How to Find Roof Pitch With a Speed Square

What roof pitch means

Roof pitch describes how steeply a roof rises. It is written as a ratio such as 6/12, which means the roof climbs 6 inches for every 12 inches it runs horizontally. The first number is the rise. The second number is always 12 in this system.

A speed square is built around this idea. The scale printed along its long angled edge is labeled Common, and each number on it matches a rise per 12 inches of run. That is why the tool can give you a pitch reading directly, with no math.

What you need

The job takes very little gear, but a few details affect how accurate your reading is.

• A speed square with a pivot point and a Common rafter scale. Plain squares that only mark 90° and 45° lines will not do this job.
• A torpedo level or any small level that sits flat on the square's heel edge.
• A pencil, plus paper or your phone to record the reading.
• A sturdy ladder, and roof-safe footwear if you plan to go on the roof.
• Work gloves if the rafters or shingles are rough.

Pick the right square

Pitch readings depend on a few features, and not every square has them. On our best speed square page, the Swanson S0101 is a 7 in die-cast aluminum square with a pivot point, rafter tables, and scribe notches, so it covers this task. The Swanson 2-pc kit also lists a pivot point, rafter tables, and a 0° to 90° etched scale.

The lightweight 8 in plastic Speedlite in the lineup lists scribe notches but no pivot point or rafter tables. That makes it a better fit for siding and general layout than for reading pitch. Check the feature list before you buy if pitch work is your main goal.

Measure from the roof surface

This method works on a roof that is safe to stand on. Pick a spot on a flat, clean section of the roof, away from vents, valleys, and damaged shingles. Debris under the square will throw off the reading.

Follow these steps:

• Set the square on the roof surface with the pivot point touching the shingles. The long angled edge with the Common scale should face you.
• Place the level on the thick heel edge of the square. This is the edge that will end up horizontal.
• Hold the pivot point in place and swing the square until the level bubble sits dead center.
• Look at the Common scale where the roof surface line meets it. The number there is your rise per 12 inches.
• Check the reading a second time, then repeat at another spot on the same roof plane to confirm.

Measure from the attic

The attic method is safer and often more accurate, because you are working against a straight rafter instead of uneven shingles. Use it whenever the roof is steep, wet, icy, or very high.

Find a rafter with a clean, straight top edge. Set the pivot point of the square against that top edge, put the level on the square's heel edge, and swing the square until the bubble centers. Read the Common scale where the rafter edge crosses it.

Be sure you are measuring the rafter's top edge and not a notched or damaged section. Watch your footing on the joists, and step only on framing, never on bare drywall or insulation between joists. Use a work light and wear a dust mask in dusty attics.

Convert pitch to degrees

Some projects, such as solar mounts or flashing, call for an angle in degrees instead of a ratio. Squares with an etched 0° to 90° scale, like the Swanson kit listed on our page, let you read the angle directly. If yours does not, this table covers common pitches. The values come from basic trigonometry and are rounded.

A pitch of 6/12 is about 26.6°, and 12/12 is exactly 45°. Roofs under about 4/12 are generally considered low slope, which can change which roofing materials are suitable, so check the material maker's minimum before you plan a job.

Common mistakes and safety

The most frequent error is reading the wrong scale. Use the Common scale for pitch, not the Hip/Val scale, which is for hip and valley rafters. Another is letting the pivot point drift while you level the square. Keep that corner anchored and rotate only the body.

Roofs are dangerous. Never climb a wet, icy, mossy, or steep roof, and do not work alone at height. Set the ladder at a stable angle, extend it well above the roof edge, and have someone foot it. For steep or tall roofs, use fall protection or hire a pro. A matte finish, like the one listed on the Swanson S0101, reduces glare on a bright roof deck, which helps you read the scale. If you are comparing squares for roof layout, our speed square comparison lists each model's features side by side.

Frequently asked questions

Do I need a level to find pitch with a speed square?

Yes, in practice. The square only reads correctly when its heel edge is truly horizontal. A small torpedo level on the heel lets you confirm that before you read the scale.

Can I use a speed square without a pivot point?

Not reliably for pitch. The pivot point is the fixed anchor you rotate around. Without it, and without a Common scale, you would need to measure rise and run by hand and calculate the ratio.

Why is pitch always out of 12?

Builders use a 12 inch run as a standard base, so the rise number alone describes the slope. It also matches the 12 inch-based Common scale printed on a speed square.

How accurate is a speed square reading?

It is good enough for planning, ordering, and rough layout. Small errors come from uneven shingles or a drifting pivot. Take two or three readings in different spots and use the one that repeats.