I have created a playhouse deck that is 8'x 8' and 5' off the ground. It is supported by 10' 4x4s on all four corners.

I want to place a 12' beam across the top of two of 4x4s in order to place a simple climbing rope.

Question: Are there any charts or other guidance that I can use to determine:
1) Is this possible?
2) How much bracing is needed?

  • 3
    What's in the playhouse? Will you adopt me? Commented Mar 26, 2015 at 17:31
  • Ha, I'm using a plan from Ana White's site as a base plan and then trying to increase the awesomeness as much as my HOA regs allow. ana-white.com/2011/03/kids-playhouse-and-slide
    – RWL01
    Commented Mar 26, 2015 at 17:33
  • If I bring the 4x4 and the simpson strong ties to put in the rope, will you let me install a zip line on the other side ?
    – Some Guy
    Commented Mar 27, 2015 at 2:02
  • 3
    I'd be wary of a rope hanging from a cantilever unless you poured footings for the playset. I recommend using through-bolts and following Chicago's deck codes.
    – Mazura
    Commented Mar 27, 2015 at 3:33

1 Answer 1


Simple in theory. Assuming you are mounting the beam to the side of the post at the top. You have an ideal situation for a cantilever. It even follows the old rule of 1/3 (cantilever), 2/3 (back span).

To size it, find any floor joist chart and figure the span of 8'. A 2x8 spans upward of 12'.

Load wise you will probably never hit over 200-300 lb on the rope swing so a couple 3/8"-1/2" through bolts at each post through the beam would be overkill. With those loads, even 6-8 long screws would be more than enough (@ aprox 70-100lb+ per screw).

Biggest problem will be rot between the beam and the post over time as water gets trapped between the two. For this we usually use the through bolts and put a small stack of galvanized washers (5-10) between the post and the beam.

As for bracing it laterally, using a wider member like a 4x would help. So a 4x8 would probably be ideal, and a 4x6 should work well enough. But then you need through bolts for sure.

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