Measuring roof pitch is one of those tasks that looks simple—until you’re the one standing on the roof trying to make an accurate measurement through uneven shingles, irregular surfaces, and the wind blowing your tools around. Whether you’re ordering a pitched rooftop curb or verifying an existing slope, accuracy matters. A miscalculated roof pitch can quickly result in a misaligned curb, a compromised RTU installation, and costly delays.
This guide walks through the basics of pitch measurement, explains common pitfalls, and offers practical improvisations when the ideal tools aren’t available.
Tools You Need to Measure Roof Pitch
You only need a few simple tools:
- A level at least 12 inches long
- A tape measure
- A straight 2×4 around three feet long (to help average shingle height)
How to Measure Roof Pitch Correctly
Most roofers and HVAC installers refer to pitch using a rise over 12 inches of horizontal distance—for example, 2″ in 12″ or 1/4″ in 12″.
However, the key detail is this:
The “12 inches” MUST be measured level (perpendicular to gravity), not along the surface of the roof.
Many installers mistakenly measure directly along the slope (Figure 1A). This will produce an inaccurate pitch value. Instead (Figure 1B), use a level to establish a perfectly horizontal 12-inch reference measurement.
Step-by-Step:
- Take your level and mark a point at exactly 12 inches from one end (Figure #2).
- Place that end flat against the roof surface.
- Lift the opposite end until the bubble is centered and perfectly level.
- Measure the vertical distance from your 12″ mark straight down to the roof surface.
- That number is your pitch (e.g. 2″ in 12″).
Dealing with Shingles & Imperfections
Shingles, dips, and roof texture can distort your measurement.
To correct for this:
- Lay a straight, three-foot 2×4 flat along the roof slope.
- Place your level on the 2×4 instead of directly on the shingles (Figure #2).
- Repeat the measurement.
This averages out surface irregularities and gives a much more accurate reading.
Applying Your Measurements to a Pitched Curb
Once you have your pitch, you need to apply it correctly to the rooftop curb you’re ordering.
Different curb manufacturers vary in:
- Supply/return opening positions
- Orientation
- Reference points
For MicroMetl curbs, simply contact Customer Service to get the exact pitched curb worksheet for your specific RTU model (Figure #3). The arrow labeled “Pitch” always indicates the high side of the roof.
Alternate Methods — When You Only Know the Angle
If you know the angle of the roof, you can convert it to pitch using basic trigonometry.
Example from Figure #4:
- Roof angle = 18.5°
- Horizontal reference = 12 inches
Formula:
12 × TAN(angle) = rise
12 × TAN(18.5°) = 4.015 ≈ 4″
So the pitch is:
4″ in 12″.
Or When You Have the Pitch and Need the Angle
Use the inverse tangent:
Given:
- rise = 4 inches
- horizontal = 12 inches
Formula:
TAN⁻¹(4 ÷ 12) = 18.435°
Angle ≈ 18.5°
Improvised Measurements (When You’re Missing Tools)
Yes, this sounds ridiculous, but every field tech has been here:
No tape measure?
Pre-measure body reference points ahead of time:
- Finger joint = roughly 1 inch
- Width of three fingers = roughly 2–3 inches
- Wrist to elbow = close to a foot
- Bottom of shoe = stable known length
Just ensure you measure these accurately first—never rely on estimates.
No level?
Use a clear glass of water:
- Mark a line around the glass at a constant height.
- Place it on a flat board that spans the roof.
- Align your measurement with the water level mark.
Or use a water tube level (old-school but accurate):
Water always finds its own level—perfect for leveling long distances.
What If You Measured Wrong and the Curb Doesn’t Fit?
This is a common scenario, and the consequences can be serious.
If the RTU isn’t level:
- Compressor oil may shift to one side
- Internal components lose lubrication
- Overheating and premature failure occur
- Efficiency drops significantly
If the pitch is way off:
- Do NOT install the unit
- Do NOT cut your roof opening
Instead:
- Order a new curb, or
- Use an angled curb adapter if the error is moderate
- Use shims only for very slight corrections
Protect the equipment and your reputation—level matters.
Final Thoughts
Roof pitch calculations are simple when done correctly and costly when done incorrectly. Always:
- Use a true 12-inch level reference
- Average out roof surface inconsistencies
- Verify with the correct curb layout for your RTU
- Double-check before cutting into the roof
And if you’re unsure—call the manufacturer. MicroMetl’s team is always ready to help ensure your pitched curb is designed and installed accurately.
