How to Calculate Trusses

Updated February 21, 2017

Calculating the length and end angles of roof truss pieces seems like a major chore. A well drawn blueprint should include this information, but even a simple scale drawing can be used to derive the needed dimensions. Laying out a full scale mockup of the truss in chalk on a driveway or garage floor is the best way to ensure that all pieces will be cut to the correct dimensions and mitres for tight joints.

Draw a horizontal line across your driveway or garage floor the width of your roof truss, along its bottom line. Use a tape measure to measure out the distance and mark both ends. Stretch a chalk line between the two points and snap a chalk line to form the bottom horizontal edge of the truss mock up.

Use the tape measure to locate the centre of the line and mark it with chalk. Snap a second line, perpendicular to the first, equal to the height of your trusses, from the bottom edge of the joist, or lower cord, to the peak. Snap lines from this peak to the two outside corners of the bottom horizontal line to complete the outside triangle.

Measure in from the two diagonal lines and the long horizontal line 3 1/2 inches at each end of each line and mark the floor. Snap a line between each of these pairs of marks to form a smaller triangle, 3 1/2 inches inside the first triangle. Overlap the lines at the corners. Two-by-four lumber is actually 3 1/2 inches wide, so this creates a full scale drawing of your upper and lower cord pieces.

Use an angle finder with one arm along each angle and the centre hinge against the peak of your template. Read the angle on the indicator in the centre of the angle finder and divide it by two. Write this number down, it is the angle you will use to cut the ends of all truss pieces.

Measure from the peak, along the outside diagonal line to where it intersects with the parallel line you drew 3 1/2 inches above the bottom horizontal. This is the length of your upper cord, or rafter pieces. Each end will be mitred to the angle you took from the peak. You will need two of these for each truss. Measure along the bottom edge of the triangle from corner to corner. This is the length of your joist, or lower cord piece, you will need one for each truss.

Use the tape measure to measure from the inside bottom horizontal to the bottom of the inside peak line. This is the height of your king post, or vertical brace. The end of this piece will get cut to a peak, with the same angle you have used on the cord pieces, cut on each side, to the centre, so that it fits under the centre of the peak.

Things You'll Need

  • Chalk line
  • Tape measure
  • Angle finder
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About the Author

Mark Morris started writing professionally in 1995. He has published a novel and stage plays with SEEDS studio. Morris specializes in many topics and has 15 years of professional carpentry experience. He is a voice, acting and film teacher. He also teaches stage craft and lectures on playwriting for Oklahoma Christian University.