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I'm building a 48' long awning along the side of a barn. It consists of 5 posts spaced 12' apart.

The roof deck is complete - just waiting on delivery final metal roofing material. But I'm noticing considerable lateral sway (parallel with building).

Three part question:

  1. Would installing knee braces on the post help reduce this sway?
  2. What size braces should be installed?
  3. What's the best material for the brace?

Details: Posts are 8 ft 6x6 rough sawn hemlock. The beam running across the posts is made of 2 2x12s nailed together. The posts are notched 3'' to accommodate the beam.

My initial thought is asking a local mill to mill true 3x3 rough sawn lumber that can be cut to size for the braces. Alternatively, I could use 6x6 and notch the end meeting the beam so that it extends to the backside of the beam and face nail on the backside.

I'm also interested in thoughts on using a 4x4 for a brace - it would overhang an inch but is cheaper than the 6x6.

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    This isn't an awning. It's a "shed roof", by its purest definition. An awning is generally a fabric thing on a frame.
    – isherwood
    Commented Apr 5, 2023 at 21:35

1 Answer 1

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Knee braces aren't really the ticket here. They'd work, but this is normally handled in the roof itself*, through one of these means:

  • The steel roofing. It will provide massive diagonal support all by itself once you stick it on with a zillion screws. You'd want to square up the framing before you install it.

  • Diagonal lumber bracing over the rafters. You could've run one or two 2x4 boards up at a diagonal before you installed the purlins. You'd then fit the purlins to that where they intersect. You could still snap some chalk lines 3½" apart and cut the purlins out for this.

  • Diagonal lumber bracing under the rafters. For an easy framing fix, just run 2x4s at a diagonal under the rafters. Done deal. It won't look quite as tidy, but maybe it's a barn.

  • Steel strapping either over or under the rafters. Rob suggested coiled strap by Simpson. It should be intended for structural use and not just dollar store material. You'll want to use some leverage or tension to get it nice and snug, and it needs to be run in both directions, as opposed to lumber, since it doesn't have stiffness. X or V shapes 90 degrees apart are the goal. This option strikes me as redundant with the roofing steel, though.

Knee braces are more work and don't do as much as diagonals across the roof itself, which are end-to-end instead of essentially cantilevered as knee braces are. They can be visually appealing, though, so do what you think best.

The size of the board is less important than the length and the fastening method. They must be very well secured to the posts and beams, and they should drop about a third the post height. They could be as light as 2x4 laid over the face of the post and beam and still do the job if they're bolted on well.

Whatever you do, square up the framing beforehand and brace it in place. Measure the two diagonals and make them match. You could slap a diagonal between two posts temporarily.

* Which is why you don't normally see structural knee braces on front porch posts.

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  • A few 2x4 tacked on underneath the rafters right now will hold it square until the roofing is on. They will be helpful while you're climbing all over the thing getting the roofing on, too. Pull them when you're done & stack them in the "high-quality scrap for future use" pile.
    – FreeMan
    Commented Apr 7, 2023 at 15:13

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