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I would like to ask for your feedback on the deck frame illustrated in the image below. It is a ground-level, free-standing deck.

All your feedback is highly appreciated!

Iteration 1

Frame

[Edit] **

Iteration 2

enter image description here

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I think your second iteration is workable. I'm not sure 2x8 joists would meet code, but I believe that they'd be appropriately strong and rigid. For an 8x8 deck at that height I think that's exactly what I'd do, except I'd go 16" centers. The extra joists and decking fasteners probably aren't going to provide any real benefit (unless you expect to have more than a few people on the deck at once or will be setting a hot tub on it).

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  • I think you mean 2x8 joists would meet the code. Right? – Andrew Hanson Jul 20 '18 at 14:38
  • Not in my experience, but it depends on the jurisdiction. – isherwood Jul 20 '18 at 15:09
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Check out this residential wood deck construction guide.

Looks like 2x6 joist lumber is not an option per IRC, and for 8' joist span your 8' beam would need to be 2-2x10s minimum (or other options depending on wood species).

IRC deck framing guide courtesy of prior question by @psaxton

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    Seems to me that the guide as shown doesn't cover such a small deck. I would not conclude from this guide that 2x6 joist lumber is inadequate for these dimensions. – Jim Stewart Jul 19 '18 at 8:59
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    The beam is only spanning a couple feet at a stretch, so that's not a concern. 8' joists should be at least 2x8 in my opinion. Except for stair landings, 2x6 is not suitable for joists. They'll be too bouncy. – isherwood Jul 19 '18 at 18:50
  • @isherwood, (1) If I add blockings between the 2x6 joists, would that mitigate the problem? (e.g. african-cichlid.com/blocking.jpg) (2) Given that 2x6s aren't suitable for for joists and they should be upgraded to 2x8s, I believe I must change the beams as well to double 2x8. Right? – Andrew Hanson Jul 19 '18 at 19:24
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    Your decking already acts as blocking between the joists to distribute load. I still don't consider it enough. Ideally, flush beams are the same height as the joists (or larger). If you did that you could eliminate one support block. – isherwood Jul 19 '18 at 20:30
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    @isherwood, agreed. The code has allowances for alternate methods and materials per local AHJ, and surely there are limitless engineering solutions, but; with the materials and methods described in the question, and assuming when the OP says he will use "joist hangers" he means the typical type, the code is specific: the beam depth must be at least equal to the joist depth. – Jimmy Fix-it Jul 20 '18 at 17:29
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The challenge here is to do the minimum that will work, but not less than the minimum. One incremental change I can see would be to sister an additional 2x6 (maybe 2x8) in the middle third of each joist. This would reduce deflection in the middle.

Even better to prevent downward deflection would be to use long metal strapping on the underside of each 2x6 joist, extending over the middle 1/3 (2/3?) of each joist. This would be either flat or angle. The underside of each joist is under tension when loaded down and a metal strap nailed with multiple nails along its length would resist deformation with little increase in weight.

Real designers could surely come up with a truly minimalist design that would use half the wood that an amateur would think was necessary. For example, I'll bet trusses out of PT 2x2 lumber would work. There are youtube videos showing how to make a set of identical trusses. Gusset plates would be from PT plywood, waterproof glued, and nailed with stainless steel nails or just deck screws.

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    Metal strapping will not significantly change the joist's tendency to bounce. The stretch that occurs when a joists flexes a quarter inch is almost immeasurably small (some of the longitudinal movement is compression), and you won't get that type of tension with typical assembly done by hand with common components. There will be movement in the fasteners, for example, which completely negates any benefit. – isherwood Jul 19 '18 at 18:51
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    The problem with the 2x2 truss idea is that one single failure point takes the whole system down. Unless you're using engineered (laminated) 2x2, there will be many weak points. Gussets would need to be carefully fastened to not introduce weakness as well, and twist would be largely uncontrolled. Basically, framing is done like it is due to the very nature of wood and the inability of a field carpenter to work to a high enough level of precision. – isherwood Jul 19 '18 at 18:55
  • @isherwood, what do you think of the idea of sistering additional 2x6 in the middle third? Would this significantly stiffen the joists? Would it concentrate stress at the points where the the sistered piece ends? Remember how old wooden ladders had the metal rod supports under the treads? In the case of a shed weight is not a consideration. – Jim Stewart Jul 20 '18 at 12:36
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    I do remember the ladders, but that was for total strength, not stiffness. Those rungs are only about 20 inches wide and had substantial flex. Sistering just the center third probably wouldn't do the job, either. I think you'd need to connect the ends of any reinforcement within about 10% of each end to dramatically improve stiffness. – isherwood Jul 20 '18 at 13:14
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My instinct is that 2x8's on 16 in centers for the joists should be fine. I would probably use 2x10's for the beams. I would put a row of staggered blocking down the middle of the joists. Don't forget to put the crown UP on all the lumber. After all that, you could probably drive a car on it.

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  • Great ideas, thanks @BrianK! About the crown, is there a good way to identify it other than trying to visually spot the higher edge? I spent some good time trying to do so and sometimes I couldn't really determine the crown side for some boards. (I'm pretty much beginner to woodworking). – Andrew Hanson Jul 20 '18 at 19:22
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    Practice makes perfect for identifying crown. Hold one end of the board up, sight down the narrow edge and see what you think. Flip it over and sight the other narrow edge. You'll figure it out. Some boards have very little crown. And if you really want to check your work, use a chalkline/string beside the edge -- it'll be obvious in the middle. – Aloysius Defenestrate Jul 21 '18 at 17:27

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