What's your flavor under slab, sand or rock?

[Sedgehammer]

be pouring the slab in a few weeks. have footer dug. Will install rebar next week. Have it inspected and then run plumin and electrical conduit. I've read plenty and some are saying sand and others rock. Back home it was always sand. Down here see lots of rock, but that's usually commercial. Haven't seen many residentials. 

anyways, what's your flavor and why come? 

Thanks

[btulloh]

Rock here.  Better drainage for any water. Patios and walks are typically sand or rock dust over 4" of rock. Probably a lot of that is driven by local conditions and site orientation. 

[VB-Milling]

Definitely driven by local conditions, but I like 21A recycled crushed concrete.  I get it locally for $18/ton. 



 

 

[alan gage]

Not a pro but have prepped for a few slabs and done quite a bit of research. From what I've read large crushed stone is the ideal since it acts as a capillary break and doesn't hold water.

That being said what I normally see around here is sand. I'm sure it's cheaper and easier to work with/smooth out. Gravel is popular too.

I often compromise and use 1 1/2" crushed rock with fines, which isn't a whole lot courser than most of the gravel we have around here. The only washed crushed rock easily available here is 2"+ and it's a bear to shovel and rake.

I'm sure site location is important. In some locations you can probably get away with murder and in others better mind your P's and Q's.

Alan

[nybhh]

Architect here.  We always specify stone/gravel and I've only ever seen stone/gravel specified around here by other building professionals as well.  As others have said, drainage is the primary reason.  A dry slab that doesn't wick moisture from below and stone doesn't hold moisture to frost heave when frozen although the soils below still can.  I also believe stone compacts better as well so less chance of settling/cracks.

[VB-Milling]

I often compromise and use 1 1/2" crushed rock with fines, which isn't a whole lot courser than most of the gravel we have around here. 

That's what I primarily like about the 21A crushed concrete.  Lots of fines that pack against the rock while still allowing for drainage.  Also for compaction to minimize voids under slab.  More than enough for patios and walkways in my opinion.
@Sedgehammer if this is a slab that's going to see heavy equipment, I'd go with something similar to 21A with fines but compact with a plate compactor in 2 inch increments.

[Raider Bill]

I used 1 1/2 is rock and fines then walked the dog (  compacted) for 3 days. I also watered it in at night.
This was on red clay

[Sedgehammer]

We have an ag base which is recycled cement with dirt added so it compacts. Looks similar to what @VB-Milling uses

[Don P]

The fines are where the capillary, rising damp, action happens. 

[sawguy21]

We only used gravel due to drainage issues, the valley floor is a hard pan clay prehistoric river bed so won't absorb moisture. It's like dealing with concrete in summer. A neighbour used sand under a decorative driveway/parking pad last year and it is already cracking from frost heaves.

[alan gage]

I know it's not what you asked but I'd highly recommend insulation under the slab. Even if you don't think you need it in Oklahoma weather I think you'd notice a big difference with interior humidity levels. The concrete slab will be cold since it's in direct contact with the soil underneath which causes it to condense with hot and humid air above. It still gets hot and humid in my shops when it's hot and humid outside but the floor never sweats (like most other buildings I've been in) and I can easily keep the humidity down with a regular home style dehumidifier (1200sq ft shop).

My dad's insulated shop on the other hand (non-insulated slab) is very clammy during humid weather with sweaty floors.

And to bring it back to the original question the foam makes a really nice capillary break.

Alan

[Tom King]

Washed, crushed granite here.  If base needs to be built up, we use the same thing as railroad ballast.  For the top layer, I use fine washed crushed granite.  They go by a number each here, but I understand the numbers aren't the same all over.

I use the 88 stone (fine) as the top layer because it's so easy to grade out level.  The railroad ballast locks together, so it's hard to grade it out smooth enough, if you're doing it by hand.  The 88 stone grades easier than dirt, to a line pulled across the top, and the distance judged to the line by the grading tool you're using, like a hard toothed rake upside down, or a concrete come-along without the wire hook.

We have an endless supply of granite here though, and the nearest rock quarry is only 15 miles from us.

[Sedgehammer]

I know it's not what you asked but I'd highly recommend insulation under the slab. Even if you don't think you need it in Oklahoma weather I think you'd notice a big difference with interior humidity levels. The concrete slab will be cold since it's in direct contact with the soil underneath which causes it to condense with hot and humid air above. It still gets hot and humid in my shops when it's hot and humid outside but the floor never sweats (like most other buildings I've been in) and I can easily keep the humidity down with a regular home style dehumidifier (1200sq ft shop).

My dad's insulated shop on the other hand (non-insulated slab) is very clammy during humid weather with sweaty floors.

And to bring it back to the original question the foam makes a really nice capillary break.

Alan

Original plan was to install infloor heat. Boss nixed that, as she wants to save more for the house. Plan is to be in here only a couple of years. We'll see on that........ I know plastic stops the concrete from being wet in the summer, plus we'll be cooling this. Haven't read up on using it in a non heated floor. I'll read up on that.

Thanks

[Sedgehammer]

Under neath the the floor is packed millings. It's the area where we parked our dump trucks. This is the highest point in the drainage vacinity, there's really no possibility of water.

We'll be lifting the pad 5" where the area for parking veichles us. 8" where the kitchen/dining/bathroom is. Underneath the compacted millings is red clay. 

[barbender]

I use compacted gravel, and then the last inch or two with sand so I can screed it off. I always use poly sheeting for a vapor barrier, and I would second the insulation as well. In our basement, I have an area that I left the under slab insulation out because of a load bearing post that beared on that spot, and it always gets condensation on it. You can see the square that I left the insulation out. Compaction is important, a plate compactor and 3-4" lifts are your friend.

[doc henderson]

wet sand is a conductor, dry sand is an insulator.  put down plastic, then sand, then put pex every foot on the 6x6 wire mat and pull it to the center of the slab.  even if you do not hook it up, in 30 years it will be a selling point for someone if you sell your place.  the pex cost me about 300 bucks back in the day for my 4500 sq. ft. shop.  

[jmouton]

ive been doin concrete for over 30 yrs and we only use 21aa crushed concrete, unless otherwise specified,,  sand sucks ,,,

[Sedgehammer]

wet sand is a conductor, dry sand is an insulator.  put down plastic, then sand, then put pex every foot on the 6x6 wire mat and pull it to the center of the slab.  even if you do not hook it up, in 30 years it will be a selling point for someone if you sell your place.  the pex cost me about 300 bucks back in the day for my 4500 sq. ft. shop.  

I'd need 2" of pink then. Lord only knows what that costs now. I had heard that one of the plants when down in that freeze in TX and it solidified, as they ran out of heat, so shortage on that also. I understand on the resale. I tried to convince her even to put it in, as the heat is so, so much better.

Doc, what size pex you run? Thanks

Just checked maynards. 2" is $53. 2.5" is $35........ Now that makes a lot of sense. I suppose the demand is in the 2" and not so much with the 2.5".

[Sedgehammer]

I use compacted gravel, and then the last inch or two with sand so I can screed it off. I always use poly sheeting for a vapor barrier, and I would second the insulation as well. In our basement, I have an area that I left the under slab insulation out because of a load bearing post that beared on that spot, and it always gets condensation on it. You can see the square that I left the insulation out. Compaction is important, a plate compactor and 3-4" lifts are your friend.

In that spot you left out the poly also or that's with the poly, but no insulation?

[mike_belben]

i havent poured yet, but i did my little shop base in coarse rock (3-5" limestone)compacted on top flat, bare clay and then capped that in compacted "crusher run" (3/4" down to dust) because the topography, property lines and runoff constrained things and made proper drainage worrisome.  

anyways water can flow in one side and out the other quite impressively without the surface gravels getting wet.  my logic is that having the porosity and voids in the base rock to leave room for water expansion would prevent frost heaving if the runoff path away is somehow compromised and water migrates towards it.  also if the water can run through so can the air, and that will help dry the slab sub base much faster.

we have a lot of humidity and fungal activity that i dont want migrating into the wood. 

this is the same base ive been laying down for driveway and the big forktruck can drive over it loaded without rutting it at all, so i guess thats my seal of approval.  that machine will squish water up out of the desert. 

[alan gage]

I use compacted gravel, and then the last inch or two with sand so I can screed it off. I always use poly sheeting for a vapor barrier, and I would second the insulation as well. In our basement, I have an area that I left the under slab insulation out because of a load bearing post that beared on that spot, and it always gets condensation on it. You can see the square that I left the insulation out. Compaction is important, a plate compactor and 3-4" lifts are your friend.

In that spot you left out the poly also or that's with the poly, but no insulation?

The standard colored foams are XPS. You can also use white foam (EPS) under slabs as long as it's dense enough. I used to have to order it in special from a local company that makes energy panel buildings but now our local lumberyard is carrying it in 2". It's a little cheaper and supposedly more environmentally friendly to manufacture. It doesn't crumble like the cheap version of white foam most of us are used to.

Both sides (XPS and EPS) make the case that their type of foam is best for use under a slab. XPS says it doesn't absorb as much water. EPS says that while it might absorb more water it's also able to quickly release the water, which XPS isn't.

I don't have enough experience to say how much difference there is between poly and foam when it comes to floor condensation. But I do know that a pepsi can is vapor impermeable but it still sweats on a hot day if the contents are cold.

Alan

[barbender]

Poly won't do anything for condensation, it's only a vapor barrier. Here's a case in point- a local farm and fleet bought a building that was built as a Wal-mart. They wanted to do decorative epoxy coating on the floor, but the contractor in charge of the project advised against it because they were aware the slab had no vapor barrier under it. The company pushed for it anyways, and now that floor looks like trash. Peeling up and flaking off everywhere. What happens is vapor gets through the slab (concrete is vapor permeable) and is trapped under the epoxy coating. When they run pallet trucks over it, the epoxy basically pops and then starts to flake off.

Sedge, my floor has poly throughout, even the area where I left out the insulation.

[Sedgehammer]

@alan gage @barbender 

Been all over the interwebs trying to find some affordable 3" EPS.....
if 2" then the XPS. It looks though like the EPS is a wee bit cheaper per R, but I know she'll really not be happy on the added costs..... 
Trying to figure out a way to hide the purchase..... :D

[SwampDonkey]

House basement is drainage stone with plastic on top before pour. Garage floor is gravel 2" higher at the back, sloped toward overhead door, with plastic on top before pour.

[alan gage]

@alan gage @barbender

Been all over the interwebs trying to find some affordable 3" EPS.....
if 2" then the XPS. It looks though like the EPS is a wee bit cheaper per R, but I know she'll really not be happy on the added costs.....
Trying to figure out a way to hide the purchase..... :D

When I was ordering EPS foam from the energy panel place I was getting 4" for the same cost as 2" XPS at our local lumber yard. And I could get it in 10' sheets which helped with layout. It was always a bit of a hassle ordering from them though as it wasn't something they normally did and I was very small potatoes.

Now that our local yard carries the 2" EPS it's much easier to just get it there but cost is higher.

When you're searching for EPS foam you need Type IX to get to the same 25psi compression as standard XPS.

You better tell your wife to get used to the added costs because there's going to be a lot of them.

Alan