A plain octagonal roof would naturally be supported by intersecting trusses, spanning from opposite angles. If the ceiling below is level or unfinished, either Queen or Howe trusses may be used for wooden construction,while for steel construction a truss of the Fink type will usually be most economical, although almost any type of steel truss may be used, as with steel it is more easy to connect the half trusses at the centre.

If the ceiling is raised in the centre, as is generally the case in churches or auditoriums, trusses like those shown by Figs. 36, 42, or 176 should be used for wooden construction.

For steel construction the Quadrangular truss is usually best adapted, if the roof is to be supported on masonry walls. The three hinged arch truss is also well adapted to octagonal roofs.

If a wooden truss has a centre rod it will be easier to make the centre connection at the intersection of the ties if only two full trusses are used, with four half trusses suspended from them.

As has been previously stated, it is very difficult to join intersecting wooden tie-beams so as to obtain the necessary resistance, and especially so if the ties are inclined. Fcr this reason, when framing a wooden octagonal roof of more than 40 feet span, the writer advocates the use of trusses of the type shown by Fig. 176.

81 Octagonal Roofs 300194

Fig. 191.

using rods for the horizontal tie, and either arranging the ties so that they can cross over each other or else connecting them to a central plate.