Determine Beam and Rafter Sizing

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Why patio roofs fail - and how to build them right!

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What happens here

Understand that the size of your beam and rafters is determined by a 'span chart' and the weight it needs to support. For rafters, closer spacing allows for smaller dimension lumber, while wider spacing requires larger rafters. For the main beam, its size is determined by the square footage and load of the roof (e.g., snow load, live load), and the distance between your support posts. Fewer posts necessitate a taller, stronger beam, whereas more closely spaced posts can support the same load with a smaller beam.

Goal: Understand how to correctly size beams and rafters based on span, load, and post spacing.

Tools and materials

Chains — Used with the excavator for load demonstration
Lumber — Demonstrating rafter and beam sizing concepts
Mini excavator/Bobcat Bobcat — Demonstrating load and uplift forces on beams (for visual effect)
Span Chart — Determining appropriate lumber sizes based on span and load requirements
Structural fasteners — Making strong structural connections (mentioned)

What they say

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not tested, span chart is how you'll determine your lumber size. If these rafters are closer, you can use smaller dimensional lumber, but as the rafter spacing gets further, these rafters cannot handle the same amount of load if there was more rafters. If you have a short span from your structure, you can use smaller dimensional lumber. Because there's a shorter span, there's less weight on this lumber. If your rafters span further, your lumber will need to be taller. But regardless of how many rafters you have, it does not change the amount of weight that this beam will hold. The square footage of your roof and your loads is actually what determines beam size and the distance between posts. So less posts need a taller beam and more posts, you can have a smaller beam. If we over span these

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