# 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. – JPhi1618 May 6 '19 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 '19 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! – Gnudiff May 6 '19 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. – Alaska Man May 6 '19 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. – Gnudiff May 6 '19 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.