Horizontal adjustments first
A horizontal overhang extends the run and gains sloping length through the same multiplier. A centred ridge-board allowance may reduce the run used to the selected line, but only when the measurement convention requires it.
For example, 4 m run plus 0.3 m horizontal overhang at 6/12 gives 4.3 × 1.118033989, or about 4.8075 m before any separate ridge deduction.
No cutting inference
The calculator does not determine plumb cuts, birdsmouth dimensions, fascia geometry or stock length. Those depend on the actual assembly and competent design.
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.
Detailed field guide
Horizontal overhang is added to horizontal run before applying the factor. A 0.30 m horizontal extension at 6/12 adds 0.335410 m of slope line, not 0.30 m.
A 38 mm ridge entered as horizontal thickness reduces each symmetric run by 19 mm when the intended endpoint is the near ridge face. Different detailing may require a different convention, which must be stated.
Do not mix horizontal and sloping adjustments. If a source supplies an along-slope tail dimension, it cannot be entered in the horizontal-overhang field without conversion.
These controls still do not calculate fascia, birdsmouth, plumb cut, member depth or stock allowance. They only alter the ideal horizontal endpoint used by the triangle.
Checks before using the result
- Preserve the original measurement, unit, endpoints and drawing revision.
- Keep different pitches and unsupported intersections separate.
- Compare one representative result with an independent calculation.
- Do not access a roof merely to obtain calculator inputs.
Name the direction of every adjustment
An overhang may be reported horizontally or along the roof plane. RoofFigured’s rafter field expects a horizontal projection. At 6/12, 0.30 m horizontal adds about 0.335 m of slope line; entering 0.30 m already measured along slope would overstate the endpoint.
Use a consistent ridge reference
A centred ridge thickness of 38 mm creates a 19 mm horizontal deduction on each symmetric side when the target ends at the near ridge face. If the source run already ends at that face, entering the deduction again would shorten the line twice.
Distinguish eaves from plan growth
For rectangular area calculations, a uniform horizontal overhang expands both ends of each plan dimension. A 0.3 m overhang turns 10×8 m into 10.6×8.6 m. Where only selected edges project, calculate the actual eaves boundary rather than using the uniform control.
Retain an endpoint note
A useful record says “outside wall to near ridge face plus horizontal eaves projection,” not merely “run.” That note makes later checking possible and prevents a value derived for one detail from being reused for another assembly with different endpoints.
State the direction and reference for every adjustment
An overhang may be specified horizontally, along the roof slope or by the position of a fascia or eaves edge. These are not interchangeable. RoofFigured’s rafter adjustment expects a horizontal projection because it modifies the right triangle’s run before the slope factor is applied. At 6/12, a 0.300 m horizontal extension adds about 0.335 m to the ideal slope line. Entering 0.300 m that was already measured along the slope as though it were horizontal would extend the result again.
Draw a small dimension arrow and label its direction whenever the source wording is ambiguous. Record the starting and ending references—wall face, support centre, ridge centreline, near ridge face, fascia or eaves edge. A number without endpoints cannot be audited. If the project drawing uses a different convention from the calculator field, transform it explicitly and retain both the source value and the derived horizontal adjustment.
Handle ridge references without double deduction
A centred ridge member with a stated horizontal thickness may create a half-thickness deduction from each symmetric run when the target line stops at the near ridge face. For a 38 mm thickness, that deduction is 19 mm per side. But drawings often dimension directly to a ridge face or use a centreline for a different purpose. If the source run already ends at the desired face, entering another deduction shortens the line twice.
Keep the base run, ridge adjustment, eaves adjustment and final adjusted run as separate record fields. Apply the pitch multiplier after the horizontal adjustments, not separately to a mixture of horizontal and slope lengths. This sequence produces an ideal line between the documented endpoints; it still does not determine a plumb cut, birdsmouth, bearing or physical stock length.
Expand plan boundaries edge by edge
For area work, a uniform horizontal overhang changes each affected plan dimension at both ends. A 10 m by 8 m wall-line rectangle with 0.3 m projection on all four sides becomes 10.6 m by 8.6 m at the eaves. If only selected edges project, use the actual expanded boundary rather than a uniform control. On multi-plane roofs, assign each added strip or corner to one plane so adjacent sections do not overlap.
Do not assume gutters, fascia faces, parapets or soffits form part of the sloping roof plane. Measure and estimate those components under their own documented method. The roof-area calculator and workbench describe planar geometry only. When an endpoint affects fabrication, weathering or structural design, the governing detail and competent professional remain authoritative.
Worked adjustment ledger
For a 3.500 m base run at 7/12, suppose the target line stops 0.020 m before the ridge centreline and extends 0.250 m horizontally beyond the wall. Record base run 3.500, ridge deduction −0.020 and eaves addition +0.250, producing adjusted run 3.730 m. The factor for 7/12 is about 1.157704, so the ideal adjusted line is about 4.318 m. The ledger shows exactly which assumption changes if the ridge or eaves detail is revised.
Do not replace the ledger with “add 230 mm to the rafter.” The net horizontal change is 230 mm, while the corresponding change along the slope is larger. Nor should this ideal line be treated as stock or cut length: bearing, plumb cuts, fascia and fabrication allowances remain outside the formula. Where a drawing supplies a direct slope dimension to the desired endpoints, retain it as independent evidence and compare it with the calculated line before deciding which project record controls.
Final verification before use
Review adjustments by reading the ledger aloud: source boundary, direction, sign, value and resulting endpoint. Confirm that horizontal values are applied before the factor and that a direct slope measurement has not been entered as horizontal. Compare the adjusted line with an independent drawing dimension where available. Preserve disagreement for review rather than adding another allowance until the figures appear to match.
For area, mark which plan edges receive an overhang and expand those edges geometrically before applying pitch. A projection on one side adds one strip; projections on both ends change the full dimension twice. At corners, avoid counting the same added rectangle in two strips. A quick plan sketch with the expanded outline is usually clearer than a percentage allowance and preserves the relationship between the source footprint and the eaves boundary.
Keep ridge and eaves adjustments out of material waste. They change the geometric boundary, while waste describes product-specific effects after area is known. Combining them prevents either stage from being checked independently. The final record should show the unadjusted geometry, explicit boundary adjustments, ideal result and later coverage assumptions as distinct quantities.