Foundation for back side of "treehouse"

Started by jcr122, March 14, 2017, 07:04:23 PM

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jcr122

Hello there! Long time lurker, first time topic-starter.

I'm building an 8x16 "treehouse" cabin this summer in SW Washington and am looking for feedback on my design.  It's really only a "half-treehouse" because the back will actually be supported by an 8 - 12' long foundation and short stem wall, while the middle-front and front will be supported by twin beams spanning two 3' diameter doug. fir.   Although they're big trees and the anchors will be close to the ground, I'm going to use real Garnier Limb treehouse hardware and treehouse design principles to make sure the beams have flexibility to move under the structure.

Here's some imagery:







My main reason for posting is I am not sure how I want to approach the in-ground foundation.  Because it's only 10-16 feet uphill from the base of the trees, I'm hesitant to dig a proper footing because of the potential for root damage.  I've been looking at helical piles (such as TechnoMetalPost), but it's become clear that will be outside my budget.

My DIY thought is to dig and pour three shallow footings (18" deep or so) perpendicular to the slope and span that with a pressure treated timber/sill plate.

If I integrate a bunch of ground anchors like these http://www.homedepot.com/p/ShelterLogic-30-in-Earth-Anchors-Set-4-Piece-10075-0/202719050 below the footing and at 45 degree angles into the slope and tie everything together with rebar, do you think that will be enough to keep the footing from heaving or sliding down the slope over time?  Here's an image of the uphill side with that idea:



Anyway, I'll listen to any feedback, but the foundation is my big head-scratcher right now. 

P.S. the frost line in this area is 12" which seems kinda shallow for my experience of the climate...

ChugiakTinkerer

Welcome to the forum!

I take we're way outside the constraints of the residential code here, so help please help us understand the basics of treehouse design.  The structure will be supported by the two fir trees and will be subject to movement, however slight.  The beam at the far end of the house must support about 40% of the load and will not be moving with the wind.  Do you have a way to anchor the house to the beam (or sill plate) that will allow for lateral movement when the trees sway?

Of the two foundation designs you show, I favor the one continuous wall.  If you construct it with a proper footing it will be well anchored and will not move downslope.  I would suggest building it no longer than is needed so that the structure will shield it from most of the effects of weather.   But roots does indeed pose a problem.  Perhaps in this case a couple of concrete pilings would work.  Then again, the whole slope may be a mass of roots.  What about keeping the support above ground?  Maybe use cribbing set on top of a gravel pad.



For the structure itself, the cantilever overhang in front leaves the front right corner unsupported.  If you are planning on using 16' joists running lengthwise, you may need to double up on that corner to keep it from bending down.  Or else run girders the length of the structure and set your 8' joists crosswise on top of the girders.

It looks like a fun project and it's good to do everything you can to ensure it is built safely.  Wind uplift probably isn't a big concern at that site.  How about the trees?  Is it worth investing some money in a professional to come out and assess the health of the trees?  Both the two you want to use for support as well as the others in the vicinity?
My cabin build thread: Alaskan remote 16x28 1.5 story


rich2Vermont

This is awesome, and timely, for me. We've been looking to do something not unlike what you've shown here, building a small bunkhouse treehouse. Our problem is that we don't have real good trees for TAPs. There is one group of 3 trees, but 2 are hemlocks which are not great for treehouses. Instead, I've looking at separate beam supports using the Diamond Pier foundation system(www.pinfoundations.com), which seems ideal for creating a decent anchor without disturbing roots. Using cement isn't an option for us, so I'm interested to see how things develop for you. Good luck and welcome,

Rich

cbc58

I am also considering a tree house and have a similar type slope.  Was planning on concrete piers where needed and don't see a problem with them tilting/leaning/slipping down slope as long as they go deep enough.  You could use those anchors or perhaps some rebar at various angles (up slope/down slope/left and right) for peace of mind.  You could also dig deeper on the up slope side and create a hook-type footing for added grabbing power.

I had the opportunity to stay in a tree house through airbnb and it was very cool.  There are many for rent on that site and they have multiple pics - some showing what type of foundational support they have. 

jcr122

Thanks for all the feedback everyone!

Yes, I don't expect this to fully meet code but I would like to build with code in mind wherever practical. 

ChugiakTinkerer, yes, the idea would be that the up-slope fixed foundation and the structures connection to the sill plate would be "immovable."  The plan would be to make sure the areas where the tree beams intersect with the joists had metal on metal contact, so if the beams move, they don't cause any push or pull on the joist or structure above and thereby the upslope footing.  The other point of flexibility would be from the use of Garnier Limbs (http://treehouses.com/joomla/index.php/construction/garnier-limb-parts) where the beams anchor to the tree, so it's even less likely that there would be any tug points between joist and beam.  Here's a picture of how those anchors work.



You do bring up a good point about the overhang.  I oriented the structure to the view, but hopefully I can counter-clock it a bit in reality so the cantilever is less pronounced.  Your idea about double joists on that end is a good one. 

It does get pretty windy here, so I'm scratching my head about whether I need to worry about everything lifting off the beams and then shock loading.  Maybe cables from underneath into the ground?

I don't think I could feel good about an above-ground foundation just because of the worry of things sliding down-slope.  Digging down enough to make things level, I would already be cutting through roots, so I would probably end up going all the way down to a footing in that case.  I may just have to risk it and give the trees a big hug afterward to say I'm sorry.

Speaking of tree health, we've had a very harsh winter this season, so I like to think any trees or branches that were in bad shape would have gone down this year.  If I start worrying about all the nearby trees that could wipe this thing out someday, I'll drive myself nuts. 

cbc58, I like your hook footing idea.  That plus going deep into the slope like a snow cave + ground anchors may be what I do. 

This mix is getting complex enough that maybe I should just turn the whole thing sideways, forget the concrete and keep everything in the tree...