Run mode and span mode
Run is the horizontal distance for one sloping side. Span is the full outside-wall distance across a symmetrical gable. Only in that selected symmetric mode does the calculator use half the span as the common-rafter run.
For a 4 m horizontal run at 6/12, the unadjusted sloping line is 4.472135955 m. This is geometry, not a stock cutting length.
Visible adjustments
Ridge thickness, overhang, fascia treatment and seat details must never be hidden assumptions. This calculator keeps a horizontal ridge deduction and horizontal overhang separate before applying the multiplier.
The result is a line-length estimate. It does not account for timber grade, member depth, birdsmouth geometry, plumb cuts or structural capacity.
Check before use
These calculations describe geometry only. They do not size rafters or trusses, assess loads, approve a design, interpret building codes, specify cuts or fasteners, or guarantee material quantities.
Keep a record of the original measurements, unit system, selected mode and any adjustment. Recalculate independently before ordering material or making a construction decision.
Roof work can cause serious injury. Do not climb onto a roof merely to collect inputs. Prefer drawings, ground-level measurements, accessible interior information or dimensions supplied by a competent professional.
Inputs, units and calculation stages
Run mode expects one horizontal wall-to-ridge distance. Symmetrical span mode expects the full outside-wall span and deliberately halves it. Ridge thickness is entered as a horizontal thickness; one half is deducted from each symmetric side. Overhang is also horizontal, not measured along the slope.
Select the mode and length unit, establish the base run, deduct half the ridge thickness, add horizontal overhang, and multiply that adjusted horizontal run by the pitch factor. Each stage remains visible so endpoints can be checked.
Worked examples
A 4.0 m run at 6/12 with no adjustments uses factor 1.118033989 and gives 4.472136 m. Adding a 0.30 m horizontal overhang gives an adjusted run of 4.30 m and a slope line of 4.807546 m.
A 24 ft symmetrical span gives 12 ft base run. With a 1.5 in ridge, the one-side deduction is 0.75 in or 0.0625 ft. At 8/12 and no overhang, the displayed theoretical line is based on 11.9375 ft.
Errors, limits and responsible use
A deduction that leaves zero or negative adjusted run is rejected. The result is a geometric reference line, not stock length. It excludes member depth, bearing details, birdsmouths, plumb cuts, fascia treatment, structural capacity and fabrication tolerance.
Check the source measurements, selected unit, representation and displayed assumptions before consequential use. RoofFigured explains geometry only; it does not provide structural design, code approval, installation instructions or a guaranteed material order.
Frequently asked questions
- Why is only half the ridge thickness deducted?
- In symmetric span mode the entered ridge thickness is shared between the two opposing horizontal runs.
- Is overhang entered along the rafter?
- No. Enter its horizontal projection; the multiplier converts it to added slope line.
- Can span mode be used with an off-centre ridge?
- No. Use separately established one-side runs because the two sides are unequal.
- Does this size a rafter?
- No. Loads, material, grade, spacing and code checks are outside this calculator.
- Can I cut from the result?
- No. It omits assembly-specific cuts and allowances.
Continue the calculation
Span versus run provides the next explanation; Adjustment guide