What to use under slab?

[Thomas Pender]

I agree with removing the top soil, compaction, no organics, figuring out the code, etc. (Oh - I saw no mention of excavating below the frost line, if applicable.) One thing I would recommend is hiring a professional Civil Engineer to scope out the ground and design the foundation based upon the expected load. After many years of construction litigation, I can tell you that one of the most litigated area is dirt and what it does to concrete - including drilled piers (like for decks), footings, grade beams, etc. You can get lucky and just place 4" of concrete (with rebars on high chairs) over compacted subsurface soil and rock or you can get unlucky - fast and expensive. But, no sensible builder "wings" concrete design - the results are final and expensive.

Obviously, if you have a sandy soil without clay, some gravel and concrete will work fine. But, if you have an expansive clay soil and some clay soils, like in Texas, Lousiana, Virginia, Georgia, etc., can generate more expansive force than dried concrete when made wet, you have to get that clay away from your footings or slab and perhaps excavate for several feet out beyond your margins and then make sure wet does not get into the subsoil.

Best case - a few feet (or less) of dirt over a rock formation (which cannot expand or move) or sand. I hate to recommend someone spend money, but it could save you in the long run. (Good drawings are also nice.)

If I had my druthers, I would use post and beam over a compacted fill or gravel and use post tensioning to tie it together.

Where the land drops (as you mentioned) you will need to build a CMU wall (a designed and reinforced one is safer and better) to retain the fill. You can build up walls with block on top of footings or rock and then fill it in until it is level (it will have to be compacted). Whether mesh will work or not depends on the thickness of the concrete and how much compressive strength you will need. I prefer #4 rebar tied together in squares resting on high chairs. Regardless, I keep coming back to getting some professional help in the design.

Good luck!! -- Oh - Do you know what an elephant is?

Answer: A mouse designed by a Professional Engineer.

[Gary Lange]

I wouldn't build it without a properly constructed foundation. Everything you do the right way now will mean less headaches later. Like everyone says check the code for your area and then have a footing and foundation constructed and filled for the floor.

[Matt Ocel]

Let me digress(is that the correct word).

You have to follow basic construction methods, and local building codes.

ie. frost footings, make sure footings are on virgin soil, size of footings.

Most of these items are boiler plate stuff for your area.

The advice some are specifing may work in there part of town but not nessicarily in your part of town. You may be over building or under building.

Go in and talk with your local building official, they will help you, thats what you pay taxes for.

If your going to build a post type building, then I may have just wasted a lot of ink.

[Greg Cuetara]

Good luck!! -- Oh - Do you know what an elephant is?

Answer: A mouse designed by a Professional Engineer.

hey now...no picking on us engineers.....that elephant must look like a mouse

btw back on topic...take Matt's advice and go talk to the building official. I would suggest that you give my profession some money and hire a civil engineer but that may not be in your budget. If you can find a local engineer one man shop you may be able to get some good advice for minimal expense which would be the ideal solution.

Good Luck,
Greg