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gravel under a slab pour(?)

newbuilder | Posted in Construction Techniques on September 12, 2005 03:13am

I’m having a slab poured in a few days.  The guy’s going to come in with his compactor … and then he’s talking vapor barrier and insulation .. wire mesh … then pour.   I had to ask him about a gravel or sand underlay placed down over the compacted soil.  His response was that he “sometimes uses gravel and sometimes doesn’t depending on conditions”.  

This guy and his crew do a LOT of work in the area.  He’s a good guy and communicates ‘fairly’ well with me.  But he’s working to maximize his profits, of course, and that always, to me, indicates that I need to really watch and ask questions.  He’s run into a few ‘unexpecteds’ on this job already and so he’s feeling like he needs to keep costs down … but this is a delicate project — tall and thin —  and I absolutely CANNOT have him cutting corners on the foundation!

So my question is:  Has anyone here heard of pouring a slab after compacting WITHOUT using gravel (or sand) first?   My soil IS very very sandy and loose and light … I’m at the tippy-top of the hill and drainage is naturally/generally away from the project.  But it still makes me nervous since Ive heard that gravel is always one of the steps in putting in a slab on grad. 

Here’s the related thread that, for me, accompanies this project:  http://forums.taunton.com/tp-breaktime/messages?msg=62609.44&redirCnt=1

thanks!

Terry

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Replies

  1. FastEddie | Sep 12, 2005 03:20am | #1

    If you're concerned about groundwater trying to pass under the slab, then a perimeter drain system would be the way to go. 

    If you really want the gravel so you can sleep better at night, and you think he's leaving it out due to cost, ask him how much more it would cost to add a layer of gravel, and then agree to pay the upcharge.

     

     

    "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

    1. brownbagg | Sep 12, 2005 03:30am | #2

      it mainly depends on the area. down south we just pour on compacted soil. over the last twenty years I know of just three where gravel was used.

    2. newbuilder | Sep 12, 2005 03:34am | #3

      If a "perimeter drain system" is those long white 5" tubes with holes running along them ... then they have them on site and I'm sure they'll be going in.

      I don't know whether gravel is necessary or not ... that's the gist of my posting ... wondering whether or not it is!  I'm asking ... is it?  As to his costs ... it says in the contract, "gravel as needed" .. and now I'm being told, 'not necessary'. 

      I don't want to ask him to include it if it isn't necessary.  But if it IS considered essential to proper slab installation ... I think it should be included.  Of course if I must I would pay the difference ... but I've already been bumped up and bumped up.

      Terry

      1. FastEddie | Sep 12, 2005 07:15am | #4

        Ok, my experience, and it's admitedly limited, sand is used to level the excavation area.  Keep the moisture away from the foundation, and there's no need for under-slab drainage. 

         I'm sure they'll be going in

        Might be good to ask.  And where do they go?  To a sump, or drain to the low side of the lot?

        it says in the contract, "gravel as needed"

        Were the foundatiuon plans drawn by an archy or engineer?  Then you might want to call them and ask if gravel is needed.  I'm not trying to undermine the builder, but an independent answer from someone not financially tied to the project miught be appropriate.  No one here has knowledge of your local conditions, so the best we can do is give generic answers.

        Yes, the long pvc pipe with two or three rows of holes are for the perimeter drain syatem.  The holes go on the underside of the pipe, not the top.  It will need some type of filter fabric as well.

         

         

        "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

        Edited 9/12/2005 12:20 am ET by FastEddie

  2. Mooney | Sep 12, 2005 07:33am | #5

    It depends on several things.

    You already say you have a sandy base . That depends on how high you have to build. For up to 2 feet sand in most conditions are appropriate. 3 feet and over require larger density packing such as SB2 gravel and simalar . Where foundations over rocky ground must be cusioned and held for weight on those large rocks SB2 is an alternative. Large gravel in beds of 4 inches and over layed with rock dust is an exellent comaction method in layering. Most all heavy floors are compacted in layers. If you are talking about a residential pour that is 1 foot above ground , that would be very excessive according to specs. Your questions seems that wide open. I have required water soaking with sand however. .

    Tim

     

    1. newbuilder | Sep 12, 2005 10:55am | #6

      Thanks guys.

      We'll see.

      Terry

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