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I'm working on designing a new house and trying to determine how thick the basement slab should be. This home will have poured basement walls over the footings, so the basement slab isn't bearing the weight of the house walls.

The International Residential Code (IRC) 2012 which applies in my area states that:

Concrete slab-on-ground floors shall [...] be a minimum 3.5 inches thick

I've found that most slabs are 3" to 4" thick, excluding the footings.

But I also know that the code is a minimum, and in some cases it makes sense to go beyond what code calls for. Is there any advantage to pouring a basement slab over 3.5" thick?

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Before I'd add more concrete I'd add reinforcing - reinforcing is cheap and the benefit of it is huge, .vs. unreinforced concrete.

After that it's a matter of use. Will you have a machine shop in the basement? You might want a bit more thickness, too. Will you be playing ping-pong and/or putting in a 1970's basement bar? No benefit. Woodworking machinery? 4 inches is probably plenty. Storing random junk you probably should throw out? Your wine collection? Minimum thickness is fine.

An additional consideration (has not much to do with thickness) is whether you might ever want heat in the basement - in which case, installing radiant heat tubing in the floor, and insulation under the floor and around the outside of the foundation walls is another low cost upgrade with major benefits, if you get it done in the planning phase.

Yet another consideration: Installing exterior drains at footing level is trivial when the hole is open (just the cost of the pipe and placing it - perhaps one more trench to take it away from the house) but terribly expensive if you discover a need for them later on and have to excavate...

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The strength gained is significant, and it's not like you're adding labor. Thicker slabs also provide a greater margin for error because any sections that are slightly thinner won't be an issue. I'd be much more certain that a 4" slab was at least 3.5" thick everywhere than a 3.5" thick slab was poured over perfectly level ground smooth as a baby's bottom.

Practically speaking the total volume of concrete is going to be a consideration. It does come in trucks after all. It doesn't make any sense in my mind to do a 4" pour if you can also do 5" with a single load. Conversely, if you can do a 4" slab with one load but the 6" you have your heart set on will require two then you'll need to weigh the extra cost and logistics of that.

  • Truckloads is a great consideration, assuming you get charged more for the delivery than the extra cement. – Nick Jul 8 '17 at 4:41
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In my opinion there is none. I have seen 6" thick slabs crack. The main thing is get the proper reinforcement in there before it is poured so the cracks stay superficial. Many times a 6"X6" welded wire mesh (WWM) is used, but if you want to overkill that, got to 3/8" or 1/2" rebar in a 18" or 24" in both directions. Making sure it is supported before it is poured so it stays in the center of the slab or at least near it.

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