Steel trusses built from angle iron allow a roof to expand a greater distance than trusses or rafters built from wood. Steel trusses support more weight and maintain more integrity than wood when properly joined together. Although angle iron trusses contract and expand with changing temperatures, they are not subject to the warping or splitting that can happen to wood when it is exposed to variation in humidity and temperature. To build steel trusses from angle iron, weld, rivet or bolt angle iron pieces together. A combination of all three joining methods can be used on the same trusses.
Place two pieces of angle iron, pre-cut to the length required by your building blueprint, parallel to each other with the right angle of the iron facing inward.
Follow blueprint specifications for the angle to cut on cross pieces, or lay a piece of angle iron across the parallel angle iron pieces, forming an acute angle -- less than 90 degrees. Place your builder's square inside the long piece of angle iron with angle of the square matching the angle iron and one arm of the square extended over the short piece of angle iron. Use a nail or marker to draw a line where the square crosses the short piece. Mark the other side of the short piece in the same manner.
Cut the angle iron on the marks you just made using a chop saw. The piece of angle iron should now fit inside between the two long pieces and form an acute angle. Another piece of angle iron is marked and cut so that when it is placed between the two parallel angle irons, it forms a "V" with the other short piece of angle iron. Short pieces of angle iron are placed the entire length of the parallel pieces, forming alternate "V" shapes. This type of truss is called open web.
Secure short angle iron pieces between the parallel pieces by welding, riveting or bolting the pieces together. Follow blueprint instructions.
Drill holes according to blueprint specifications. Insert bolts and fasten with nuts as instructed. Rivets are often used in areas where a protruding bolt is undesirable.
Weld joints according to blueprint instructions using a MIG welder and wire. Wear a welding helmet to prevent injury to your eyes during welding. Allow the metal to cool thoroughly before handling.
Contact your local county building inspection department for building codes and requirements in your area.