Calculators
Roof pitch, area and rafter calculators
Pitch in any unit, true roof area from a footprint, rafter and hip lengths, truss counts, ridge height and ladder length.
A 2,000 square foot house does not have a 2,000 square foot roof. The roof overhangs the walls, and every plane of it is tilted, so a footprint of 2,000 square feet under a 6/12 pitch is about 2,240 square feet of roofing before overhangs and 2,500 after them. All of these calculators rest on the same piece of trigonometry: the pitch factor, the ratio of the sloped length to the horizontal run, which is √(1 + (rise ÷ 12)²).
These tools measure and count; they do not certify. Rafter lengths, hip lengths and truss counts come out of geometry, but whether a given board can span a given distance is a question for the span tables in the building code, the truss manufacturer or an engineer. Each page says so, and none of them asks you to climb onto a roof to get a number.
9 calculators
Roof Pitch Calculator
Pitch from rise and run, degrees or percent, with the pitch factor, hip factor and what the slope means for roofing.
Roof Area Calculator
True roof surface area and squares from the building footprint, overhangs and pitch, with dormers and suggested waste.
Roof Pitch Factor Calculator
The pitch factor and hip factor for any pitch, with a table of the common pitches and the area or rafter length they give.
Rafter Length Calculator
Common rafter length from span or run and pitch, with ridge shortening, overhang, cut angles, birdsmouth and the board to buy.
Hip & Valley Rafter Calculator
Hip or valley rafter length, run, plumb and cheek cuts, drop and the jack rafter difference for a regular hip.
Truss Count Calculator
How many trusses a building needs at 12, 16, 19.2 or 24 in spacing, with gable-end trusses, lateral bracing and ties.
Gable Area Calculator
The triangle area of a gable end from its width and pitch or peak height, for siding and sheathing, with rake trim length.
Ridge Height Calculator
Rise and ridge height above the plate and above the floor or grade from span, pitch, wall height and rafter HAP, plus the fascia height.
Ladder Length Calculator
The extension ladder size for an eave height using the 4:1 angle rule and the 3 ft extension, with the base distance from the wall.
Guides for this topic
Bundles per square and the waste nobody budgets for
The bundle count is easy; the waste is where orders come up short. Here is where every percent of waste goes and how to set it for your roof.
Cedar shakes and shingles: exposure, bundles and lifespan
A bundle of cedar does not cover a fixed area: it covers more at a longer exposure. The Bureau's rule makes the count simple once that is clear.
Flat roof membranes: EPDM, TPO and PVC quantities
A flat roof is a membrane, a slope to the drains and as few seams as possible. Here is how to count each one.
How to measure a roof from the ground (no ladder needed)
Everything a roofing order needs can be measured standing on the lawn. Here is the method, the one calculation that matters, and the checks that catch the usual mistakes.
Ice and water shield: where the code wants it and how much
One sentence of code decides how much membrane a roof needs, and the pitch decides how it reads on the deck. Here is the sentence and the arithmetic.
Ladders and roof safety: the 4:1 rule and when not to go up
Falls from roofs and ladders are among the leading causes of construction deaths. Most of the rules that prevent them are simple; the first one is knowing when not to climb.
Rafter maths: run, rise, line length and the birdsmouth
A rafter is two right triangles and a notch. Once the triangles are clear, every cut follows, and the only judgement left is structural.
Why your roof is bigger than your house
A 2,000 square foot house rarely has a 2,000 square foot roof. Three things make the difference, and each one is easy to measure.
Roof pitch explained: ratios, degrees and the pitch factor
Pitch is written four ways and used in almost every roofing calculation. One right triangle explains all of it.
Is your roof solar-ready? Area, pitch, setbacks and panels
Solar panels last 25 years or more. Before they go up, the roof under them needs to last as long, have room after the setbacks and face the right way.
Underlayment: felt vs synthetic, coverage and overlaps
The layer under the shingles is the roof's second line of defence. Choosing it is easy; counting it right means counting the laps.
Questions people ask
- What is a 6/12 pitch in degrees?
- A 6/12 pitch rises 6 inches for every 12 inches of run, which is an angle of 26.6 degrees and a slope of 50 percent. Its pitch factor is 1.118, so every foot of horizontal run is 1.118 feet of rafter. A 4/12 roof is 18.4 degrees (factor 1.054), an 8/12 is 33.7 degrees (1.202) and a 12/12 is 45 degrees (1.414).
- How do I work out roof area from the footprint?
- Measure the length and width of the building at the eaves, including the overhangs, multiply them for the footprint, then multiply by the pitch factor for your pitch. A 50 by 32 foot footprint (1,600 square feet) at 6/12 is 1,600 × 1.118 = 1,789 square feet, or about 18 squares. The roof area calculator does this and adds dormers and complexity.
- How long is a rafter for a 12-foot run at 6/12?
- The line length is the run times the pitch factor: 12 × 1.118 = 13.42 feet, or 13 feet 5 inches from the ridge to the outside of the wall. Add the overhang (also multiplied by the factor) and subtract half the ridge board thickness, then buy the next stock length up, here a 16-foot board for a 12-foot run with a 1-foot overhang.
- How many trusses does a 40-foot building need?
- At the usual 24-inch spacing, divide the length by 2 feet and add one for the first truss: 40 ÷ 2 + 1 = 21 trusses, of which the two at the ends are gable trusses. At 16-inch spacing the same building needs 31. The truss count calculator also estimates the lateral bracing.
- What ladder do I need for a 20-foot eave?
- OSHA asks for the ladder base to sit one foot out for every four feet of height and for the ladder to extend 3 feet above the landing. A 20-foot eave needs a working length of about 20.6 feet plus 3 feet, so 23.6 feet of ladder in use. Extension ladders lose 3 to 4 feet to the overlap of their sections, so that means a 28-foot ladder, not a 24.
More calculators on our other sites
The Woven Garden
Garden and yard calculators for mulch, soil, gravel, sod, pavers, fences and decks.