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