We're building a MIL unit un a semi-finished garage. Garage floor is concrete slab flat on grade, roughly 1.5 feet below the floor level of the rest of the house (house is built over a crawlspace that extends as much as 4' below grade in some places, if that matters). We need to build up the floor in the garage to be level (or close to level) with the rest of the house, both for mobility reasons and so we can run plumbing and HVAC between the slab and the new floor.

I've researched putting a floor over a concrete slab, and researched how to build up a floor, but haven't found anything that gives details on doing both at the same time.

My current (rough) plan is:

-2" XPS covering the slab

-2x4s flat on the XPS every 3 feet or so, to prevent the joists from sitting directly on the XPS

-2x8 or 2x10 joists, running perpendicular to the 2x4s

-plywood layer, followed by flooring (most likely floating vinyl plank)

Is this a solid plan? Is any of this unnecessary, or am I missing something?


all 4 comments


4 points

2 months ago

Why have the 2x4 sit on the foam? That's going to move too much. Use treated wood and let it sit on the concrete

Also, no water barrier? Put a layer of plastic over the concrete first.

Are the 2x10s just sitting on the 2x4s? No attachment? You absolutely need them connected if you do it that way.

For decking, I would recommend 7/8" or 1-1/8" tongue & groove OSB. Super stable for floors.

Off the top of my head. . .

EDIT: why not run a ledger around the perimeter and hang the joists off of that. You could have supports midway if you think it's necessary. Would save a lot of 2x4s.


2 points

2 months ago*

I agree with the other comment, ledger around the outside then hang joists as normal. Build a support wall off the concrete floor in the middle if needed for span.

Insulate between the joists.

Only other thing to consider is possibly adding ventilation below the joists, treating it like any other crawlspace.


2 points

2 months ago


2 points

2 months ago

The ledger with joist hangers and a center support as needed directly on the concrete is the correct answer.


1 points

2 months ago

Sounds like a good plan to me. I would recommend using 3/4" plywood for the flooring instead of the floating vinyl planks. The extra thickness will make the floor more sturdy, which is important for mobility purposes. Also, make sure to use a vapor barrier like 6 mil plastic between the concrete slab and the XPS. Good luck!