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I'm building a fort for my kids out of wood, 8 foot by 8 foot and trying to work out how to make it last. I can't get treated wood, and apparently untreated wood rots within a year of ground contact even if painted.

Can I make a base out of 3 inch pipe, with 2 foot legs concreted into the ground and then build my fort on it? Perhaps have the base raised a couple of inches off ground? How long could I expect it to last. I don't want to (can't afford to) build a proper foundation, just concrete in the legs so it's solid.

We get a LOT of rain here, like a monsoon season and cyclone prone, so I need it solid as it will be 15 foot tall on an 8 foot square base. Although in the event of a cyclone I'd probably unbolt it from the base and lay it down.

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  • If you can't get treated wood, consider hardwoods - many of them will last about as long as treated softwoods. It does vary, though, so you'll need to look up the species available in your region.
    – Bob
    Commented Dec 31, 2018 at 16:15
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    You could also consider redwood: homeguides.sfgate.com/redwood-rot-99622.html
    – rrauenza
    Commented Dec 31, 2018 at 17:12
  • @rrauenza I don't have a selection of timber, I'm mid pacific
    – Kilisi
    Commented Jan 1, 2019 at 3:33
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    Not sure what is available in your area, but you may be able to find used wood that may serve your needs. Things like railroad ties, dock pilings, and utility poles will resist rot and are often replaced before they fail, but may still have enough life left in them for your needs. And they are often inexpensive.
    – YLearn
    Commented Jan 3, 2019 at 6:47

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You could build big concrete posts, maybe 12" in diameter, up about 12" above ground level, and use metal post bases to keep the wood off the concrete. Be sure to treat the end grain and base of the post with the best preservative you have available, and it ought to hold up pretty well.

In the comments you mention you're in the mid Pacific. Without pressure treated available, keep in mind that some species are far more rot resistant than others. The people selling lumber may be able to help suggest what will work best. Farmers usually know, too. Some of the most rot resistant woods are tropicals that would be very expensive exotics where I am, but they may be affordable where you are.

Sometimes small utility poles are at a decent price, they are treated for direct embedment in post holes. Setting a pole is doable with nothing more than a shovel if you can hand dig the necessary depth.

simpson post base

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  • Good idea, I'll look around see if these base plates are available, save building a whole frame out of pipe.
    – Kilisi
    Commented Dec 31, 2018 at 12:55
  • I've been trying to find out about these, and they seem great for holding weight, any experience with sideways force on them. The Fort will hold up one end of a big 10 foot swing.
    – Kilisi
    Commented Dec 31, 2018 at 14:38
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    @Kilisi - the one in the picture is from Simpson, there are other similar brands. They should have all the engineering data on the company web site. Some are made of heavier gauge metal than the one pictured and may tolerate bigger shear forces. The orientation of the bracket may also make a difference - you might want the flanges in the direction of the force the swing would generate.
    – batsplatsterson
    Commented Dec 31, 2018 at 15:05
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    @Kilisi shear force probably isn't a concern; if your posts will be bending, then you'll need to pay extra for hardware rated for moment resistance.
    – Spencer Joplin
    Commented Jan 1, 2019 at 0:55
  • If I can find these brackets I'll weld something thicker and longer to go into the concrete onto the bottom of them I think
    – Kilisi
    Commented Jan 1, 2019 at 3:16
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The 3" pipe in concrete will last for generations in contact with the ground or in poured concrete. Remember this, your kids will tire of the fort in a couple years so I would not make it too permanent. We all want to build something for our kids to have fun with, but as they grow up their likes and abilities change. (think computers and the opposite sex). Both my son and I have been where you are at. my 2 cents.

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    Good answer, I got 4 kids already and one due any time now, and not finished yet, so I expect many many years of usage.
    – Kilisi
    Commented Dec 31, 2018 at 12:38
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I heard of a guy "frying" wood in wax, and I plan to try it someday. Paraffin wax is cheap, too. I plan to sand the posts smooth, treat with preservative as suggested, and then hoist the post over a pot of hot wax. The water should leave the wood when hotter than 212 F/100 C, and then the wax should soak in as it cools.

It sounds like a lot of work to me, but I like that sort of thing, and it really ought to last longer than I'd ever need.

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  • too much effort, and no idea where I'd get the stuff, I'm in the middle of the Pacific, not much choices and nothing is cheap
    – Kilisi
    Commented Jan 1, 2019 at 3:13
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You could try burning the part that goes below ground. There is evidence to support scorched wood being able to resist rot.

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You could use cheap cinder blocks or bricks as a quick & easy foundation, similar to a backyard shed, to keep your wood off of the ground.

This could vary of course depending on stuff like the water table level, soil conditions...but in general you would just dig out holes, being only 8’x8’ the four corners is all you’d need, drop some leveling stone in, and then place your cinder blocks and get them level. Backfill with more stone, or quickcrete, and do your best to get good compaction.

brickfoundation

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  • No idea where the mid west is, I'm in the mid Pacific, everything is imported and usually junk nobody wants and very expensive, so my 4 by 4's for example cost me the equivalent of 60 USD each because I got them on special and none of them are straight from end to end. How would I attach my timber to cinder blocks? I do have some but the fort has a big swing so needs to be solid for sideways forces as well as hold up weight, hence me wanting to concrete them into the ground. The swing has a metal aframe on one end so that's fine, but the other end is held up by the fort.
    – Kilisi
    Commented Jan 1, 2019 at 5:58
  • The cinder blocks I proposed are to keep wood off the ground due to your rotting concerns, not for swing stability. For that, for securing the swings, you will want to anchor it DOWN because it will want to pull UP and OUT. They make swingset ties for this. Or just fasten the wood frame to the cinder blocks with a hammer drill and concrete fasteners. Coming from a civil engineer, if you think you’re building something big enough that you have doubt over its structural integrity, do some more research first and then consider the construction methods.
    – Pipeliner_USA
    Commented Jan 1, 2019 at 6:54
  • not a bad idea, but no hammer drill or the rest. I have considered the integrity, which is why I'm concreting in the legs somehow. But can't get too fancy, just want solid and lasting. Swingset ties? Not available, I have to use what is around, which isn't much, repeat I'm not in the USA or first World and I'm on a budget.
    – Kilisi
    Commented Jan 1, 2019 at 7:32
  • I looked up cinder blocks and found they're different from concrete and none here. I do have 100 concrete blocks which I bought a couple of years ago and haven't used yet, but don't think this is the right solution for my project.
    – Kilisi
    Commented Jan 1, 2019 at 15:14
  • This works very well . It can actually be simpler/cheaper; I leveled my sandy soil and set cinder blocks directly on it. Set to keep wood about 2 " above the soil . My 8 X 12' shed is over 15 years old and shows no wood deterioration - warm ,wet location. No anchors were necessary , the weight of the shed holds it down; If we have a tornado - well it is just a garden shed, not a bomb shelter.
    – blacksmith37
    Commented Jan 1, 2019 at 20:34

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