ROOFING TRUSSES
The chapter on Bridge Building gives some suggestions as to form of
trusses, the particular types there shown being principally for wide
spans. Such trusses were made for one purpose only, namely, to take
great weight, and they were, as a consequence, so constructed as to
provide strength.
But a roofing truss, while designed to hold the accumulated materials,
such as snow and ice, likely to be deposited there, is of such a design,
principally, so as to afford means of ornamentation. This remark has
reference to such types as dispense with the cross, or tie beam, which
is the distinguishing feature in bridge building.
The tie beam is also an important element in many types of trusses,
where ornamentation is not required, or in such structures as have the
roofed portion of the buildings enclosed by ceiling walls, or where the
space between the roofs is used for storage purposes.
In England, and on the Continent of Europe, are thousands of trusses
structured to support the roofs, which are marvels of beauty. Some of
them are bewildering in their formation. The moldings, beaded surfaces,
and the carved outlines of the soffits, of the arches, and of the
purlins, are wonderful in detail.
The wooden roof of Westminster Hall, while very simple in structure, as
compared with many others, looks like an intricate maze of beams, struts
and braces, but it is, nevertheless, so harmonized that the effect is
most pleasing to the eye, and its very appearance gives the impression
of grandeur and strength.
Nearly all of the forms shown herein have come down to us from mediæval
times, when more stress was laid on wooden structures than at the
present time, but most of 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 plates (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 form is
not available for wide spans, and is shown to illustrate how the
development was made into the succeeding types.
[Illustration: _Fig. 290. Braced Collar Beam._]
THE RIB AND COLLAR TRUSS, Fig. 291, is the first important structural
arrangement which permitted the architect to give full sway to
embellishment. The inwardly-projecting members (A, A) are called _Hammer
Beams_. They were devised as a substitute for the thick walls used in
the Braced Collar Beam Truss, and small brackets (B, B) were placed
beneath as supports.
[Illustration: _Fig. 291. Rib and Collar Truss._]
The short tie beam (C), near the apex, serves as the member to receive
the thrust and stress of the curved ribs (D, D). It forms a most
graceful type of roof, and is capable of the most exquisite
ornamentation, but it is used for the high pitched roofs only.
[Illustration: _Fig. 290-1/2. Hammer Beam Truss._]
The acme of all constructions, in which strength, beauty, and capacity
for ornamentation are blended, is the _Hammer Beam Truss_. Here the
hammer beam projects inwardly farther than in the preceding figure, and
has a deeper bracket (B), and this also extends down the pendant post
(C) a greater distance.
The curved supporting arch (D), on each side, is not ribbed, as in the
Rib and Collar Truss, but instead, is provided with openwork (not shown
herein), together with beadings and moldings, and other ornamental
characteristics, and some of the most beautiful architectural forms in
existence are in this type of roof.
What are called Flying Buttresses (E) are sometimes used in connection
with the Hammer Beam Truss, which, with heavy roofs and wide spans, is
found to be absolutely necessary.