What are the best practices for residential elevated foundation 42" construction in Coastal NC? I have read alot about: soil tests, compacted fill, 4" of gravel on top of compacted fill, vapor barrier, termite treatment, etc. Where do I find the latest industry best practices for constructing an elevated slab ? Questions include: 1. What is the best soil to use for fill? 2. How do you determine if the fill has been properly compacted? 3. What purpose does the 4" of gravel on top of the compacted fill serve, is it structural or for water drainage or both? 4. If you plan on having reinforced 4" concrete slab 3000 psi do you need the 4" of gravel fill on top of the compacted fill? 5. If you have a vapor barrier do you need the 4" of gravel fill on top of the compacted fill? 6. How long is the termite treatment good for? etc.

  • This is too many questions at a time; post one question and we will attempt to help – Ack Nov 24 '20 at 2:33
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So, if this is “What is the best foundation system?” I’ll give it a go...

  1. Do not use soil for fill. Use evenly graded “crushed rock”.

  2. You have fill tested to make sure it’s compacted to 95.% density. (You can’t get 100%.) Install it in 6” layers and compact as you go.

  3. When you have that much fill, (over 18” or so) you need to use larger rocks to save money. I’d go with 6” minus. That means you’ll have big big rocks in the fill. By installing a 3” layer of 3/4” minus rock you’ll have a nice uniform layer for the concrete slab and rebar.

  4. Yes you need a base regardless. Do not use wire mesh in your slab...there isn’t an adequate amount of rebar and the slab will crack. Use temperature steel. Do you know how to calculate the amount needed?

  5. A vapor barrier may not be necessary , that high above grade. We’d need more info to decide.

  6. Length of service by your termite treatment depends on strength, quantity, etc.

Btw, if you use 3,000 psi concrete, then by Code you’ll need it tested. Why not use 2500 psi concrete and skip the cost of testing?

Also, that high above grade, why not use wood framing and skip the fill, compaction, etc.?

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