How to Measure Pitch with a Level and a Tape
Why this matters
Pitch drives the material count, the labor, the safety plan and, near the low end, which roofing products and underlayment the code and the manufacturer allow. Plans are often missing or wrong, and an estimate built on a pitch guessed from the street can be a full step off. A tape and a level get the pitch to within a fraction of a step in about a minute, if the reading is taken in the right place and the right way.
Pitch is written as rise in 12: the inches the roof climbs for every 12 in of level run. A 5/12 roof rises 5 in per foot. The measurement on this card is simply that definition done with tools: hold 12 in of level run against the roof and measure the rise.
Before anyone goes up: read it from the attic or the gable end
The safest reading is one that never puts anybody on the roof. Three places give a true pitch without walking the surface:
- A rafter in the attic. The underside of a rafter or the face of a truss top chord is at the roof's pitch, and it is smooth lumber, which reads better than shingles.
- The gable end or a rake board, from a ladder set and tied off at the gable, without stepping onto the roof.
- The edge of the roof at the eave, from a ladder, with the level resting on the first few feet of surface.
In the attic, walk only on framing or a board laid across it, never on drywall or insulation between joists, which will not hold a person. Disturbing old loose-fill insulation raises dust; if it is gray-brown and pebbly, it may be vermiculite, which can contain asbestos, so leave it undisturbed and take the reading from a rafter you can reach without moving it.
If the roof surface itself has to be measured, that is work at height. Construction work at 6 ft or more falls under 29 CFR 1926.501 fall protection, and your employer's program sets how it is met. Nobody walks a wet, frosted or mossy roof to take a reading that the attic can give.
The setup
PITCH READING, LEVEL HELD LEVEL ON THE ROOF
uphill end of level resting on the roof
|
v 12 in mark 24 in mark
o==========|===================|==== <- 24 in level,
\ | | bubble level
\ | rise at 12 in |
\ | (5 in on 5/12) | rise at 24 in
\ | | (10 in on 5/12)
\ | |
x |
roof \ |
surface \ |
\ |
\ |
\ |
\ |
\ |
\ |
\ |
x
measure plumb (straight down) from the level's
bottom edge to the roof, at the mark
The level's uphill end touches the roof. The level is brought to level with the bubble. At a mark a known level distance from the touching end, the rise is measured plumb down from the bottom edge of the level to the roof. Rise divided by that distance, times 12, is the pitch.
The steps
Pick a clean, straight stretch of surface. On a rafter, pick a section between any blocking or hangers, away from a splice. On the roof, avoid ridge caps, vents and a course where the shingles are lifted. Why: the reading assumes the surface under the level is a straight line; a bump under the touching end tilts the whole reading.
Set the uphill end on the surface and bring the level to level. Hold the level square across the slope, pointing straight up and down the roof, not at an angle across it. What breaks if you skip it: a level skewed across the slope measures a shorter rise over the same marks, and the pitch reads low.
Measure plumb from the bottom edge of the level at the mark. Keep the tape vertical; a tape leaning downhill reads long. Read to the nearest 1/16 in.
Use the 24 in mark if the level is long enough, and halve the reading. The rise at 24 in is twice the rise at 12 in on the same roof, but the error in placing the end and reading the tape is about the same size either way. Halving the 24 in reading halves that error. A 1/4 in error at 12 in is a quarter of a pitch step; at 24 in it becomes 1/8 in after halving, an eighth of a step.
Take a second reading at another spot and flip the level end for end once. Turning the level around catches a level that is out of true: if the two readings differ, average them and get the level checked.
Convert and record. Pitch = rise per 12. From pitch, rise over run is pitch / 12, the multiplier is the square root of 1 + (rise / 12) squared, and the angle is the arctangent of rise over run. See related: Roof Pitch Multipliers for Any Sloped Surface, which owns the multiplier table, and Slope Four Ways: Percent, Ratio, Inches Per Foot and Degrees.
Worked example: an attic reading against a roof-surface reading
A roofer is estimating a reroof with no plans. The first reading is taken at the eave from a ladder, with a 12 in mark on the shingles: 5-1/4 in. That reads as 5.25/12.
Before writing it down, the roofer reads a rafter from the attic hatch with a 24 in level: 10-1/8 in at the 24 in mark.
- Halve it: 10.125 / 2 = 5.06 in per 12.
- Flip the level end for end and reread: 10 in, so 5.00 per 12.
- Average of the two: (5.06 + 5.00) / 2 = 5.03 per 12.
The rafter says 5/12. The shingle reading was 1/4 in high at 12 in, and the reason is the surface: shingle courses step at every butt edge, so a level resting on one course and measured down to the next can catch a step of a shingle thickness or more. The rafter is a straight piece of lumber with no steps. Built roofs are framed to whole or half pitches, so a reading of 5.03 is a 5/12 roof read with about 1/32 in of slop, and 5/12 goes on the estimate.
Convert the 5/12:
- Rise over run: 5 / 12 = 0.4167.
- Multiplier: 0.4167 squared is 0.1736, plus 1 is 1.1736, and the square root is 1.0833.
- Angle: arctan(0.4167) = 22.62 degrees. Check backward: tan(22.62) = 0.4167, which is 5/12.
Does the 1/4 in matter here? On the area takeoff, barely. Had the estimate used 5.25/12, the multiplier would be the square root of 1 + 0.1914, which is 1.0915. Against 1.0833 that is 1.0915 / 1.0833 = 1.0076, so 0.76 percent more roof. On a house with 1,800 sq ft of plan area, 1,800 x 1.0833 = 1,950 sq ft against 1,800 x 1.0915 = 1,965 sq ft, a difference of 15 sq ft, which the waste factor swallows. See related: The Materials Waste Factor Worth Measuring.
Where a quarter inch does change the call
The same reading error matters a great deal when the pitch sits near a threshold. The common one for asphalt shingles: the IRC, as adopted by your jurisdiction, permits asphalt shingles only on slopes of 2/12 or greater and requires a double layer of underlayment, applied the way the code describes, on slopes from 2/12 up to less than 4/12 (IRC R905.2.2 with R905.1.1 in the 2021 edition; check the edition your jurisdiction has adopted). The shingle manufacturer's installation instructions may set their own low-slope requirements, and those apply on top of the code.
Take a porch roof read at the eave with a 12 in mark: 4-1/8 in, so 4.125/12. On that reading the porch clears 4/12 and gets standard underlayment. Then a 24 in reading off the porch rafter gives 7-3/4 in, halved to 3.875/12, and a flipped second reading gives 7-5/8 in, halved to 3.81. Average: (3.875 + 3.81) / 2 = 3.84/12.
That porch is under 4/12. The surface reading was high by about 0.28 in at 12 in, the same direction and about the same size as the error on the main roof, and here it flips which underlayment the job needs. When a reading lands within half a step of a threshold, always confirm it off framing with the longer level before quoting the material.
Other tools, and what each needs
- A digital level or angle finder reads degrees directly. Convert with pitch = tan(degrees) x 12. A reading of 22.6 degrees gives 0.4163 x 12 = 5.0 per 12. Calibrate it on a flat surface and check it by reversal the same way as a bubble level.
- A phone with an inclinometer works on the same principle and is only as good as the flatness of the phone's case against the surface. A case with a raised camera bump rocks; lay the phone on the level instead of on the roof.
- A framing square held with the blade level, the blade's 12 in mark resting on the roof uphill and the tongue hanging plumb from the heel, gives the rise directly where the tongue meets the surface.
- A pitch gauge with a built-in bubble reads pitch directly and is subject to the same surface-step error as a 12 in level on shingles.
The slips that pass a quick look
Measuring along the roof instead of level. Laying a 12 in ruler on the slope and measuring up from it reads a rise per foot of slope, not per foot of run. On a 5/12 roof that reads 12 x sin(22.62) = 4.62 in instead of 5, and the roof reads as about 4.6/12.
The touching end on the downhill side. If the level's low end rests on the roof and its high end floats in the air, the measurement has to be taken up from the roof, which is awkward and easy to read wrong. Put the touching end uphill and measure down.
A sagged or bowed rafter. An old rafter can carry a sag along its length. Read it near the plate and near the ridge; if the two readings disagree, the roof is not a straight plane, and the estimate should note it, because a sagged roof is also a structural observation worth passing to the owner.
One reading. A single number carries no check. Two spots and one flip cost thirty seconds and catch a bad level, a bump and a misread tape.
References
- See related: Roof Pitch Multipliers for Any Sloped Surface
- See related: Roof Slope, Pitch, + Measurement Reference
- See related: Slope Four Ways: Percent, Ratio, Inches Per Foot and Degrees
- International Residential Code, Sections R905.1.1 and R905.2.2, in the edition your jurisdiction has adopted, and the roofing manufacturer's installation instructions
- 29 CFR 1926.501, duty to have fall protection, for construction work