Roof Pitch Calculation Dos and Don’ts

Here at Curbco we’re happy to create curb for sloped roofs as long as we have the data we need. (See two downloadable forms on our website, “Welded RTU Curbs” and “Calculating the Pitch of a Roof,” for a visual representation of this data.) In principle it’s straightforward enough. The contractors set the level on the roof slope, raise one end until the level reads true, measure the vertical difference distance between the end of the level and the roof surface, then divide that vertical measurement by however many feet the level is long. Voila, you’ve got your pitch.

But as simple as this procedure may sound, people can and do get it wrong, with disastrous results. If we build a curb adapter to the incorrect roof pitch, your RTU just won’t sit correctly. We then have to build an extension — an adapter for the adapter, so to speak — just to correct the imbalance. It’s a waste of time, money and labor, and it’s completely avoidable if we can have the right pitch measurements to begin with.

How do contractors get this measurement wrong? For one thing, they may assume their level is a certain length when in fact it isn’t. Ideally you’d want to use a one-foot level so you don’t have additional math to cope with on that side of the equation. But levels come in various sizes — two-foot, three-foot, four-foot, eight-foot, you name it. The contractors may also measure from the topside of the level to the roof surface, instead of from the bottom side. Or they may assume all the roof’s various slopes have the same pitch as the one they measured. Oops!


Make sure your contractors take the right steps to calculate roof slope. We’ll take it from there!