How to Design Your Storage Buildings With a Hip Roof
A hip roof has four sloped sides. Designing a hip-roofed structure poses no special difficulties, and designing a simple hip-roofed storage building is a good project to start with. Calculating the length and angle of the hip rafter is a little trickier. A good carpenter can use your design drawings to come up with calculations. If you do the calculations yourself, however, you'll know your material needs and better understand your building. Does this Spark an idea?
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Things You'll Need
- CAD architectural software or drafting tools such as a drafting table, T-square, triangle and protractor
Instructions
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Designing a Hip Roof
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Establish a plan view of your rectangular building, which is the view of the building's footprint and walls seen from above. Draw an elevation view to show one of the two narrower faces. Establish the height of the top plates---the horizontal two-by-fours or two-by-sixes that tie your studs together at the top of the wall. Draw the two vertical sides from the base to the top plate.
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Establish the top of the ridge by drawing an angled line upward and inward from the top plate at one side of the building and continuing past the center line of the elevation. Measure this angle with a protractor or with your CAD software. Using the same angle in reverse, draw another angled line upward and inward from the top plate at the other side of the building and continuing until the two lines intersect.
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Draw an elevation view showing one of the two wider facades from the base to the top plate. At the ridge height, draw a horizontal line across the drawing from one side to the other. From the outer edges of the opposing plates, draw identically angled lines upward and inward to meet the line drawn at ridge height. Designers often make these lines the same pitch or angle as the lines drawn to create the ridge.
Determining the Length of the Hip Rafter
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Establish the rises and runs of the four hip rafters you have designed to calculate their lengths. One solution is an adaptation of the method described by Miller and Leger in "Audel Complete Building Construction," with an emphasis on the design geometry more than on a carpenter's layout.
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Draw a vertical line downward from point B---where the hip rafter meets the top of the ridge, and draw another line running horizontally from the top plate at the corner of the building (point A) until it meets that vertical line. Label that point C. Draw a diagonal line from point A to point B. You've drawn the right triangle, A-B-C. A right triangle is a triangle with one 90-degree corner.
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Give the line from A to C the name "b," because it's opposite point B. Give the line from B to C the name "a," opposite point A. Give the diagonal line from A to B the name "c," opposite point C. Square the value for "b" by multiplying it by itself. In the same way, square the value for "a." Add the two squared values together. Note that the sum of a squared plus "b" squared equals "c" squared, and that the square root of that sum equals "c," the length of the hip rafter.
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You can calculate this square root, or you can look up the value with an online guide, such as Paul Hsieh's Square Root page. For example, if "a" = 7 and "b" = 15, then (7 x 7) + (15 x 15) = (49 + 225) or 274, and the square root of 274 is 16.553, the value of "c," the length of the hip rafter.
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Tips & Warnings
Create a scratch drawing of the hip roof and rafters as described to better visualize and understand these methods.
If you plan to cut the hip rafter yourself, you will need to find the angles between the hip rafter and between the hip rafter and rise. Go to a source like mathforum.org to find both angles from the lengths of the two sides. Plug in the values of the horizontal and the vertical sides of your A-B-C triangle. The result will be two angles; one will be the angle between hip rafter and run, and the other will be the angle between hip rafter and rise.
References
Resources
- Photo Credit roof image by Eldin Muratovic from Fotolia.com
