The symmetric gable case
A 10 m outside-wall span with a centred ridge gives a 5 m run on each side, before explicit horizontal adjustments. Apply the pitch multiplier to 5 m, not 10 m, for one common-rafter line.
If the ridge is not centred, the two runs differ. If the roof is a shed or mono-pitch form, the single run can equal the full supported span. Select the correct mode rather than assuming.
Check the labels
Confirm the measurement endpoints and whether overhang is included. The phrase roof width can refer to several different lines.
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
For a centred symmetrical gable, full outside-wall span is divided by two to obtain each wall-to-ridge run. A 10 m span therefore produces two 5 m runs before explicit ridge or eaves adjustments.
An off-centre ridge creates unequal runs. A 10 m span might divide into 4 m and 6 m; each side then has a different rise unless pitches also differ. The symmetric span shortcut is inappropriate.
A mono-pitch roof can use the full supported width as its one horizontal run. This is why a field labelled roof width is insufficient: the form and endpoints determine its meaning.
Ridge thickness is not automatically part of span. State whether measurements end at ridge centreline, ridge face or another reference, and apply only the horizontal deduction supported by that convention.
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.
Identify the three lines
Span is the full horizontal distance between the selected supports. Run is the horizontal distance for one roof plane. Slope length follows that plane. On a centred symmetric gable, run is half span; that shortcut is a consequence of symmetry, not a general definition.
Treat asymmetric roofs explicitly
If a 10 m span has a ridge 4 m from one support, the runs are 4 m and 6 m. Equal pitches would then produce unequal ridge heights, so the actual geometry must supply either different pitches or another reference. Calculate each side separately and keep their labels tied to the drawing.
Set endpoints before adjustments
State whether span is measured outside-to-outside, bearing-to-bearing or to another line. Establish the base run first; then apply a supported ridge-face deduction and horizontal overhang. Never halve an already one-sided run, and never subtract an along-slope dimension from a horizontal line.
Reconcile the answer
For a symmetric roof, two base runs should add back to the entered span before ridge or eaves changes. For an asymmetric roof, the separately established runs should reconcile with the same supported width. This arithmetic check catches many mode-selection errors before any trigonometry is applied.
Locate span and run on the same section
Span is a full horizontal width between defined supports or wall references; run is the horizontal distance associated with one roof slope. On a simple centred, equal-pitch gable, the geometric run for either side is half the selected span. That shortcut depends on symmetry. A 10 m span produces two 5 m runs only when the ridge is centred and both roof planes terminate at the same reference lines. Label whether the span is measured between outside wall faces, centres, structural supports or eaves, because changing the reference changes the line being calculated.
A mono-pitch roof normally uses the full horizontal width as its run, not half. An asymmetric gable has two different runs that add to the supported width. Intersections, stepped walls and offset ridges require separately established geometry. Before pressing calculate, sketch a horizontal line for the selected run and mark both endpoints. If those endpoints cannot be identified on the drawing or survey, the number is not yet sufficiently defined for an auditable rafter or area calculation.
Keep ridge and eaves adjustments outside the base span
A building span commonly stops at a wall reference, while a desired slope line may stop at a ridge face and extend to an eaves endpoint. Those are different boundaries. Begin with the documented span or run, then apply only adjustments whose direction and meaning are known. In symmetric span mode, RoofFigured halves a stated horizontal ridge thickness across the two sides and adds a stated horizontal overhang before applying the pitch multiplier. It does not interpret member depth, plumb cuts, fascia or bearing details.
Avoid double adjustment. If a drawing dimension already runs from the near ridge face to the eaves edge, do not subtract ridge thickness or add overhang again. Conversely, a wall-to-wall span may omit both. Record a short endpoint note such as “outside wall face to near ridge face, plus 300 mm horizontal eaves projection.” This is more useful than a bare number because another reviewer can reconstruct why the adjusted run differs from half the building width.
Use reconciliation as an error detector
For a centred gable, double the unadjusted base run and compare it with the stated span. For an asymmetric roof, add the two separately established base runs. The result should reconcile with the same horizontal reference width before ridge or eaves changes. If it does not, investigate the ridge position, wall offsets, units and endpoint definitions. This check is independent of pitch and often catches a wrong mode selection before trigonometry turns it into a precise-looking slope length.
Span and run describe geometry, not structural capacity. Halving a span does not determine a compliant rafter length, because real members depend on support conditions, loads, species, grade, section, restraint and connection design. Use RoofFigured to document the ideal right-triangle line only. A competent designer or fabricator must establish the physical member, cuts and allowances from the governing project information.
Worked boundary comparison
Consider a symmetric building with an 8.000 m outside-wall span, a centred ridge 38 mm thick horizontally and a 300 mm horizontal eaves projection on each side. Half the wall span is 4.000 m. If the required ideal line stops at the near ridge face, subtract 0.019 m; if it reaches the eaves edge, add 0.300 m. The adjusted run is therefore 4.281 m. At 6/12, multiplying by 1.118033989 gives an ideal line of about 4.786 m. Each figure can be traced to one boundary decision.
If an eaves-to-eaves plan instead reports 8.600 m, halving it already includes the 300 mm projections. Adding them again would incorrectly produce a 4.600 m run before ridge treatment. Likewise, a drawing dimension to the ridge face may already include the 19 mm deduction. This example shows why calculator fields cannot decide the correct adjustment from a roof label. The user must identify the source endpoints, and the record must retain them so a later reviewer can reproduce or challenge the choice.
Final verification before use
A final check should be drawable: show the full selected span, the ridge position and the one-side run on a horizontal line. Confirm that two symmetric base runs equal the span before adjustments. Then show ridge and eaves changes separately. If the drawing cannot support that sequence, span mode is not justified and the individual runs must be established another way.
Test the selected mode with a simple example. A 9 m centred span gives 4.5 m base runs; the same 9 m width on a mono-pitch roof normally gives one 9 m run. At 6/12 those choices produce ideal lines of about 5.031 m and 10.062 m respectively. The twofold difference is not a rounding issue—it reflects a different roof model. Confirm the roof form before accepting whichever number looks familiar.