How to Measure Roof Pitch and Slope with Your Phone

Why You'd Need to Measure Roof Pitch

Knowing your roof pitch matters in more situations than most homeowners expect:

• Ordering roofing materials — the steeper the pitch, the more tiles or sheets you need per square metre of floor area
• Checking planning permission — many permitted development rules specify a maximum or minimum pitch for extensions and outbuildings
• Fitting roof windows or skylights — manufacturers specify minimum and maximum pitch ranges for every product
• Installing solar panels — the angle affects optimal orientation and expected output
• Snow load calculations — pitch determines how much snow accumulates
• Checking gutters and drainage — a flat or low-pitch section needs different detailing than a steep pitch

Traditionally you'd measure this with a speed square and a spirit level, or a dedicated pitch gauge. Your phone's inclinometer gives you an equally accurate reading in a fraction of the time.

Understanding Roof Pitch Formats

Roof pitch is expressed in three different ways depending on your country and profession, and you'll often need to convert between them.

Degrees

The simplest format — the angle of the roof surface from horizontal. A flat roof is 0°, a perfectly vertical wall is 90°. Most domestic roofs fall between 15° and 50°. This is what your phone's inclinometer reads directly.

Rise/Run Ratio (x:12 in the US, x/12 pitch)

Common in North America: the number of inches the roof rises for every 12 inches of horizontal run. A 6:12 pitch rises 6 inches for every 12 inches horizontally — equivalent to about 26.6°. Steeper roofs have higher numbers; a 12:12 pitch is 45°.

Percentage Slope

Used in civil engineering, drainage design, and many European building codes. A 100% slope is 45°. A 10% slope is about 5.7°. This is the "Percentage of Inclination" mode in the Bubble Level app.

Conversion Table

Degrees measured by your phone, converted to the other common formats:

Method 1 — Measuring from the Roof Surface (Most Accurate)

This works if you can safely access the roof, loft space, or a rafter from inside.

1. Open Bubble Level and select the inclinometer mode (portrait / vertical mode).
2. Hold the phone flat against the roof surface — on a rafter, a tile, or the underside of the roof deck in the loft. The phone needs to lie parallel to the slope direction, not across it.
3. Read the angle. Switch to "Percentage of Inclination" mode if you need the percentage slope.
4. Take two or three readings on different rafters and average them; slight variations between measurements are normal.

Safety note: only access a roof if you have the correct equipment and experience. For the main roof pitch, measuring from inside the loft against a rafter is both safer and equally accurate.

Method 2 — Measuring from the Ground

You can get a useful approximate reading without going anywhere near the roof, using the phone's camera and a bit of geometry.

1. Stand back from the house so you can clearly see the roof slope in profile (gable end view).
2. Hold your phone up and align its long edge visually with the roofline.
3. Switch Bubble Level to inclinometer mode — the angle displayed is your roof pitch.

This method is less precise than a direct measurement (expect ±2–3°) but is perfectly adequate for estimating material quantities, comparing pitches, or getting a quick answer before deciding whether to investigate further.

Method 3 — From a Ladder at the Eaves

A middle-ground option: climb a ladder to eaves height and hold the phone against the fascia board or directly on a tile surface near the edge. You get a direct-contact reading without needing to go fully onto the roof.

Measuring a Flat Roof Slope

"Flat" roofs are never truly flat — they need a minimum fall for drainage, typically 1:80 (1.25%) or about 0.7° for built-up felt, and 1:40 (2.5%) or about 1.4° for single-ply membranes. Place your phone flat on the roof surface and switch to the percentage inclination mode to check whether the existing fall meets the specification, and in which direction it drains.

On a large flat roof, check multiple points — ponding water is often caused by a section that has no fall or a reverse fall.

Saving Your Reading

The Bubble Level app lets you lock the current reading so you can step off the ladder or roof and read it safely. You can also set a custom reference angle — useful if you need to check whether multiple roof sections are all at the same pitch, or whether a new extension roof matches the existing one.

Quick Summary

• Use inclinometer mode for direct degree readings from any surface
• Use percentage inclination mode for drainage and civil engineering specs
• Measuring from inside the loft on a rafter is the safest and most accurate method
• Ground-based measurement gives ±2–3° accuracy — good enough for estimates
• Use the conversion table above to move between degrees, rise:run, and percentage