tes (H, H) support the roof
rafters (A, B, C); I, I are the vertical tie rods.

This type is probably the oldest form of truss for building purposes,
and it has been modified in many ways, the most usual modification being
the substitution of posts for the tie rods (I, I).

Following out the foregoing forms, we may call attention to one more
type which permitted ornamentation to a considerable degree, although it
still required the tie beam. In fact the tie beam itself was the feature
on which the architect depended to make the greatest effect by
elaborating it.

This is shown in Fig. 287, and is called the _Arched_, or _Cambered, Tie
Beam Truss_. It is a very old type, samples of which have been found
which take it back to a very remote age.

[Illustration: _Fig. 287. Arched, or Cambered, Tie Beam._]

The tie beam A, in wide spans, was made in two sections, properly tied
together, and sometimes the outer ends were very wide, and to add to the
effect of the arch, it might also be raised in the middle, something in
the form shown by the dotted line (B).

_The Mansard_ is what may be called a double-mounted roof, and it will
be seen how it was evolved from the preceding types. It will be noted
that the simple truss formed by the members (A, B, C) is merely
superposed on the leaning posts, the tie beam also being necessary in
this construction.

[Illustration: _Fig. 288. The Mansard._]

But the most elaborate formations are those which were intended to
provide trusses for buildings wherein the tie beams were dispensed with.

The simplest form known is called the _Scissors Beam_, illustrated in
Fig. 289. This has been utilized for small spaces, and steep pitches.
Each rafter (A) has an angled beam or brace (B), springing from its
base, to the opposite rafter (A), to which it is joined, midway between
its ends, as at C.

Where the two braces (B) cross each other they are secured together, as
at D. As a result, three trusses are formed, namely, 1, 2, 3, and it
possesses remarkable strength.

[Illustration: _Fig. 289. Scissors Beam._]

BRACED COLLAR BEAM.--This is a modification of the last type, but is
adapted for thick walls only. The tie rod braces (A, A) have to be
brought down low to give a good bracing action, and this arrangement is
capable of considerable ornamentation.

The steeper the pitch the higher up would be the inner and lower brace
posts (B, B) which were supported by the top of the wall. This