How do you calculate gambrel roof geometry?
Calculate symmetrical gambrel roof slope lengths, ridge rise, surface area, roofing squares, bundles and underlayment from actual geometry. Symmetrical two-slope roof geometry only. Enter actual roof extents including overhangs. No gambrel truss, rafter sizing, connections or snow-load design is provided.
Which measurements should you enter?
Enter the full covered roof span and length including any overhangs you intend to measure. Both roof sides are symmetric in this model.
Which equation gives the result?
Upper run per side = span/2 − lower run; lower and upper slope lengths = run × sqrt(1 + (pitch/12)²); roof area = 2 × roof length × combined side lengths
What does a worked example show?
For a 20 ft span, each half has 10 ft of horizontal run. A 4 ft lower run leaves 6 ft for the upper slope; each sloping length is calculated from its own pitch.
Which assumptions affect the estimate?
Lower and upper pitches are rise per 12 units of horizontal run. A slope length uses horizontal run times the square root of one plus the squared pitch ratio.
What should you verify before using the result?
Bundle and underlayment counts use actual product coverage and round up to whole packages. Geometry does not supply engineered truss joints or structural design.
How do you keep units consistent?
Select measurement units before entering dimensions. Keep density, price and product coverage units attached to their labels. The calculator converts geometry but does not invent a missing supplier value.
Which input units apply to this worksheet?
The table lists the field units and the condition in which each input is used.
| Input | Units | Condition |
|---|---|---|
| Roof length including gable-end overhangs | ft / m | all modes |
| Total horizontal roof span including eave overhangs | ft / m | all modes |
| Lower slope horizontal run on each side | ft / m | all modes |
| Lower slope rise per 12 run | as labeled | all modes |
| Upper slope rise per 12 run | as labeled | all modes |
| Roofing coverage allowance (%) (optional) | as labeled | all modes |
| Actual roofing bundle coverage (optional) | ft² / m² | all modes |
| Actual usable underlayment roll area (optional) | ft² / m² | all modes |
| Roof covering price per square foot (optional) | as labeled | all modes |
How can you compare and save scenarios?
Calculate a baseline, add it to comparison and change one project value. Copy or share the result, download the order sheet or print inputs and results. Reload and clear controls return the worksheet to blank numeric inputs.
Which tools help with the next calculation?
Continue with Roofing Calculator and Roof Pitch Calculator. Check scope and units before transferring the result.
