How to Design a Gambrel Pitch for a House

Design a gambrel roof based on the width, or span, and the pitch, or angles of slope, for the two sections. Divide the span in two to calculate the run -- the distance each side must support. The pitch of each slope will vary and can be altered to meet the design. Another key element is the type of internal bracing required; some gambrels are braced between sloping rafters to afford more space while others are braced on a horizontal joist between walls at the side rafters. Bracing does not affect the basic design, however.

Start a gambrel design with a half-circle. Draw a circle on graph paper with a compass. Split the circle in half horizontally. You now have the basic outline for a gambrel. Mark a peak at the top of the semi-circle and draw a line from that peak to some point on the outside of the circle then make another line from that point to the bottom edge of the semi-circle.

Use a protractor to determine the angle on each of the lines noted in Step 2. Alter the slopes until you arrive at a design that pleases you. The top angle can go halfway down the roof, just a short distance from the peak or almost to the edge of the roof. You can make two short top slopes if desired. Mark the top angle first then connect it to the bottom of the semi-circle.

Add the angles. These must add up to 90 degrees. A traditional gambrel, for instance, has connecting points for the rafter pairs on each side of the roof at 22.5 degrees; that is, the ends of each rafter are cut at 22.5 degrees. Or if the top rafter ends are cut at 30 degrees, the bottom rafter must be 60 degrees. Another way to figure rafters is to use the pitch. If the top slope is a 7/12, rising 7 inches per foot, the bottom slope must be 12/7. Reverse the pitch of the top element to calculate the bottom portion.