NOTE: Within a heating system, the use of fiber-class duct is restricted by the adhesive used to affix the protective outer coating to the fiber  glass.  Check  the  specifications  and  ensure that it will not fail when exposed to heat over 250 degrees. FABRICATION To fabricate a rectangular/square duct, you must first  measure  the  duct  board  accurately.  Next,  the grooves must be cut. Ensure they are at the proper locations and cut straight because this allows the board to be folded to create the desired rectangular/square shape. When cutting the board, you will need to leave an overlapping tab that is pulled tight and stapled (fig. 2-104, view A). Tape is then applied and the joint is heat-sealed   (fig.   2-104,   view   B).   Joints   between sections  are  fabricated  by  pulling  the  shiplap  end sections together and finished by stapling, taping, and heat sealing the joint (fig. 2-104, view C). INSTALLATION The very nature of fiber-glass duct requires that it be  supported  with  1-inch  by  1/16-inch  galvanized steel   strap   hangers.   These   must   be   supplied   or fabricated to fit the duct precisely whether the duct be rectangular/square or round. Rectangular/square ducts up  to  24  inches  (span)  can  be  supported  on  8-foot centers. Ducts larger than 24 inches must be supported on 4-foot centers. For round ducts the supports must not  be  less  than  6-foot  centers. SAFETY Some  of  the  safety  precautions  applicable  to sheet-metal tools and equipment have been mentioned throughout  this  chapter.  Here  are  a  few  additional precautions that should be carefully observed when you  are  working  with  sheet  metal. 1. Sheet metal can cause serious cuts. Handle it with  care.  Wear  steel  reinforced  gloves  whenever feasible. 2.  Treat  every  cut  immediately,  no  matter  how minor. 3. Remove all burrs from the metal sheet before attempting to work on it further. 4. Use a brush to clean the work area. NEVER brush  metal  with  your  hands. 5. Use tools that are sharp. 6.  Keep  your  hands  clear squaring  shears. of  the  blade  on  all 7. A serious and painful foot injury will result if your foot is under the foot pedal of the squaring shears when a cut is made. 8. Do not run your hands over the surface of sheet metal that has just been cut or drilled. Painful cuts can be received from the burrs. 9. Get help when large pieces of sheet metal are being cut. Keep your helper well clear of the shears when you are making the cut. 10. Keep your hands and fingers clear of the rotating parts on forming machines. 11. Place scrap pieces of sheet metal in the scrap box. 12. Always remember to keep a clean shop. GOOD HOUSEKEEPING is the key to a safe shop. 13.  Do  not  use  tools  that  are  not  in  first-class condition-hammer heads loose on the handle, chisels with  mushroomed  heads,  power  tools  with  guards removed, and so forth. 14.  Wear  goggles  when  in  the  shop. 2-39