True 6" slab with 2x6 forms?

[78SC4X4]

Ok, This seems like a really basic question, but I can't find the answer anywhere. I told my builder I intended to pour a 6" slab. They installed a 2x6 perimeter board at ground level around the posts for the forms. But a 2x6 is only 5 1/2". How do I get a true 6" slab?

 

[The Cobbler]

raise the form up 1/2" or nail on a strip to the top of the form.
or , do you really need a full 6"?

 

[78SC4X4]

I'm planning on radiant heat. So, after 2" of insulation, I'm only left with 3.5". So, is a 4" slab actually 3.5" in depth. I could go 1/2" below the form, but door sills will be at the top of the 2x6 so I can't go up.

 

[The Cobbler]

remove 1/2" of stone then

 

[bczygan]

A proper base on properly prepared earth and then 6 true inches of slab is what should be in the written contract.

And let the contractor know that before he gets a penny of payment, it will be checked with cores and if insufficient he will have to tear it out and replace it.

So he should post a bond in an amount sufficient to cover that.

All should be in writing.

Bill

 

[joes169]

As a concrete guy, I can tell you that the height of the form or "rat board" has little to no bearing on the actual thickness of a floor, the grade is the only thing that matters. And, when we grade, we always and only use the top of the form, never the bottom, to set grade. I can pour a 12" thick slab with nothing more than a 2x3 and some dirt backfill, and conversely, I can pour a 4" thick floor with a 2x12 for a form. Now, if their setting the "rat board" down at grade, you may have something to worry about.

 

[joes169]

A proper base on properly prepared earth and then 6 true inches of slab is what should be in the written contract.

And let the contractor know that before he gets a penny of payment, it will be checked with cores and if insufficient he will have to tear it out and replace it.

So he should post a bond in an amount sufficient to cover that.

All should be in writing.

Bill

That sounds like a really quick way to spend thousands of needless dollars on a residential job when there's so many better, more realistic way's to verify the thickness before it's poured.......

 

[The Cobbler]

^^^ several years ago buddy had a driveway poured, was supposed to be 6" thick... 2x6 forms , but the centre was heaped up with gravel, I bet it was 3-1/2" in the centre. I mentioned it to him, I dunno of he ever discussed it with the contractor, I did my part.

 

[larry4406]

As a concrete guy, I can tell you that the height of the form or "rat board" has little to no bearing on the actual thickness of a floor, the grade is the only thing that matters. And, when we grade, we always and only use the top of the form, never the bottom, to set grade. I can pour a 12" thick slab with nothing more than a 2x3 and some dirt backfill, and conversely, I can pour a 4" thick floor with a 2x12 for a form. Now, if their setting the "rat board" down at grade, you may have something to worry about.

I think this is spot on.

 

[walrus]

You can't have a 6inch slab with 2 inches of foam included in the 6 inches. Form boards should be on top of foam. I agree with joes169, grading is the key. I work some guys who do flatwork over the top of gas tanks and around gas islands. They set final height or forms and do final grades inside of forms by stretching strings across top of form boards and measuring with tape measure. They fill low spots with pea stone or gravel and then compact. They make sure its never high as its easier to add than subtract compacted gravel by hand.

 

[rburke65]

I don’t know but sounds like someone’s planning wasn’t thought out.

 

[78SC4X4]

As a concrete guy, I can tell you that the height of the form or "rat board" has little to no bearing on the actual thickness of a floor, the grade is the only thing that matters. And, when we grade, we always and only use the top of the form, never the bottom, to set grade. I can pour a 12" thick slab with nothing more than a 2x3 and some dirt backfill, and conversely, I can pour a 4" thick floor with a 2x12 for a form. Now, if their setting the "rat board" down at grade, you may have something to worry about.

Thanks. This is helpful. The "rat" boards are not down on grade, so I have room to work with. Since I'm preparing the base we should be just fine.