Concrete slab over sandstone?

[Paul Becker]

Hi, I'm going to build a small (11' x 9') shed in my back yard and have some questions about pouring a concrete slab for it to sit on...

The dirt is only about 4 inches deep, then I hit solid & relatively flat sandstone. The sandstone is soft enough that I can gouge a groove in it fairly easily with a shovel. I have dug out a 11.5' x 9.5' area down to the sandstone - the original idea was to get a flat surface for the slab. Here are my questions:

Would it be a bad idea to pour directly onto this sandstone bedrock, or should I have some sort of gravel or something on top of it first?

The sandstone is soft enough that a masonry bit can drill a 6" or deeper hole fairly easily into it, so should I do this on some sort of grid pattern and put some anchor bolts in there prior to pouring the concrete to help anchor it to the bedrock?

Thanks for any help, Paul

[Paul Furman]

[Roger]

Sandstone, providing it is porous - you can test that - makes a wonderful base. This will be the "Shed of Ages".

[chibiabos]

Man, I wish I knew a detailed answer to your question but, assuming there's no outrageous decomposition going on with the sandstone, you gotta figure it's been there for a couple million years at least. If it were expanding or contracting you'd see righteous cracks. Absent those, I think it's not going anywhere anytime soon and you have very little to worry about.

Does your house sit on the same stone? If so, how did they bed that structure?

-chib

[Paul Becker]

Good points, and I must be stupid, because yes, my house is only about 30 feet away and is a slab foundation. For the much newer houses nearby with basements, they had to blast (which apparently did nothing to my foundation or the visible part of the sandstone).

Concrete on sandstone it will be, then.

Thanks to all who replied. Paul