Figure 2-103. - Duct installed to a built-up steel roof.

Figure 2-104. - Fabricating rectangular/square fiber-glass duct from duct board.

Of extreme importance is the selection of the proper board size to fabricate the duct before cutting and grooving. In all applications the inside diameter of the duct is the determining factor of the board size. Use table 2-3 to determine board size.