Swingset beam between two trees

Question

I've got two pine trees (3ft diameter) 13 feet apart. I wanted to build a swing set between them. I was thinking of using a treehouse lag screw in each tree, perpendicular to the beam of the swing, drill a hole in each end of the beam and hang it on the trees. Then suspend 3 swings from it.

I was looking at one 12 inch long, 1.25 inch thick lag bolt in each tree and a 16ft long board (2×12) of ground-contact Hem Fir (maybe 2 of those bolted together? Making a ~4 inch wide, 12 inch tall, 16 foot long beam spanning about 15 feet from screw to screw.)

Is it sturdy enough?

I'd like to add a small climbing platform (octagonal donut around the tree with railing) to one end in a few years.

Answer 1

Double 2x12 is certainly stout enough for the span. The bigger concern is the anchors to the trees. Trees move, and rigid connections often get torn apart by nature. Also, one anchor leaves the potential for rocking with swing activity.

So...

• Use a slotted, loose connection to allow for tree sway. Big washers over a routed slot a few inches wide would make this possible.

• Use two or more anchors at each tree, oriented vertically with respect to each other to prevent roll of the beam. This would also allow you to bush out the beam at one screw or the other to get the beam vertical in case of an imperfect tree surface.

• Use reasonably sized hardware. 1¼" is for lifting train cars. You'll do fine with ½" or ⅝" lags having adequate penetration. 3-4" into healthy wood (beneath the bark and cambium) is plenty.

• Consider accommodating tree growth. Short, stiff springs under the screw heads would keep the beam snugly in place without becoming a problem with annual cambium expansion. You'd still have to back the lags out every couple years, but they wouldn't start to grow in quite so readily.

I'm just putting this here because it's freaking hilarious. Be aware though, that to properly use this absurd thing you need to pilot for the full screw shank to the depth that it'll be inserted, then beyond that slightly smaller than the shank for the full depth of the screw. Who has bits like that??

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