the joist where you take your measurement.

3. Measure the distance from the bottom of the joist to the laser lever at the high point.

4. Then repeat this process at the low point.

5. Measure the distance between the two points and repeat steps 3 through 6 above to find the slope.

Inside the Building With a Digital Level

The third option is to measure the slope from inside the building using a digital level. Digital levels are relatively inexpensive and can usually be found at any home supply store.

1. Set your digital level to the roof slope setting.

2. Set the level on the bottom of the roof joist

3. Read the slope displayed on the level.

We don’t just recommend setting the level on the roof itself because the joist will give you a more accurate measurement. There can sometimes be irregularities and undulations in the roof that could otherwise throw off a level.

What If Your Roof Has No Slope

Standing Water on Flat Roof  |
If your building was built between 1950 and 1980, do not be surprised if the roof has no slope. The old built-up roof systems built during that time typically did not require slope. It was the school of thought at the time that ponding water on the roof would help keep the building cooler. Now it is known that having a roof slope to evacuate water will make the roof last longer.


While it is helpful to know the slope of your commercial flat roof, it can be tricky to measure because the surface on a commercial rooftop is so large. We recommend using one of the three methods explained above to gain a quick measure of your roof’s slope using a simple tape measure, laser, or digital level.


Expert Advice

Whether you’re a beginner, need advanced information about flat roofing topics, or a refresher,  this is where you can learn everything you need to know about commercial and industrial flat roofing.

Contact us for more information or pricing.




Jared Van Vranken is Vice President and lead estimator at Flat Roof Solutions in Malvern, PA. Jared has directed the installation, repair, recovery, and replacement of commercial flat roofs for more than a decade at Flat Roof Solutions. He guides clients in selecting the best roofing materials for their needs and budget.

You Might Also Like


Submit a Comment

Your email address will not be published. Required fields are marked *

Pin It on Pinterest

Share This