How To Calculate Roof Pitch Ratio For Existing House or Building - Simple Construction Math
Knowing your roof's pitch – its slope or angle – is handy for various projects, from estimating shingle needs to planning a dormer addition. But how do you figure it out without calling in the pros? Fear not, DIYers! Here's a guide to becoming a roof pitch detective, complete with simple methods and examples.
Understanding Roof Pitch
Roof pitch is expressed as a ratio of rise (vertical height) to run (horizontal distance), typically in the format X:12. For example, a 4:12 pitch means the roof rises 4 inches for every 12 inches of horizontal distance.
Methods to Find Your Roof Pitch
1. The Classic Rise-Over-Run
Tools: Level (at least 12 inches long), tape measure
Steps:
Access your attic and find a rafter.
Place the level horizontally against the rafter's bottom edge.
Measure 12 inches along the level from where it touches the rafter.
Measure vertically from that 12-inch mark up to the rafter. This is your "rise."
Your pitch is the rise value:12 (e.g., if you measure 6 inches, your pitch is 6:12).
2. The Angle Finder App
Tools: Smartphone with an angle finder app (many free options available)
Steps:
Go into your attic and hold your phone against the underside of the roof.
Open the angle finder app and align it with the roof slope.
The app will display the angle. Use an online calculator to convert the angle to a pitch ratio (search for "roof pitch calculator").
3. The Roof Pitch Gauge
Tools: Roof pitch gauge (available at most hardware stores)
Steps:
Climb onto the roof (observe safety precautions!)
Place the gauge against the roof surface.
The gauge will directly indicate the roof pitch ratio.
Examples
Example 1: Using the rise-over-run method, you measure 8 inches vertically at the 12-inch mark on your level. Your roof pitch is 8:12.
Example 2: Your angle finder app shows a 30-degree angle. An online calculator converts this to approximately a 7:12 pitch.
Example 3: Your roof pitch gauge shows a reading of 5:12.
Why Find Your Roof Pitch?
Knowing your roof pitch is essential for:
Estimating roofing materials: Accurate measurements for shingles, underlayment, etc.
Planning additions or alterations: Matching the pitch of new roof sections to the existing roof.
Assessing drainage: Determining if your roof slope is sufficient for proper water runoff.
Evaluating solar panel suitability: Some roof pitches are better suited for solar panel installation.
Safety First!
Always prioritize safety when accessing your roof or attic. Use appropriate safety gear and consider enlisting a helper. If you're uncomfortable with heights or unsure about any part of the process, consult a qualified roofing professional.
