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This is really a 1.5 part question. The first half has to do with whether it's okay t use single post for one side of a swingset (the other side will be attached to a play structure). I've seen some designs with 6x6s and maybe that's beefy enough when sunk 3' and concreted in. Any thoughts about that would be appreciated. It seems like the top might swing a little and the forces (especially if an adult is swinging) could loosen the concreted block in the ground even if the post doesn't sway.

The 2nd and primary question is whether I can save money on a 6x6 post and make build one up with 4 2x4s. It will also be easier to handle (bring home from home depot) and cut and I can get pressure treated 2x4s in lots of lengths including 10ft so I can sink it into the ground far. I realize it won't be 6x6 (5.5" x 5.5") but it'll be 5x5. x-section of 2x4 post
And because it's hollow it won't be as strong as a 6x6 but should be much stiffer than a 4x4. I guess I'd need to cover the top so water doesn't get in. I guess I could stiffen it up by putting a few blocks internally as well.

Any guesses on the strength of this approach and downsides?

update: here's a picture of the layout of the swingset enter image description here

Another set of options is (1) attaching the post to the garage so that it is more stable to the back and forth motion when swinging or (2) attaching the horizontal beam directly to the corner of the garage.

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    I do not think that a composite post (as in your diagram) can be as strong as a single piece. Imagine the twisting force that could collapse the post made by your diagram. You will also need a lot of fasteners and preferably some glue. Vertically it may hold up great but if on an angle or twisted, by my non-engineering opinion, it will be very risky. I would not do it.
    – Chris Paveglio
    Commented Mar 27, 2018 at 20:09
  • The first half of your question isn't clear. We have no idea what the design of your set is. The second half is unclear for the same reason. Length and load are critical data points. As Chris hinted, by the time you buy enough lag bolts to build the post you'll probably have spent more than the 6x6 would cost.
    – isherwood
    Commented Mar 27, 2018 at 20:27
  • Just thinking about digging a 10-12" hole 3' deep and sinking a 6x6 in it makes me want to just use a standard A frame. Works great for the back and forth of swings, little to no digging, and cheaper. Much cheaper.
    – JPhi1618
    Commented Mar 27, 2018 at 20:30
  • thanks for the comments so far. the reason for not going with an A frame is shown in the picture I appended to the original question. One side is very close to a detached garage. Also we wanted to minimize the footprint and just have a clean space (given the small size of our bay area back yard).
    – Chris_engagingdata
    Commented Mar 27, 2018 at 21:04
  • not sure about the twisting as the bottom will be in concrete and the top will have a long beam(s) attached to them. I guess one other question is whether or not the swing set can be attached directly to the garage with no post. It's relatively new construction (about 10 yrs old, i.e. not 100 years old).
    – Chris_engagingdata
    Commented Mar 27, 2018 at 21:12

2 Answers 2

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I would emphasize that it is not possible to engineer a solution to your problem without more concrete information about loads and the existing structures. But I can share some thoughts that I hope are useful.

Start with an A-frame and modify to suit your situation. This is a common solution to the problem of handling the dynamic vertical and lateral loads of a swing set. It is also a much easier DIY project than a vertical post anchored into the ground. You could build this framing on both sides of your swing and simply attach it to the play structure. If you want a more elegant solution start with this concept in mind and adapt it to use elements of the existing structures. Example A-frame structure

Can the play structure handle it's half of the load from a swing set? Specifically, is it anchored to the ground and/or braced to handle the lateral loads? If there is no cross-bracing add an internal cross brace between the post you attach to in your diagram and the one below it. This is one half of the "A". Consider adding a second diagonal cross brace from the same post into the ground (up in your diagram). This is the other half of the "A".

On the garage side take the same approach. If you can anchor into the garage framing that's great. To keep things lined up you can anchor one or more 2x6 members against the garage so that the cross-bar attaches at a right angle. Install a diagonal brace from this point to the ground (down in your diagram). This is half of the "A". Inside the garage, especially if you already have access to the framing, install a diagonal brace to form the other half of the "A".

About making a 6x6 post from 2x6 boards and then burying it. Besides the good points raised in comments to your question I would point out that boards (2x4, 2x6, etc) are typically used for framing above ground while timbers (4x4, 6x6, etc) are routinely buried as fence posts or covered with dirt to build retaining walls. You will find pressure treated varieties of both but the boards are almost always rated for above ground exposed use only while the timbers are rated for ground contact. In the example below the top label is ground contact and the bottom is above ground only.
Compare PT labels

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  • Thank you for your detailed answer and I think this basically is spot on for all of the issues raised and reasoning provided. I think I will go with one or two pressure treated 4x4s since I can't get a 6x6 and sink those and provide an attachment point to the garage corner. I will also put in a brace inside the garage framing perpendicular to the beam for swings. The play structure hasn't been made yet, but will also have a post sunk in the ground and will be braced in the direction of swinging. I'll try to avoid bracing the post upward from the play structure, but can always add it later
    – Chris_engagingdata
    Commented Mar 28, 2018 at 3:47
  • If the play structure does not yet exist I would consider locating the top post in line with the garage. This way the cross-frame for the swing would attach to the left hand side of the garage rather than the bottom. Assuming you can bolt into the garage framing then the swinging motion would but sheer forces on the bolts rather than trying to rip them off the garage. This is the stronger setup for attaching to the garage. (You still need to make sure the garage frame itself can handle the lateral forces.)
    – Stanwood
    Commented Mar 28, 2018 at 17:31
  • If the play structure is well-anchored to the ground by burying the main posts like a fence then you can brace diagonally from a vertical post to horizontal beam or build an X brace. This avoids expanding the footprint of your playset.
    – Stanwood
    Commented Mar 28, 2018 at 17:43
  • On the garage side, it may be good design practice to add an external brace that extends the footprint downward in the diagram. This discourages little ones from running close to the swings when they are in use. The path you are blocking in front of the swings is "dead space" anyway.
    – Stanwood
    Commented Mar 28, 2018 at 17:45
  • Thanks for your suggestion about relocating the post (to the left rather than the bottom). Due to the fact that I've already dug the hole and another constraint (in ground conduit) I will keep it on the bottom but I will place a bracket/connector of some sort from the left side of the garage to the beam so that the bolts in the garage are in a shear configuration rather than a pull out configuration
    – Chris_engagingdata
    Commented Mar 28, 2018 at 20:36
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Based on your plan you do not have a significant torsion problem. You have essentially a flag pole that will only bend in one direction. You would be better off just glue nailing two 2x8s together and orient the 8" side perpendicular to the swing-set beam. Nailing two boards together will be much simpler than trying to nail four boards into a post

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