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Because the angle is odd and the trees are not parallel my idea was to put everything up as is, tack the decking on and then strike a line on the joists to cut them exactly so they're even. You can see in this picture that everything is up and all I have left is to cut the joists to the right size, but, after having it like this for a few days we like the exposed looks of the joists and don't really want to add a rim joist. Is this a problem for cutting them to length but not adding a rim joists? I know it's there to provide lateral support but for this application do you think it's necessary? What could happen if I leave it off?

tree house platform

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  • FWIW, I did a similar, though smaller and lower tree house that I closed off the ends with a standard rim joist treatment.
    – SteveSh
    Commented Jun 22, 2020 at 14:02

3 Answers 3

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Do you have to?

No.

What will happen if you don't?

The unsupported ends of your joists will:

  1. sag
  2. twist

The decking on the left of the single tree (from the angle of the picture you provided) will become uneven, making tripping hazards for the occupants up top. You'll also end up needing to replace those deck boards more frequently, since not only will they want to do some warping and cracking of their own (it's wood, that's what it does), but they'll be forced into even more warping by the joists moving underneath them.

An additional consideration - without the rim joist, what are you going to attach the railing to? Having the nice straight, even surface of the rim joist gives a good mating surface for screwing the railing posts to (with some serious sized structural screws, not decking screws (they're not designed for that kind of load), and especially not drywall screws). Without it, you're going to have to fabricate some pretty unique and beefy brackets to hold the railings to the ends of the joists. You'll probably have to cut through that last deck board to attach them. Then, you'll get to watch the whole railing get ripped apart as the joists twist with age.

But, you don't have to.

Maybe OP had thought about this, but it just occurred to me: I'd put a rim joist at the far end, too, even though each joist is directly supported by the beam and attached to them with the hurricane clips (kudos for that!). Those metal clips will help prevent the bottoms from moving when the joists want to twist (if it twists away from the metal, a joist can pull itself off the nail, leaving the metal behind), but will leave the tops free to move. A rim joist on the far end will help prevent warping all the way along the ends of the joists.

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    He could attach the posts for the railing directly to the floor joist.
    – SteveSh
    Commented Jun 22, 2020 at 14:01
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    Indeed, @SteveSh. But, as the joists twist, they're going to take the posts with them, racking the railing. The railing will provide some resistance to warping, but I'm not sure it can be counted on the way a rim joist would.
    – FreeMan
    Commented Jun 22, 2020 at 14:08
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yes you should use them

As you stated they are a part of the lateral system and the answer about that is no you don't need the rim joists here for that reason.

However, the rim joists serve other purposes. For one, they are also part of your gravity system. Look at the picture you posted an ask yourself, what stops all the joists from rolling over together like a connected string of dominoes? Yes, you do have the blocking in the middle (kudos! because that is serving the same purpose here, like a rim joist in mid span) but remember that wood is very ductile and the support is localized. That is why we use blocking at 8' oc maximum. I do note that loads are a big factor and this is more important if there are walls, roof and maybe more levels bearing on the end of the joists (building edge) and here that won't be the case. However, the roll over can still happen, slowly over time...

Other purposes for the rim joist is to provide overall integrity, alternate load paths and so on. To attach railings, and so on

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I see you are using “Dry” lumber (rather than “Green” lumber) so drying out and twisting probably won’t be a problem.

However, standard construction practices would require a rim joist. If you follow the Building Code, the Code says, “Joists shall be supported laterally at the ends of joists by blocking, rim joists, etc. to prevent rotation. (See ICC R502.7) However, this requirement is under the general heading: “Lateral restraint at supports”. Key phrase here is “at supports”.

In addition, the Code requires bridging or blocking at 8’ on center when the size of joists exceed 2x12’s. (See ICC R502.7.1) Your framing doesn’t “exceed” 2x12’s, but it does eliminate lateral rotation of the joists.

All these Code requirements are based on standard construction, including design load of 40 lbs. per square foot. I doubt you’ll have that unless you add furniture, etc.

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