Calculate a truss angle by first measuring the truss's base, the horizontal piece parallel to the unit's ceiling. Look for the horizontal distance from the edge of the base to the point directly below the peak. For symmetrical trusses, this equals one half of the base's length.

Measure the height or the vertical distance from the apex to the base. Next, calculate the angle of the truss's lower corner. This angle derives from the base and one of the sloping edges. Use the trigonometric function of inverse tangent to find this angle. The tangent is the opposite edge divided by the adjacent angle.

Symmetrical trusses have equal base angles. If the trusses are not symmetrical, find the left-side base angle to produce the right-side base angle.

The truss's top angle derives from 180 degrees minus the two base angles. The sum of angles within a triangle always equals 180 degrees; therefore, calculating the two base angles provides the peak angle by subtracting the total from 180.

Calculating roof angles always comes from the width and height of the roof. This relationship is absolute because the roof section of a home or building is often triangular, with a height in the middle, a base width and two sloping sides.