Prefer existing information
Check approved drawings, survey records, truss documentation or dimensions supplied by the contractor. Confirm whether a stated width is total span, horizontal run or slope length before using it.
An accessible attic may expose useful geometry without roof access, but only use areas that are safe and intended for access. Do not improvise work-at-height methods.
Official safety boundary
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.
For US guidance consult OSHA Residential Fall Protection. For UK guidance consult HSE Roof work and HSG33. RoofFigured links to those sources rather than paraphrasing operational instructions.
Detailed field guide
Prefer approved drawings, survey records, truss documentation or dimensions supplied by a competent person. Confirm scale, revision, units and whether dimensions run to walls, eaves, ridge centreline or another endpoint.
A safely accessible interior can reveal a right triangle only where access is intended and the surfaces represent the required roof plane. Ground-based surveying may provide dimensions without roof access, but instrument setup, sight lines and uncertainty must be documented.
Take repeated readings where practical and compare them. Small errors in a short run can create a large pitch error. Do not add decimal places beyond the accuracy of the source.
This guide deliberately contains no ladder, edge-protection or roof-access procedure. Consult current OSHA residential fall-protection guidance or HSE roof-work guidance and use competent planning for any work at height.
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.
Use records before physical access
Begin with the latest approved drawing, survey, truss schedule or dimension set. Confirm its revision, scale, unit and reference points. A dimension to the ridge centreline is not interchangeable with one to a ridge face, and a wall-line width is not automatically the eaves width.
Create a measurement record
For every input, write down the source, date, plane name, two endpoints, unit and estimated uncertainty. Repeat independent readings where practical. If repeated values disagree beyond the instrument or drawing tolerance, stop and resolve the source rather than averaging incompatible geometry into a deceptively precise pitch.
Prefer non-roof methods
Ground-based survey equipment, supplied plans and safely accessible internal geometry can avoid roof access. Each method still has limitations: perspective can distort photographs, timber faces may not represent the finished plane, and a drawing scale can be invalid after resizing. Document the method rather than treating a number as self-proving.
Know the safety boundary
RoofFigured does not provide a ladder, fall-protection or access procedure. Do not step onto a roof to obtain a calculator input. Work at height requires competent planning and current local controls; consult the linked HSE or OSHA guidance and the responsible professional for the actual site.
Use existing information before taking a new measurement
The safest useful pitch measurement is often one that already exists. Begin with construction drawings, survey information, a truss profile, an as-built record or dimensions supplied by a competent contractor. Check the document revision and confirm that the stated roof plane matches the part of the building being assessed. Older properties and later additions may not match the original drawing. Record the source, date and plane label alongside the number so the calculation can be repeated without someone returning to the building merely to rediscover where the value came from.
If drawings provide rise and horizontal run, those values can be entered directly once their units and endpoints are understood. If only an angle is shown, convert it but retain the original angle in the job note. Photographs can help identify which plane a record refers to, but perspective makes them unreliable as the sole basis for a precise pitch. Satellite or mapping imagery may help with orientation, yet it should not be treated as measured roof geometry unless the provider states an appropriate method and accuracy for that use.
Prefer accessible interior and ground-based evidence
Where a new measurement is necessary, use a location that does not require roof access and is already safe and authorised. An accessible interior profile may expose a rafter or roof line from which rise and horizontal run can be measured, provided finishes, ceiling slopes and structural members are not mistaken for the external roof plane. A ground-based survey instrument or competent measured survey may provide better evidence for larger or consequential work. The method, instrument and measurement uncertainty belong in the record just as much as the final ratio.
Do not improvise access, lean from openings, climb ladders or step onto a roof to obtain calculator inputs. Roof work has serious fall risks, and a geometry website cannot assess access conditions, fragile surfaces, weather, edge protection or competence. In the United States consult current OSHA material; in the United Kingdom consult current HSE roof-work guidance. Those authorities address work controls. RoofFigured intentionally provides no ladder, fall-protection or positioning procedure and should never be interpreted as permission to proceed.
Check the measurement before calculating
Confirm that rise is vertical and run is horizontal. Measuring two convenient sloping or diagonal points does not create a valid rise/run pair. Use the same unit for both values and take enough distance to reduce the effect of small reading errors. For example, an error of 3 mm is proportionally much larger over a 100 mm run than over a 1,000 mm run. Repeat the observation where practical and investigate inconsistent results rather than averaging measurements that may belong to different planes.
After calculation, perform a reasonableness check against the building profile and any independent source. A result of 6/12 should look and behave like a ratio of 0.5, about 26.57 degrees and multiplier 1.1180. If one representation is surprising, revisit the endpoints, units and selected input mode. Keep the measured values separate from rounded display results. The calculation can be exact for the supplied numbers while the supplied numbers themselves remain uncertain, so the record should never imply greater field accuracy than the source supports.
Create a measurement note someone else can repeat
A useful note records the property or drawing reference, roof plane, observation location, date, instrument or document used, units, endpoints and the person or organisation that supplied the information. It should also state whether the value was measured directly, converted from an angle or inferred from a documented rise and run. Add a brief uncertainty note where appropriate. “Main south plane, truss profile A3, 600 mm horizontal run and 150 mm vertical rise, drawing revision C” is auditable; “roof pitch 3/12” alone is not.
Before using the result, compare it with another safe source where reasonably available. A truss profile might be checked against a section drawing; an accessible interior observation might be compared with a documented angle. Differences should be investigated, not hidden by averaging. Confirm that both sources concern the same plane and revision. If the discrepancy cannot be resolved, preserve both observations and state that the geometry is uncertain. The appropriate response to missing reliable evidence is to pause the calculation or obtain competent measurement—not to take greater physical risk for a cleaner number.
Final verification before use
The final decision is not whether a measurement can be taken, but whether it can be obtained from an authorised, suitably accurate source without creating roof-access risk. Record unresolved uncertainty and escalate the measurement task to a competent person when necessary. RoofFigured remains useful with existing plans and safely supplied dimensions; it never requires direct roof access.
Review the finished note for source, revision, plane, endpoints, unit and method. Confirm that an interior line actually represents the external roof plane and that a documented angle has not been confused with percentage or rise per 12. If two safe sources disagree, retain both and investigate. Do not select whichever produces the expected answer. The calculation should expose uncertainty instead of encouraging a more hazardous attempt to remove it.