To plan a gambrel roof, measure four angles for the peak, the walls and where the two truss chords connect. A gambrel roof is the same design as a barn roof, and has either double or triple slopes on either side of the peak of the roof. It is important to calculate the angles of the trusses to create the correct measurements for either style.
Using a graph and a protractor, create a semi-circle and denote the peak on the top of the arch. From the peak, draw two lines on either side using the deign you want for your roof. Take the protractor and measure the angles of the lines at the peak, where the walls are to be and at where the walls intersect at the top.
Choose a pitch for the slope on top and one for the bottom angle. Many gambrel roof pitches are a rise of 7 inches per foot. Determine this by continuing the roof blueprint on the graph paper, making it to scale and measuring the rise from the midpoint connection up to the peak.
With a two-sloped gambrel roof, the angles of the cuts for four chords need to equal 90 degrees. One simple way to do this is to have all four chords cut at 22.5-degree angels. For a three-sloped roof, the cuts for six chords also need to equal 90 degrees. Use the graph paper and protractor to find the correct angles.