How to calculate sizing for treehouse footing?

I would like to build a small free-standing treehouse without digging into ground and/or pouring concrete.

Here is what I have in mind:

It is, of course, for kids, so I don't think I need it to be able to support more than ~100kg (220lbs) with the reserve. I made it more wide than tall in order to reduce the instability, but what kind size poles I should need for to support both the overlying deck and house+roofing constructions + kids? [material is most probably pine for everything, as it is the only which is readily available, the blue timber is assumed to be pressure-treated]

The width of the house I had in mind around ~2.5m (8ft).

• With this outside in the weather, I would use pressure treated for everything unless you're going to have a somewhat water proof "house" area with paint, etc. May 6, 2019 at 14:49
• How wide is it in the other direction (13' x ???). You can fit a lot of kids on a 13' span, even if it's only 5' wide. And kids are pretty much the definition of live loads - 5 50 pound 8 year olds already blow past your 220 lb assumption. Then they start jumping. Then they jump off the roof onto the decking. Then they bring up some buckets and fill them with water. Plan conservatively.
– Drew
May 6, 2019 at 15:18
• @Drew Thank you! I had in mind around 13'x8'. And in normal times it is only 2 kids who will use it, one currently at around 40lbs, and the other around 88 ~= 130lbs. There could be cases when guests come, of course, thanks, I will have to consider that! May 6, 2019 at 16:17
• @Gnudiff The footing for a tree house is a TREE. ( meaning a tree that is still standing and not a tree that has been milled into lumber ) If it is not in a tree then it is not a treehouse, it is playhouse. May 6, 2019 at 17:36
• @Alaskaman Probably. Forgive me, I am not a native English speaker. Plus, of course, the house most likely will have a tree sticking somewhere through the deck. I am just not going to use the tree for any kind of support. May 6, 2019 at 18:23

Hmmm...no digging in the ground and no concrete.

Sounds like the four perimeter beams will be your foundation. They’ll need to be treated for ground contact (pressure treated) as you indicate.

Your total load will be about 220 lbs. (Live Load) as you indicate, plus about 800 lbs. in lumber and material (Dead Load). I did not add snow load as I’d assume if there’s snow, then there’d be no “kid” load.

If you don’t remove the topsoil, your soil bearing value will be about 200 - 400 lbs. per square foot.

Assuming a total load of about 1,020 lbs. distributed at the perimeter: 13’ x 13’ x 4” wide beam for a footer allows a total footprint of about 17 square feet.

So, with a total load of 1,020 lbs. / 17 sq. Ft. = a soil bearing value of 60 lbs. per square foot for 4x beam footings.

Option 1: Use pre-cast square concrete footings at the corners. So, 4 - 16” x 16” footings will give you a footprint of about 7 square feet and a soil bearing value of about 145 lbs. per square foot, which is also very reasonable.

• Thank you! If I take 13' x 8' ft perimeter, would it still work with 4" beams? I am a bit confused whether reducing perimeter will make for enough less dead load, because the live load will stay the same, obviously. Good to know about pre-cast concrete, the issue is that anything with dug-in foundations is covered by building codes, which require tons of red tape to get over in this country. May 6, 2019 at 16:36
• Changing the size to 13’ x 8’ reduces the footprint from 17 square feet to about 9.5 square feet of surface using a 4” wide beam footing. Therefore, the 1,020 lbs. will be distributed over that 9.5 square feet yielding a net soil load of about 107 lbs. per square foot, which is well below the estimated soil resistance of 200 - 400 lbs. per square foot for topsoil. May 6, 2019 at 17:24