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?
Best Answer
No.
The unsupported ends of your joists will:
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.