Joists for a treehouse

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Forum topic by gbarron1 posted 03-29-2013 12:58 AM 11875 views 0 times favorited 11 replies Add to Favorites Watch
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9 posts in 1938 days

03-29-2013 12:58 AM

I’m building a free standing treehouse out of mostly recycled lumber from a deck I had rebuilt last year.
It will be a 16×16 deck with a 12×12 fort atop the deck. Probably 6 feet above the ground.
I’m building over a 9×12 concrete slab that was just sitting there when I bought the house and I’ll dig post holes and use concrete footers to complete the 16×16 footprint.
Most of the 2×6 deck boards were salvaged but the 2×8 joists were hauled away. Are 2×6’s sufficient for joists?
How about 4×4 posts?


11 replies so far

View RonInOhio's profile


721 posts in 2892 days

#1 posted 03-29-2013 01:24 AM

If it were just a deck 2×6s would probably be ok. Though 16 feet is quite a span. With the 12×12 structure built on top I would probably use 4×4s . Not an expert though so take it for what its worth.

Seems like a long span for just having perimeter footings with nothing but the joists supporting the span and the additional load of the structure on top. Do you get snow ?

Maybe you could use (4-6) 4×4s to span under the ends and underneath the part that the fort will rest on and use 2×6s for the perimeter joists around the fort. Or you could use doubled 2×6s.

View Dallas's profile


3599 posts in 2515 days

#2 posted 03-29-2013 01:52 AM

This would be a great question to ask over at Home Refurbers! Your same ID and password will work there too.

-- Improvise.... Adapt...... Overcome!

View devann's profile


2246 posts in 2720 days

#3 posted 03-29-2013 04:53 AM

gbarron1, A drawing or diagram showing post layout and span would help. What size are your beams?

1st. question is no.

2nd. question no.

Personally I don’t like to use anything smaller than a 4×6. I’ve had 4×4s twist to many times and they’re a pain to replace.

-- Darrell, making more sawdust than I know what to do with

View Tim's profile


3812 posts in 1989 days

#4 posted 03-29-2013 04:15 PM

I don’t have a degree in this, I just spent a ton of time looking at these tables and equations for maximum span.

If used as a joist, 4×4’s would actually have approximately the same span and load capabilities as 2×6’s according to the tables in the wood structural design data publication. It’s a little old, but wood itself doesn’t change much. The depth of a joist matters a lot more than it’s thickness, a 2×8 is a lot better joist than a 4×4 but it’s almost the same amount of wood for example. Basically whether this would work depends on how far the actual span is between supports and what else you support it with, etc. The table on page 256 in that document gives a maximum floor joist distance of 6’1” for 24” joist spacing and the weakest wood to 12’11” for 12” joist spacing and the strongest available wood. Salvaged outdoor deck wood also couldn’t be counted on to have the same strength as new. It would take a lot of work to figure out that document and know the appropriate safety margins to build in, so I agree, home refurbers is a good place to ask. I couldn’t tell you if that load (50 psf) is appropriate for example.

View rustfever's profile


752 posts in 3338 days

#5 posted 03-29-2013 04:31 PM

You could use 2×6 is you were making a temporary trampoline. 2×6 will be very ‘Springy’ until they fail.

-- Rustfever, Central California

View gbarron1's profile


9 posts in 1938 days

#6 posted 04-02-2013 02:45 AM

Thanks for the input, everyone. I’m going with 6×6 posts. i dug post holes around ghe slab to accommodate posts spaced at roughly 4” around the perimeter. Is this overkill? Doubled 2×10 beams over either a notched posts or caps.
2×8 joists with hangers spaced at 16” oc.
I’m thinking of either or both beam to joist cross and 2×4 cross braces under the joists.
All thoughts are welcome.


View Grandpa's profile


3259 posts in 2703 days

#7 posted 04-02-2013 02:50 AM

as a floor joist in a house a 2×6 will span 6’10” and a 2×8 will span 14 feet. That is 16 inches on center on both of them.

View stan3443's profile


301 posts in 2303 days

#8 posted 04-02-2013 02:57 AM

it takes a 2×10 to span 16 feet

-- If your not supposed to have hair on your face......why does it grow their

View gbarron1's profile


9 posts in 1938 days

#9 posted 04-02-2013 03:23 AM

Thanks. Since I’ve already bought the 2×8s and the framing for the fort, will a narrower joist span do the trick with some additional joists? I could go with a 14” span but I’d prefer to stay at 16 if only for the space to work on while building the fort.

View scotsman9's profile


134 posts in 1916 days

#10 posted 04-02-2013 03:25 AM

For a non living space 2×6 joists will do just fine. You need to alternate the joist runs, and use a center post support. And use the proper joist hangers

-- Just a man and his opinion.

View gbarron1's profile


9 posts in 1938 days

#11 posted 04-03-2013 02:53 AM

Thanks but I’m using 2×8 joists. And I was planning on spacing the posts roughly 4” apart in all directions imcluding the center – 20 posts in all. Seems like overkill but I thought it would provide more stability.

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