Peaked
roofs can be built from trusses or rafters depending on whether the roof
has an |
open
attic area or not. The most common design uses prefabricated
wooden trusses. The |
following model shows how these trusses are arranged to form the L-shaped
roof structure |
. |
|
|
|
Fig. 212 - Prefabricated truss |
roof framework |
|
truss details ► |
(scale visualization model) |
click image to enlarge |
|
. |
of a
single story ranch house with attached garage. This is a closed roof
design with no |
appreciable attic space since web members of the truss penetrate the
interior. The lower |
chord of
the truss ties its legs together to resist their outward thrust.
Horizontal braces, |
called purlins, colored red in the model, help keep the |
|
trusses from tipping over during
construction. The model |
to the right is a shelter
with a hip roof showing how the |
purlins and the roof
sheathing stabilize the structure. |
Fig. 213 - Train station with
prefabricated roof trusses ► |
(scale visualization model) |
click image to enlarge |
|
|
If the
roof area is to be an open attic, long wooden rafters are used to span the
distance |
between
the top plate of the bearing walls to the peak. They are evenly
spaced and either |
blocked
or cross braced like the floor joists shown before. The top ends of
the rafters are |
joined
together by a wooden plank, called a ridge board, that forms the
peak. Since the |
roof
area is open, joists are used to support the floor of the attic as well as
the ceiling |
of the
room(s) below. The joists tie the ends of the rafters preventing any
outward thrust. |
. |
|
|
 |
Fig. 214 - Rafters used |
for an open attic |
|
rafter details ► |
(scale visualization model) |
click image to enlarge |
|
. |
Taken as
a whole, light wood frame construction can be idealized as a combination
of a |
lattice
framework enclosed in a plate shell. The triangulated nature of the
lattice helps to |
concentrate forces in the members and joints of the framework by means of
lattice action. |
And the
shear panels distribute the forces over the surface by means of plate
action. Lattice |
and
plate action serve a complementary, dual role in stabilizing the
structure. Each |
accentuates the strengths while mitigating the weaknesses of the other. |
. |
Back
to Knowhere |
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Page 127
- Building stability - Light wood frame construction |
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