Beam strength determines the spacing between supporting posts.
It's logical to want your patio cover's posts spaced as far apart as possible; if they're too close, you'll block the outlying landscape, which defeats the purpose of an outdoor living space. However, if the spacing is too wide, the patio cover's beams will bow under the weight of the joists and roof covering materials. The determining factor of post spacing is the allowable span of your beams: in other words, the maximum distance it can cover without support from posts. Allowable spans vary according to wood species and the loads that the beam will bear, such as snow loads or the "dead loads" of construction materials.
Understanding Patio Cover Framing
The basic idea behind the structural design of patio covers is to safely transfer weight from the top of the patio cover to the foundation. The chain of load distribution begins at the roof covering material, which exerts force on the cover's ceiling joists. The ceiling joists transfer their loads to ledgers or beams, the beams transfer loads to the posts and the posts to the foundation. The size and spacing of the framing members depend on the amount of weight that they are capable of supporting and the distance over which they can support that weight without breaking or bowing. This relationship between strength and unsupported distance is called a span.
The Relationship Between Joists, Beams and Posts
For any particular load, the allowable span of patio cover joists and beams depends on wood species and board size. For any species, the allowable span generally increases as the joist or beam increases in size. When you refer to a beam's allowable span, what you're really describing is the allowable spacing between posts. As the designer of the patio cover, this means that you are in charge of determining the spacing between posts; the larger the beam the wider the spacing between posts.
Sizing Your Beam and Spacing Your Posts
Aside from species and span, beam sizing depends on loads, including the dead weight of the roof structure, if any, and seasonal factors, such as snow. Local building authorities sometimes provide information regarding load calculation, or you can consult a contractor or architect for assistance.
Common Post Spacing
While your beam size ultimately depends on design factors, such as whether you include a covered roof structure or open lattice strips, a familiarity with conventional beam spans gives you an idea of how far apart you can space your posts. The smallest beams for patio covers are typically 4-by-6. In general, the maximum span for a 4-by-6 beam is 6 feet between 4-by-4 posts. If you move up to 4-by-8, you're usually allowed to span up to 10 feet between posts. A 4-by-10 can usually span up to 12 feet between posts. Keep in mind that these figures depend on the size and spacing of joists and the overall structural loads.
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