How to Calculate Angle Cuts for Deck Roof Joists

Pitch

• The pitch, or degree of slope, is the most important element in calculating rafters, which are often called joists on flat or low-slope roofs. Deck roof joists perform the same function as rafters or top truss chords on a gable roof -- they are the framework to hold roof decking and shingles or other covering such as metal or vinyl panels.

Run

• The run of each rafter is the second key element. This is the distance each rafter must support the roof from one end of the roof to the other. On a shed roof, it is the length between the house wall connection and the outer band of the roof frame. Roof joists can be attached to the outer band in two ways, with metal rafter hangers or with triangular notches in the joists to fit over that support.

Framing Square

• Use a framing square to calculate angles for a specific roof. Put the point, or heel, of the square at the bottom of one end of a joist, typically a 2-by-4-inch board. Set the inch mark on the thin tongue of the square at the pitch, 2 inches for a roof that slopes 2 inches per foot, for example. Put the 12-inch mark of the wide blade at the top to make an angle at the end of the board for the top or plumb cut of the joist to connect at the roof top.

Length Determination

• Figure the length of the joist with a table on the blade, called "length of common rafter." It will show under the pitch mark the differential or extra space a sloped joist needs to cover the run. On a 2-pitch roof, that is 12.16, meaning each joist has to be about 12 1/4 inches for every foot of run. That would make a joist for a 10-foot wide roof about 10 feet 2 inches between end supports.

End Calculation

• A sloped joist has to have angle cuts on each end unless it is being set on top of the outer deck support. You calculate that end the reverse of the top cut, with the framing square at the top rather than the bottom of a joist. For a joist that will sit on top of a wall cap, use the length of rafter to determine the bearing point where the joist will meet the end and figure a triangle to fit exactly over that board, a 2-by-4 set horizontally or a 2-by-6 installed vertically. Figure that triangle by marking the outside bearing point, where the joist meets the outer edge of the support. Measure 3 1/2 inches back for a 2-by-4 notch or 1 1/2 inches for 2-by-6 notch; 2-inch lumber is actually 1/2-inch less than that.