Roofing Calculator

About the Roofing Calculator

Before anyone can price a new roof, they need to know how big it is — and a roof is always bigger than the house underneath. It overhangs the walls at the eaves and gables, and it slopes, so every square foot of floor plan becomes more than a square foot of roof. On a steep roof the difference is large.

Measuring a roof directly means climbing onto it, which is neither safe nor necessary. The area can be worked out on the ground from the footprint of the building, the overhang, and the pitch of the roof. For any roof whose slopes all share one pitch, the result is exact.

This roofing calculator does that calculation and gives the roof area in square feet, square metres and roofing squares — the 100 ft² unit the trade prices in — with a waste allowance set by the roof's shape.

How to Use the Roofing Calculator

Choose imperial (feet) or metric (metres).

Enter the building's length and width, measured at the outside walls.

Enter the overhang at the eaves and gables, on each side.

Enter the pitch, either as a rise in 12 (for example 6 for a "6 in 12" roof) or in degrees.

Choose the roof shape — gable or hip — to set the waste, or enter your own.

How the Roof Area Is Worked Out

  plan area     =  (length + 2 × overhang) × (width + 2 × overhang)
  pitch factor  =  √(1 + (rise ÷ 12)²)
  roof area     =  plan area × pitch factor
  squares       =  roof area ÷ 100 ft²   (9.29 m²)

Step-by-Step Example

A 40 × 30 ft house, 1 ft overhangs, 6 in 12 pitch, gable roof.

  Plan area:     (40 + 2) × (30 + 2)        =  1,344 ft²
  Pitch factor:  √(1 + (6 ÷ 12)²)           =  1.118
  Roof area:     1,344 × 1.118              =  1,503 ft²
  Squares:       1,503 ÷ 100                =  15.03
  With 10% waste:  15.03 × 1.10             =  16.53  →  17 squares

The Pitch Factor

Pitch is how steeply the roof rises: a 6 in 12 roof rises 6 inches for every 12 inches it runs horizontally, about 26.6°. The pitch factor is the length of the slope for each unit of horizontal run — the hypotenuse of that triangle.

A steep 12 in 12 roof has 41% more area than its plan; a shallow 3 in 12 roof only 3% more.

Why Gable and Hip Roofs Have the Same Area

A gable roof has two slopes and two triangular walls at the ends. A hip roof slopes on all four sides. It seems as if the hip roof should have more roof — but if every slope has the same pitch, each part of the roof covers its patch of plan at the same angle, so the total is the plan area times the same pitch factor either way.

What differs is the waste. Along every hip, each course of shingles is cut at an angle, and much of the offcut cannot be used. So the calculator allows about 10% for a gable roof and 15% for a hip roof. Roofs with valleys, dormers, skylights or chimneys need more.

Measuring Pitch From the Ground

If you do not know the pitch, it can often be measured safely from inside the loft: hold a level horizontally against a rafter, measure 12 inches along the level, and measure straight up from that point to the underside of the rafter. That height is the rise in 12. Pitch apps that use a phone's tilt sensor against a gable end are another option. The roof pitch calculator converts between rise, degrees and ratio.

Low-Slope Roofs

Asphalt shingles shed water by overlapping, which only works when the roof is steep enough. Most manufacturers set a minimum of 2 in 12, and require a double layer of underlayment or a full ice-and-water membrane between 2 and 4 in 12. Below 2 in 12, the calculator reminds you that the roof needs a membrane system such as EPDM, TPO or modified bitumen.

Getting Quotes Per Square

Roofers quote by the square because it makes jobs comparable. A quote should state the number of squares, the waste allowance, and what is included — tear-off of the old roof, deck repairs, underlayment, flashing, ventilation and disposal. If two quotes differ wildly in the number of squares, one of them has measured the roof differently, and it is worth asking how. Steeper roofs also cost more per square to fit, because they are slower and need more safety equipment, so a steep-roof surcharge is normal above about 8 in 12.

Understanding Your Result

Squares to buy is the roof area plus waste, rounded up to whole squares.

Roof area gives the sloped area in square feet or metres and in squares.

Pitch shows the rise in 12, the angle and the pitch factor.

Underlayment gives the area of underlayment, with 10% for overlaps.

Worth knowing covers minimum pitches and what a square is.

When Should You Use This Calculator?

Before getting roofing quotes, to check the squares they are pricing.

Planning a reroof or a new build, to order shingles and underlayment.

Comparing pitches when designing an extension or garage.

With the shingle calculator, which turns the area into bundles.

Common Mistakes

Using the floor area. The roof is larger, by the overhang and the pitch.

Forgetting the overhangs. On a small building they add a lot.

Using gable waste on a hip roof. Hips need about 15%.

Guessing the pitch. A 4 in 12 and an 8 in 12 roof differ by 14%.

Ignoring dormers and valleys. Add waste for every complication.