Crushed Limestone Foundation

Has anyone used crushed limestone as a pad for outbuildings before?  Working on a design for a new herb processing outbuilding, and wondering if I can park my metal-clad SIP structure directly on packed crushed limestone.  My resource suggests verifying there won't be any moisture problems.  They usually put them directly on concrete, so I would think crushed limestone would be advantageous, as it is water-permeable.

If this works, I'll roll out trials for mycelium-core, wood-clad SIPs, as well as site-fabricated cross-laminated timber.  I wonder if the wood would benefit from some low skids to allow a tiny bit of airflow underneath.

If blow-away is a concern, I can anchor to ground screws from below.

Will this work?  What am I missing?

What is crushed lime? Limestone or crushed lime based mortar or just lime from the bag that got partially hardened?

Cristobal Cristo wrote:What is crushed lime? Limestone or crushed lime based mortar or just lime from the bag that got partially hardened?

Crushed limestone.  Available cheap as a byproduct of mining and processing where I live.  Less energy is intensive than concrete or lime to produce, as it's literally just limestone crushed to maybe an eighth to 3/4 inch, depending on your needs.

Typically you'd excavate down to undisturbed subsoil, fill with crushed limestone, wet it, and run a vibrating plate to compact.  

My impression is that it is advantageous to concrete in several ways:
1) far lower carbon footprint
2) the resulting slab is moisture-permeable, playing better with precipitation and water table stuff
3) it's not solid, so it will readily self-heal, rather than crack like concrete
4) far cheaper, usually

Will you make it level with the ground or try to somehow elevate it, so it would be protected from inflowing dirt?

I've used crushed limestone for foundations for an outbuilding, rain water tanks and retaining walls. For the outbuilding I did pour a concrete perimeter grade beam. If I didn't do a grade beam I would put the structure on skids.

Aaron Yarbrough wrote:I've used crushed limestone for foundations for an outbuilding, rain water tanks and retaining walls. For the outbuilding I did pour a concrete perimeter grade beam. If I didn't do a grade beam I would put the structure on skids.

Aaron, thank you, exactly what I'm looking for.

Can you tell me more about your reasoning for the concrete perimeter grade beam?  I am wondering if it is to provide more reliable contact and unsettling, uniform support, as well as the opportunity to anchor the structure to the beam.

My questions:
Are there different ways that moisture behaves with a concrete perimeter grade beam than it would for a curshed limestome slab?
The reason for the skids: Moisture-related?  Structural?

Cristobal Cristo wrote:Will you make it level with the ground or try to somehow elevate it, so it would be protected from inflowing dirt?

Planning to make the limestone pad just a couple inches elevated from grade to assist with incidental runoff.  The floor-SIP level is 6 inches width, so there will be a single step up to enter the strucutre, or possibly a ramp.

Here in N. Ohio, we have:

-57 limestone.   Which is limestone pieces 1/2" to 1' in size. It is clean and free of limestone dust or powder. It's used for footers under cement walls or pads. And for driveways. It's clean and doesn't track into buildings.
-411 limestone.  Which is limestone no larger than 1" on down to limestone dust or powder. It's especially good for packing into voids and crevasses. It's "dirty" and messy and tracks everywhere if you're not careful.

Limestone has edges and angles, so it doesn't tend to slip or slid, like regular gravel can do. It's also generally cheaper than gravel. 411 limestone packs harder than 57, almost like cement, but will wash away if exposed to water.

I've moved a lot of buildings. Sometimes without having much money. Generally speaking, a cement footer is best and lasts the longest. But, 411 is quite solid below ground, if you tamp it. It can spread at ground surface as the surrounding dirt expands and contracts from getting wet or from freezing. 57 doesn't pack as solidly. Sometimes 411 is cheaper than 57, but not by much. What I have done at times is to dig down to the solid clay layer and then pour in 411. Then tamp it hard. I then set cement blocks on top of the 411 to rise above the dirt surface, sometimes one block above surface. But for my purposes, two blocks, so I am high enough above the dirt surface to be able to mulch heavily. I pound in rebar every so often then fill the blocks with 411. If you look on Craigslist or Facebook Marketplace, you can find used block for much less expense than buying new at Lowes. Usually, I'll put two layers of asphalt roof shingles on top the blocks, to water protect the wood sill plates of the building foundation. Pack the dirt back in around the block, and make sure the drainage is away from the foundation, and you're good to good. One other thing is getting the whole thing square and level. I have all kinds of water tube levelers, transits, and such. But the easiest way I have found is to just pour in water to your ditch. Then pound in wood stakes every 5' or so. Figure out how high out of the ground you want your block to come, then mark your sticks all the way around, starting at the shallowest water part of the ditch. For me, the least amount of 411 I use is 5" deep. More is better.

Jim Fry wrote:What I have done at times is to dig down to the solid clay layer and then pour in 411. Then tamp it hard. I then set cement blocks on top of the 411 to rise above the dirt surface, sometimes one block above surface. But for my purposes, two blocks, so I am high enough above the dirt surface to be able to mulch heavily. I pound in rebar every so often then fill the blocks with 411. If you look on Craigslist or Facebook Marketplace, you can find used block for much less expense than buying new at Lowes. Usually, I'll put two layers of asphalt roof shingles on top the blocks, to water protect the wood sill plates of the building foundation. Pack the dirt back in around the block, and make sure the drainage is away from the foundation, and you're good to good. One other thing is getting the whole thing square and level. I have all kinds of water tube levelers, transits, and such. But the easiest way I have found is to just pour in water to your ditch. Then pound in wood stakes every 5' or so. Figure out how high out of the ground you want your block to come, then mark your sticks all the way around, starting at the shallowest water part of the ditch. For me, the least amount of 411 I use is 5" deep. More is better.

Thank you, Jim.  Illuminating.

Do you infill the void under building center with the blocks?  Or leave it vacant down to the 411?  I will need to provide some support for the floor at at least one run, midway, perhaps two.  These metal-clad sips are structural, but they can sag under weight if unsupported.  

Beau M. Davidson wrote:Can you tell me more about your reasoning for the concrete perimeter grade beam?  I am wondering if it is to provide more reliable contact and unsettling, uniform support, as well as the opportunity to anchor the structure to the beam.

Yes, all those things, but primarily to anchor the building to the ground, create separation from the ground and to make a good seal between the foundation and the building to keep out insects and other creatures

Beau M. Davidson wrote:Are there different ways that moisture behaves with a concrete perimeter grade beam than it would for a curshed limestome slab?
The reason for the skids: Moisture-related?  Structural?

Concrete wicks water where gravel sheds water. However, over time the gravel will get infilled with sediment reducing its ability to drain.

Either way, I think installing a sill gasket under your sill plates would be wise.