Truss framing is figured with the base 12" and any rise to meet the objective point. The rules given in rafter framing will give any cut in truss framing, by raising and lowering the rise, the hypotenuse is the length of brace in one base foot.
Truss (fig. 61) is 16' long, 4' 6" high. The outside braces have 3 1/2 base feet and 3' 6" rise. There are as many inches rise to one base foot as there are number of inches in the rise divided by the number of base feet. Thus, 3' 6"=42"divided by 3% base feet=12" rise to one base foot. The inside brace has 3 base feet and 2'10" rise equals 34" rise divided by 3 base feet.=11 1/3" rise to one base foot. The length of. brace is 12" base and 11 1/3" rise laid on the timber three successive times. Mark on the base at top and bottom, add the seat of brace, beyond those marks in the proportion of 4" to 12". (See fig. 61.)
There are 1" iron rods with plates that extend from the top beam at the corner through the bottom beam. Where the outside brace is seated an iron clamp or stirrup laps over the bottom and is bolted to the beam. The beam is three thicknesses of 2"xl2".
Truss No. 2 is 20' span with 12" rise to 1 base foot. See rule for common rafters for length and cut of main or outside rafters. Place a cross tie from the plate to the opposite rafter 1/3 the distance down from the point of rafter to the plate. Put a horizontal tie up 1/3 the length of the rafter from the plate, and a tie from the ridge to the bottom tie, spiked between cross ties and at bottom. (See fig. 62.)
Truss No. 3 spans 20', 3" rise to one base foot. See rule for common rafters for length and cut. Tie from ridge to cross tie. Place a brace from the center of through tie to 1/2 way up the rafter, and a perpendicular tie from the center of the rafter to the cross tie.
Trusses No. 2 and No. 3 are used mostly in roof construction.