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I am currently in the process of designing/building a play structure for my son. The structure will include a 5' high elevated deck, that is 6'x4'. The supports for the decking will be 24" center-to-center.

Reading through load tables, it is recommended to use 2x4 or 2x6 stock as the decking, if one is spanning 24". To use 5/4" decking, requires 16" or smaller spans.

The plan for this play structure is, it is for kids, with the occasional "mom or dad wants to play too!" So, in this case, would using 5/4" decking be appropriate? The 2x6 decking seems to be a bit overkill for a kids play set, and the trap-door/ladder I am planning to incorporate would get quite heavy for little guys to manipulate.

  • When we were little and were building those kinds of things, we just used whatever lumber we could get our hands on. Of course we didn't have a clue about loads and such. As long as it looked like it would hold, it was good enough. The point is - I would just follow common sense... – Vitaliy Jul 17 '12 at 15:51
  • Is it possible to reduce the span? – Tester101 Jul 17 '12 at 16:02
  • @Vitality - Of course common sense goes a long ways, and is what I am trying to do here- common sense with the pocket book. Cedar, unfortunately, is not the cheapest material in the world. I'd prefer not to go get a bunch of 5/4 decking, just to find it deflects too much, and need to spend more/make another trip to the lumber yard ;) – MarkD Jul 17 '12 at 16:48
  • @Tester101 - Not easily, due to the nature of the design (I am hanging the deck on a 4x4 mortise and tenon frame, with columns spaced at 24". Adding cross members at the beam center points would be possible, but would double the number of mortise and tenon joints I'd need to make.). – MarkD Jul 17 '12 at 16:51
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Before writing anything else, let me warn you that I'm not a structural engineer, and I don't claim any competence in these matters. I'm just some random guy on the Internet.

Because I couldn't find a good load table (at least not quickly), I tried to calculate deflection and bending stress, and I can't see anything wrong with using 5/4" x 6" cedar boards (actual dimensions 1" x 5.5") across a 24-inch span:

Decking load calculations

Perhaps I did something wrong in my calculations? Do you have a load table that you trust?

  • The ironic thing is- I am a structural engineer by education.. But like you couldn't find any good loading information for Cedar. One minor change (and I'll update my question above), I'll be using western red cedar, which doesn't have quite the strength of white cedar IIRC. I was getting my loading info from here: cedar-deck.org/building-and-finishing/construction/… , which indicates that a 5/4 x 6 would require less than 16" spans. – MarkD Jul 17 '12 at 16:45
  • Another source for red cedar data—alas, only valid for 2" nominal thickness and higher. – Vebjorn Ljosa Jul 17 '12 at 17:29
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It is not officially recommended, and any chart you will find will list 16" as max span (I've even seen a couple that said 12").

As far as safety goes, it would feel a little "bouncy" to an adult but I'd like to sit back with a beer and watch you try to break it.

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I'm a code official and you should ask your authority having jurisdiction. The answer depends on if you need a permit. The 2x4 or 2x6 decking is likely based on the requirements of the International Residential Code which prescribes a 40 psf floor load on any permitted structure, section R105.2. A Play Structure, however, is exempt from a permit under the IRC, therefore, you can use any material as long as it is safe and need not follow the prescriptions of this code. You need to design it for the loads intended, worst case.

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Place a couple 2x4's on the ground at 24" spacing and lay some decking on it. Stand on it, and let the kids jump on it. It should be fine. I was a framer for years. It's only a kids playhouse. Just make sure your beam end connections are strong.

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“Can I get away with...” and you’re trained as a structural engineer??? You know you design for 1) critical loads not normal loads, 2) “span” means net clear span, 3) impact loading, 4) grade of wood, 5) wet condition of “use”, 6) surfaced two sides or four and 7) repetitive use or single use.

...And you want to increase the span by 1/3 more than what is recommended.

Enjoy that slice of birthday cake at your daughter’s next birthday party, while those crazy neighbor teenagers are dancing on that deck.

  • Oops...yikes, 50% more span. – Lee Sam Oct 26 '17 at 1:56
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    Most of the regs for spans etc are for maximum acceptable deflection (people tend not to like bouncy floors) rather than strength. You don't want it to fail with critical loads; you don't want too much deflection with normal loads. I think he will be fine. – Martin Bonner supports Monica Oct 26 '17 at 13:33
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work backwards---------Use a max deflection of 1/32 per foot (1/16" for a 24" span ) then solve for a uniform load per foot. Suggest that you use tongue&groove deck boards .

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Your numbers are wrong, visually looking at them. As a civil non structural engineer, I know you neglected to design for influence lines or worst case scenario loading where your load is distributed at the center of every other joist. This loading causes a moment of force or torque acting on both ends of the "simply supported beam."

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    Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jul 8 at 0:49

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