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Charlie: Larry writes to us, “In a recent segment on This Old House they were building a new house in New Hampshire for a disabled war veteran. They put in 12″ of sand before placing the concrete for the foundation. Would sand be as good or better than structural fill for slabs here?”
Tom: What they’re doing there is they’re just putting filler. Sand is always the best way to go. You do put sand under slabs here too as long as the soil is okay. If you have to bring in structural fill, what you usually end up doing is taking soil out, bringing structural fill back in so the slab can sit on that because the slab was designed for it.
Now let’s say you do piers in the ground. You need to bring in structural fill, but you have to bring the house up a foot or two so the top of the concrete, your finished floor is high enough so it doesn’t flood. You would end up using bank sand under the slab for that. It has its purpose. Up in the north area they don’t have the expansive clays and stuff. They’re digging very deep because what they have to do is go down below the frost line. To fill up and just fill it up cheaply, sand is the best way to go.
Charlie: Bank sand is a sharper sand?
Tom: It’s actually a cheap sand. It’s a dirtier sand.
Charlie: Okay. All right.
Tom: As opposed to real fine sand.
Charlie: It hasn’t been rounded off in the grading process.
Tom: Yeah, it’s pretty much just dug out of the ground.