Hybrid Concrete-Free Slab and Pier Foundation for Shed
I’ve been pouring over the concrete free slab articles and am really intrigued. I am building a 16×24 shed and propose a sort of hybrid system. Since it will not be permitted I am not sweating every detail.
The perimeter will be made up of pressure treated 4×12 lumber supported at points by poured concrete piers with embedded Simpson post brackets. This “beam foundation” will be close to the ground but not in direct contact with moist earth. I could then fill with the proper layers: drain rock, screed layer, foam, vapor barrier, sleepers, and subfloor.
Does this sound like a reasonable plan? The main concern is that the perimeter is not sealed like it is with a concrete perimeter foundation so bugs do have a way into the center. Should this be a concern?
The other option would be to run joists between the perimeter beams for a more traditional system. Thoughts?
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If you are in a cold climate you will have frost movement issues with this design. frost protected piers and unprotected "slab". You could possibly mitigate this with foam. look up frost protected shallow foundations. But for a shed this it going way too far. There are much easier ways to do this
What would be your "much easier way"?
The supports would be in concrete below the frost line. We don't have a big problem frost movement here in 4c.
That sounds fine then, as far as frost heave. It wasn't clear what kind of piers you were using. You could also use helical piles to get away from concrete completely.
helical piers sound great but I need a machine to drive them? and what happens when you repeatedly hit medium sized stones?
How will you use this building? Will it be heated. How is your budget? What about the soil drainage.
The building will be multipurpose but mainly a small wood-shop. It will not be heated only when needed. I'm considering installing a small wood burning stove.
Budget is flexible but the idea is essentially as cheap as possible while not skimping on necessary details.
Drainage at parent soil is not great, but drainage in the assembley after adding rock would be very good.
Joe, I've used a similar system, without the sonotubes, for garden sheds. It worked well enough for that but if you're going for a bug-free sort of enclosure I would stick with a more traditional approach.
Does bug migration really pose any risk? I'm not sure this is a problem. I just see it as a potential possibility since the perimeter is open.
I think critters of various descriptions would be the main challenge for this design overall. Carpenter ants and termites can be destructive; most others are merely annoying. But then there are borrowing mammals who might dig under. I've gradually been adding more and more galvanized hardware cloth around my shed to stop the chipmunks and groundhogs. The groundhogs have moved their headquarters to under the wood pile instead, but the chipmunks are still finding ways to get under the shed. And the mice find ways in too, although they probably always will.
I think concrete is a necessary evil for the time being. Think long term... if the building has critter issues or what will the future uses be? future owners uses? You could DIY a slab like this with a little bit of research, recycled xps/eps... not a bad option
I do not think using all those manufactured products are very green. I think a concrete slab floor is not the end of the world. I built a workshop (16X20). Pressure treated post on concrete footings. A pressure treated skirt board all the way around. Inside filled part way with processed gravel. Then a concrete slab. Use long eaves, and all back fill should be gravel. If the outside grade is higher than the bottom of the skirt board, it discourages critters. Thirty years and no problems.