Replies

1. 
   MYBuilder | May 02, 2014 08:16pm | #1

   Addition on the house. You will need a foundation around the slab so it won't tend to settle away fom the house. The 30" side needs to be deeper than frost for your area and the whole foundation could be that deep or deeper depending on the houses foundation. Inside the foundation, we would use granular fill. It tends to not settle. You can use sand, crushed stone, recrushed asphalt or concrete too. The difference is in the compaction. Depending on hauling price and labor price we sometimes do different things. If you use something that needs compaction and all you have is a rented plate compactor go up in 3" lifts. Heavier compactors can do larger lifts. Don't forget the vapor barrier, 6mil plastic. You may also want to put perimeter insulation if you want a comfy area. The slab can have fiber in the concrete or wire mesh. We have luck with both. The fiber is a little harder to finish but it doesn't sink to the bottom like mesh tends to. Good finishers can do either equally well. Good luck.

1. 
   tractorsteve | May 03, 2014 03:46pm | #2

   Thanks for all the info, there will be a footing and stem wall and 15 mill visqueen between gravel and slab.  The concrete contractor puts 3/8' rebar in the slabs 18'"or 24" o.c. both ways. What size and type gravel should be used under the slab,  I read some where that the gravel should be clean with no fines, does that mean like 3/4" french drain gravel and why clean?

1. 
   MYBuilder | May 03, 2014 10:14pm | #3

   I don't know why clean would matter. "Gravel" to me implies crushed rocks. 2's, 53's, 73's etc. All those can be "clean" or with "fines" or "crusher dust", but they tend to settle and or crush under the weight of traffic. They would be compacted either way. Granular fill, "pea" fill, non-settling fill does not tend to settle because of the size and shape. It fills it's own voids, so to speak.

2. 
   AndyEngel | May 05, 2014 09:45am | #6

   You've got two things going on. First, you need fill that will compact well and not settle afterwards - That takes hard fill in a mix of sizes that compacts to something like a solid. You've gotten good advice about that. The second part is the clean stone. This is simply crushed stone that's graded and washed. Typically in my experience, 4 inches of 3/4 in. clean stone is used directly under slabs. You could use larger stone, but it's harder to hand rake. The purpose is twofold - First, it's easy to level out with rakes and shovels, so you can fine tune the grade below the slab and achieve a consistent slab thickness.

   The second purpose is why clean stone is used - It's a capillary break, which means it prevents ground moisture from rising through capillary action to the bottom of the slab. Water only moves by capillary action from larger spaces to smaller ones. Consequently, water can often find a capillary path upwards in the fill, with its mix of particle sizes. Clean stone creates a layer with larger spaces between the stones, short circuiting capillary action.

2. 
   DanH | May 03, 2014 10:34pm | #4

   I'd suggest that you ask your contractor what he recommends.  There are differences based on local soil conditions, the local availability of materials, and, quite simply, the preferences of the guy who will ultimately do the work.

3. 
   BossHog | May 04, 2014 03:37pm | #5

   As someone else said - It would pay to check locally.  We don't know where you're at or what exactly is available there. Your local codes may even specify something.

   Many years back when I built my own garage I had to do roughly the same amount of fill.  There was a subdivision not too far away that was putting in a sewer, and was giving away the clay they dug out if you could haul it away.  (In this part of Illinois, clay packs like a rock, so it makes a good base)

   I borrowed a truck, hauled a load over, and dumped it.  Before I got another load I spread it out so it wasn't too thick.

   When I came back with another load, I drove the loaded truck back and forth over the fill to pack it down. It was about the right moisture content, so it worked really well.

   That's a very un-scientific approach, I know.  But it worked like a charm.  When I sold that house about 15 years later the garage slab was perfect - Not the tinyiest crack anywhere.

4. 
   tractorsteve | May 05, 2014 11:54am | #7

   Thanks for everyones input.  My site is sandstone rock and a mix of clay and sandy dirt, of which I have mutiple piles.  I purchase uncleaned  river rock  for packing my driveway, it's peagravel up to fist size rock with a good amount of clay in it and packs well and costs me less then sand  or gravel.  Should I use ether of those for fill, my concern with clay is expansion and contraction. Our frost depth is 18" and rarely goes deeper than 6"-8".  I don't have a plate compactor but do have a bammer that packs clay well. Thanks again for your advise.

1. 
   AndyEngel | May 05, 2014 01:45pm | #8

   The trouble with clay is that it expands and contracts as the moisture level changes. If there aren't big swings in moisture, it's probably not a problem. One issue with river rock is that it tends to move under dynamic loads because it's rounded. Crushed stone stays put better because it's angular and locks together. Which isn't likely to be a huge problem under a garage slab, IMO.