Whether you are estimating on the spot or checking a result somebody else gave you, the gambrel roof calculator is the quickest way to get from alpha1, building length (l), lower roof angle (θ), lower rafter length (r1), lower rise height (y1), lower run length (x1), lower run length (x1), eaves overhang width (e), gable overhang length (g), upper roof angle (φ), upper rafter length (r2), upper rise height (y2), upper rise height (y2), upper run length (x2), tan, sin, cos, atan and acos to total lower rafter length (r1t), upper roof segment area (a2) and lower roof segment area (a1).
What Is the Gambrel Roof Calculator?
The gambrel roof calculator turns alpha1, building length (l), lower roof angle (θ), lower rafter length (r1), lower rise height (y1), lower run length (x1), lower run length (x1), eaves overhang width (e), gable overhang length (g), upper roof angle (φ), upper rafter length (r2), upper rise height (y2), upper rise height (y2), upper run length (x2), tan, sin, cos, atan and acos into total lower rafter length (r1t), upper roof segment area (a2) and lower roof segment area (a1), so you can order the right amount of material the first time.
Material estimates are where projects go over budget. Getting the quantities right before the delivery arrives is worth more than any discount on the order.
How to Measure for the Gambrel Roof
Measure everything in the same unit and take the largest dimension you can reasonably use rather than the average. Formwork, ground conditions and cutting waste all push the real figure above the theoretical one.
Where a plan calls for a specific thickness, check it against local building regulations before you order. The calculator reflects the relationship, not the code.
Allowing for Waste and Errors
Order a little more than the calculation suggests. The gap between the theoretical quantity and what actually arrives on site is normally waste, spillage, over-excavation and uneven grading, and all of it comes out of the same budget.
Write the figures down and compare them with what you actually used on site. That is how you build an estimate you can trust for the next job.
A Worked Example
Working through an example, with alpha1 set to 10, building length (l) set to 5, lower roof angle (θ) set to 2, the calculator returns lower roof angle (θ) of -3.4292, and building width (w) of 14, and lower rise height (y1) of -1.47906. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out total lower rafter length (r1t)?
Enter alpha1, building length (l), lower roof angle (θ), lower rafter length (r1), lower rise height (y1), lower run length (x1), lower run length (x1), eaves overhang width (e), gable overhang length (g), upper roof angle (φ), upper rafter length (r2), upper rise height (y2), upper rise height (y2), upper run length (x2), tan, sin, cos, atan and acos. The calculator then works through the relationship and reports total lower rafter length (r1t), upper roof segment area (a2) and lower roof segment area (a1).
What does the gambrel roof actually calculate?
It takes alpha1, building length (l), lower roof angle (θ), lower rafter length (r1), lower rise height (y1), lower run length (x1), lower run length (x1), eaves overhang width (e), gable overhang length (g), upper roof angle (φ), upper rafter length (r2), upper rise height (y2), upper rise height (y2), upper run length (x2), tan, sin, cos, atan and acos and converts them into total lower rafter length (r1t), upper roof segment area (a2) and lower roof segment area (a1) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter alpha1?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.