f the stone and metal buildings grew out of the
wooden prototypes.

Now the prime object of nearly all the double-roofed trusses was to
utilize the space between the rafters so as to give height and majesty
to the interior.

A large dome is grand, owing to its great simplicity, but the same plain
outlines, or lack of ornamentation, in the ceiling of a square or
rectangular building would be painful to view, hence, the braces, beams,
plates, and various supports of the roofed truss served as ornamental
parts, and it is in this particular that the art of the designer finds
his inspiration.

Before proceeding to apply the matter of ornamentation, it might be well
to develop these roof forms, starting with the old type Barn Roof, where
the space between the rafters must be utilized for the storage of hay.

[Illustration: _Fig. 284. Gambrel Roof._]

_The Gambrel Roof_, Fig. 284, requires a tie beam, (A), as shown, but
the space above the beam is free of all obstructions, and gives a large
storage space. The roof has two sets of rafters (B, C), and of different
pitch, the lower rafters (B) having a pitch of about 30 degrees, and the
upper ones (C), about 45 degrees.

A tie bar (D) joins the middle portion of each of the rafters (B, C) and
another tie bar (E) joins the middle part of the rafter (B), and the
supporting post (F). The cross tie beam (G) completes the span, and a
little study will show the complete interdependence of one piece upon
the other.

[Illustration: _Fig. 285. Purlin Roof._]

_The Purlin Roof_ is a type of structure used very largely throughout
the United States, for wide barns. (A) is the cross beam; (B, B) the
purlin posts; (C, C) the purlin plates; (D, D) the rafters; and (E, E)
the supporting braces.

The rafters (D) are in two sections, the distance from the eaves to the
comb being too great for single length rafters, and the purlin plates
are not designed to make what is called a "self-supporting" roof, but
merely to serve as supports for the regular rafters.

_The Princess Truss_, on the other hand, is designed to act as a support
for the different lengths of rafters (A, B, C), and as a means for
holding the roof. It is adapted for low pitch and wide spans.

[Illustration: _Fig. 286. Princess Truss._]

The main truss is made up of the cross beam (D), rafters (E, E) and
thrust beam (F). Purlin posts (G, G) are placed at an angle intermediate
the ends of the rafters, and the purlin pla