Roof Span Calculator

Calculate the horizontal span of your roof from rafter length, pitch, rise, or run measurements for framing, trusses, and structural planning

Calculate horizontal span from rafter length and roof pitch

Quick presets

ft

Total Roof Area

0 sq ft

0.0 roofing squares • Pitch multiplier: 1.054

PRO

Professional Calculator

Calculate total area for complex roofs with multiple sections

ft
ft

Total Roof Area

1,389 sq ft

Sections

2

Total Squares

13.9

Cost Breakdown

Section 1: $1,054 (76%)
Section 2: $335 (24%)

Detailed Breakdown

Section 11,054 sq ft
Dimensions40 × 25 ft (footprint: 1,000 sq ft)
Pitch4/12 (×1.054)
Section 2335 sq ft
Dimensions20 × 15 ft (footprint: 300 sq ft)
Pitch6/12 (×1.118)
Total Roof Area1,389 sq ft
Total Squares13.9
Bundles (10% waste)46
How to Use This Calculator
Start on the From Rafter tab if you know your rafter length (measured along the slope from ridge to wall plate, not including the tail/overhang). Enter the rafter length in feet, select the roof pitch, and choose whether the roof is a gable (two-sided) or shed (single-slope). The calculator instantly returns the horizontal run and total span.

Switch to the From Rise & Run tab if you have measured the vertical rise (ridge height above the wall plate) and horizontal run directly. This is common when measuring from inside the attic. The calculator computes the span and also back-calculates the pitch and rafter length for your reference.

Use the Building Width tab when you need to know the total roofline width including overhangs — important for lot setback calculations, gutter and fascia ordering, and determining whether the roof will clear adjacent structures. Enter rafter length, pitch, and overhang distance on each side.

The Formula
Pitch Angle = arctan(Rise / 12) where pitch is expressed as Rise/12 Run from Rafter = Rafter Length x cos(Pitch Angle) = Rafter Length / Pitch Factor Pitch Factors: 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414 Span (Gable) = 2 x Run Span (Shed) = 1 x Run Total Building Width = Span + (2 x Overhang) for gable, Span + Overhang(s) for shed Rafter Length from Rise & Run = sqrt(Rise² + Run²)
Example Calculation
For a gable roof with 16-foot rafters at 6/12 pitch and 18-inch overhangs:
• Pitch factor (6/12): 1.118
• Run: 16 / 1.118 = 14.31 ft
• Span: 2 x 14.31 = 28.62 ft
• Overhang each side: 1.5 ft horizontal
• Total building width at roofline: 28.62 + (2 x 1.5) = 31.62 ft
• Rise: 14.31 x (6/12) = 7.16 ft above wall plate

Frequently Asked Questions

What is roof span and how is it different from run?
Roof span is the total horizontal distance from the outside of one exterior wall to the outside of the opposite exterior wall — the full width that the roof covers. Roof run is half of the span on a symmetrical gable roof, measured from the outside of one wall to the center of the ridge. For example, a house with a 30-foot span has a 15-foot run on each side. On a shed (single-slope) roof, the span and run are the same distance because there is only one slope.
How do I calculate span from rafter length and pitch?
Use the formula: Run = Rafter Length x cos(pitch angle), then Span = 2 x Run for a gable roof. For a 4/12 pitch, the angle is 18.43 degrees, so cos(18.43) = 0.9487. A 15-foot rafter gives: Run = 15 x 0.9487 = 14.23 ft, Span = 2 x 14.23 = 28.46 ft. You can also use the pitch factor directly: Run = Rafter Length / Pitch Factor, where the pitch factor for 4/12 is 1.054.
What is the maximum roof span without intermediate support?
Maximum unsupported span depends on the truss or rafter design, lumber size, species, and local building codes. Common limits for standard dimensional lumber rafters are: 2x6 rafters span up to 10-14 ft of run, 2x8 up to 14-18 ft, 2x10 up to 18-22 ft, and 2x12 up to 22-26 ft. Engineered trusses can span 40-60+ ft without interior bearing walls. Always consult a structural engineer or truss manufacturer for spans exceeding 30 ft or unusual load conditions.
How does overhang affect total building width?
Overhang extends the roofline beyond the exterior walls but does not change the structural span. Total building width at the roofline = Span + (2 x Overhang) for a gable roof, or Span + Overhang for a shed roof with overhang on only one side. A 30-foot span house with 18-inch overhangs on each side has a total roofline width of 33 feet. Overhang matters for lot setback calculations, gutter placement, and fascia board ordering.
Why do I need to know my roof span?
Roof span is critical for: (1) ordering pre-fabricated trusses — the span is the primary specification, (2) sizing ridge beams and headers on vaulted ceilings, (3) determining whether interior load-bearing walls are needed, (4) calculating rafter sizes per building code span tables, and (5) verifying that existing framing can support a heavier roofing material like tile or slate. Accurate span measurement prevents costly errors in truss fabrication and structural engineering.

Related Calculators

Roof Run Calculator

Calculate horizontal run from pitch and rise

Roof Pitch Calculator

Determine pitch from rise and run measurements

Rafter Length Calculator

Calculate rafter length from span and pitch

Roof Truss Calculator

Estimate truss count, spacing, and cost

Roof Area Calculator

Calculate total roof surface area from span and pitch

Related Guides & Resources

How to Measure a Roof Understanding Roof Pitch Roof Pitch Chart