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

.

prefabricated_roof_truss.gif

 

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

prefabricated_roof_truss.gif

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.

.

rafter.jpg

 

rafter.jpg

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.

.

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Page 127 - Building stability - Light wood frame construction

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