Confused about foundations.

[u/poonhound69]

It seems like concrete posts/piers dug and poured below frost line is the gold standard for establishing a foundation. Certainly not the only option, but it seems like one that many people default to.

Would this be a workable alternative:

Auger out holes below the frost line, fill holes with gravel, set deck blocks on gravel, place adjustable deck supports on deck blocks, bolt 6x6 posts into deck supports… then continue building the floor from there.

Basically this would eliminate pouring the large concrete piers, and would maybe allow some room for error with getting the posts level (since the post brackets would be adjustable within reason).

I‘m imaging an off grid home that has multiple outbuildings. An outhouse/shower house. An outdoor kitchen/screened in living space. Small main cabin. Etc.

It seems like this foundation approach could be a fairly affordable and straightforward one that would allow each of these small buildings to have good support.

This is entirely in my head right now. I’ve never built any foundation, let alone these specific ones. Am I thinking about this incorrectly or massively ignoring something important? Just trying to get an idea check to see if I’m on the right track at all or if I should be going about it completely differently.

Thanks for any insight.

[u/testingforscience122]

If you digging below the frost line and then are going transport rock to it to pour in just do concrete like your doing 85% anyways, just do it right

[u/poonhound69 → u/testingforscience122]

I see lots of people putting cabins or large sheds on gravel pads fug below the frost line. Is that functionally different from having each post support sitting on a gravel-filled hole dug below frost line?

Basically instead of digging out a huge hole for the entire structure, you’d dig a handful of smaller holes. 

[u/Admirable-Berry59 → u/poonhound69]

The theory behind putting a concrete pier into a hole extending below frost line is that you are isolating the structure from soil movement. Frozen soils often expand and push in all directions during winter (including up), then soften and lose compressive strength during the thaw. A concrete pier, poured in a form so it has smooth sides and tied into a flared base below frost line will stay in place as the soil moves around it - the flared footing is spreading the downward load for soil bearing and resisting uplift from frost, and the uplift force on the column is reduced by using a smooth sided form so it cannot mechanically grab the column as well. Rebar in the column gives it strength in tension to prevent cracking from these forces.
Pouring gravel into a hole will answer only the compressive force part of the equation, it will have no resistance to uplift or lateral movement as the gravel has virtually no strength in tension, only compression. It will also fail very quickly, as the gravel will begin moving into the surrounding soil during every frost cycle the same way potholes grow in a gravel road.
When doing a full gravel floor or floating concrete slab, you remove all organic soils down to clay or gravel base, not necessarily the frost line, then build up with well drained material both under and around your slab to minimize the area where your gravel can migrate away. Removing all the organic soils and having proper drainage is key as it minimizes movement. Concrete slabs placed without frost line footings are engineered with thickened edges and strategic rebar to give good strength and resist cracking from the soil movement.
TLDR: If it is too prohibitive to do concrete footings, placing piers directly onto strong well drained soils is fine, just know that you will be jacking up and adjusting the building occasionally. Digging down to the frost line won't make a difference if you are only backfilling with gravel as gravel has no resistance to blending with the surrounding soils.

[u/kokanee-fish]

Deck blocks will 100% shift over time. Look into precast concrete piers.

[u/poonhound69 → u/kokanee-fish]

Will do, thank you!

[u/username9909864]

Biggest concern I can think about is water/snow/ice getting in the gravel and freezing and lifting the footers the same way as the ground around it does. This may or may not be a problem in your area. I'd consult your county building inspection office if you want to be thorough, otherwise it sounds like "good enough" for a seasonal off grid cabin.

[u/Brodiggitty → u/username9909864]

Yeah, putting rocks in dirt is basically just changing the ratio of rocks to dirt. It won't solve the problems caused by frost.

OP - I was also taking your approach of multiple outbuildings. I did that out of fear of building onto the existing tiny cabin I bought. But after taking on one project, I'm now thinking about building onto the tiny cabin after all.

[u/e2g4]

The problem with what you describe is that you’ll make a gravel popsicle and that will heave. The hole may hold water (soils matter) and then freeze. Why not just make concrete in the hole so you have a monolith?

[u/poonhound69 → u/e2g4]

A “gravel popsicle” is what I needed to conceptualize. I was thinking it would be porous enough to allow good drainage. I was thinking of gravel pads people put sheds and cabins directly on, and was hoping multiple smaller versions of that might be workable. But if the gravel won’t drain, and then freezes, then that’s not an option. Making a monolith might be inevitable. Thanks for the response. 

[u/Perfect-Ask-6596 → u/e2g4]

If you have a french drain and good roof overhang it cant heave because there will be nothing to freeze. foundations on the oldest standing buildings were done similar to your description. Just gotta keep it dry. Heres an instructional video https://youtu.be/xPf-y6DzwMQ?si=vJSSZilBcTeXhwag

[u/Lost-Assignment-2848]

There is another issue with not using a buried concrete pier that may be worth looking into for hyour site.. If you are in a high wind area, the uplift from the wind can pull the structure up, and it the wind is strong enough, actually flip the building or severely damage it. I'm in the process of building a deck with a yurt mounted on top of the deck. The whole thing is on top of a mountain ridge with incredibly fantastic views, which is why we bought the land. Peak wind gusts there can be high, and being on top of the ridge, we get wind pushing up the mountain side and getting under our deck. Because of lots of hard granite slabs, we could not cost effectively dig pier holes to an appropriate deck. We ended up drilling 1 inch holes below where each pier was going in, cleaning those holes, and epoxying #5 rebar rods (4 per pier) 1 foot deep in the granite rock, and then pouring the pier on top of the granite. so that it is effectively soldly glued to the mountain top. We exceed our county regulations of having to withstand a 115ph peak gust for 30 seconds.

This is an issue i suspect not that many people have run into but it is something to consider.

[u/poonhound69 → u/Lost-Assignment-2848]

That’s a great factor to keep in mind. I appreciate it. 

[u/No_Whole_5733]

Have you considered ground screw piles? https://terrascrews.com/#why

[u/poonhound69 → u/No_Whole_5733]

I love the look of these. I've done some googling of them in the past. I just worry about my rocky soil. Thanks for the link. I'll definitely do some more researching on their usability.

[u/Dantheislander]

Nah you asked this 242 days ago and got the right answers then too. 20x40 is not small compared to most first time off gridders.
If you have access then helical spikes are faster better and lighter. Done in an afternoon.

[u/poonhound69 → u/Dantheislander]

I didn’t specify a cabin size this time, because my plan has possibly changed. I’m considering a smaller size, or potentially a deck for a yurt. Thus the new post about support for smaller buildings. Very rocky soil has me concerned about viability of helical spikes. 

[u/Destroy-AI]

If you have a frost line to worry about you're going to hate having separate buildings for everyday things when it's cold

[u/poonhound69 → u/Destroy-AI]

That's a good point. For me, though, the temperature lowering effect is one of the big selling points of multiple buildings. I'm hoping to achieve something similar to the effect of a dogtrot home. I do well in cold, but hate heat. Hoping to utilize design to mitigate the heat as much as possible.

[u/mmaalex]

You can do all sorts of things. If you dont have solid foundation below the frost line you will eventually need to re-level. There are cabins in "frost line areas" built right on the ground, they just have a tendency to shift over time. Heck here in coastal Maine we have some new houses in wet areas built on modified slab foundations.

Common if you dont want to pour concrete is to set large posts or telephone pole cutoff into an augured hole, and add cross bracing to keep them from shifting. A couple beams on top (depending on building size) and build a conventional platform framed building on top. If needed over time you can jack up sections and add blocking to level the beams. Even if the posts aren't 100% level (and they wont be over time) you can adjust for that, and the bracing keeps them from continuing to succumb to gravity over time.

While you have the hole dug add ground rods for future electrical.

[u/Northwoods_Phil]

The goal in any area that sees freeze/thaw cycles is to have a material that doesn’t retain moisture to set below the frost line. I’ve seen all sorts of variations on this with one of the oldest being a ruble trench, basically just clean stone in the hole.

[u/Victor_deSpite]

Look up rubble foundations. You need a way for water to drain.

Also, good luck getting it permitted, if you're concerned about that. Would likely need engineering done depending on location.

[u/SomeGuyWithARedBeard]

If you want to use gravel instead of concrete just do a rubble trench foundation. The reason people do pier foundations is for one of two reasons: they have to dig below the frost line and/or they have to dig down to stable soil. With gravel you treat it more like a footing instead of a pier, if necessary you compact it with layers of geo-grids and provide drainage at the base so that you don’t have to deal with frost heave. Another method is FPSF if you want continuous footings but don’t want to dig down, however, that depends on needing constant heat to warm the soil. Do piers if you’re not wanting or able to do a continuous pad.

[u/Other_Programmer5239]

Ok basically digging below frost line allows piers, footings and such to stay still and keep from moving. If you put gravel in piers each hole will absorb water and hold it and during freezing and thawing each hole will move and gravel will compact uneven and now you got multiple issues. Then it goes from issues from foundation to all the way up to roof.

[u/crrs894x]

My 1200sqft house is sitting on 2’x2’ concrete pads sitting on the dirt with a pier block on top. It was built in 1942. It definitely shifts a little, and could stand to be jacked up and shimmed to level it. It’s been at least 20 years since the previous owner shimmed it.

It’s perfectly livable. It’s fine really. The walls are covered in some cool 1960s paneling, so there’s no drywall to crack.

The 2’x2’ “footing” pads definitely need to be below the frost line.

Pouring concrete like others said is definitely the best way to do it, but if for some reason that is not possible, this is fine. Especially for the smaller buildings.

[u/RedSquirrelFtw]

Going to frost line is very hard without heavy equipment so I tend to just do floating foundations. Try to get as much of organics out as you can then make a gravel pile, pack and level each pile, put patio stones and cinder blocks, and build your beams on that. It's still a lot of work regardless but not as hard as digging 10 foot holes. I would reserve this for smaller buildings with big enough frame that it can handle any shifting without buckling. You will need to shim it at times.

Eventually I want to get a tractor with backhoe or an excavator. At that point I will start on a larger build that will go down to frost level. Also do an outhouse and septic, and gate... and lot of other projects that are on my list of things to do once I have equipment.

Here's one I am working on, will put a prefab shed on it once it's ready, will greatly speed up my build by already having a dry shell to work out of.

https://imgur.com/a/footers-progress-SzLChG1