I have a room below my garage (suspended slab). I want to finish the room/ceiling. The room's dimensions are approximately 24' x 23.5' (this is to the walls, which are already framed/dry-walled). I don't know if I can drill into the concrete ceiling, and would prefer to avoid that if at all possible. I would like to do 2 inches of closed cell spray foam insulation. What I'm hoping to be able to do is span the 24' length with 2-4 beams, then anchor 2x4s perpendicular to the beams to create a suitable backing for something like drywall or another lightweight finish (lightweight shiplap, etc). How do I calculate a beam size that only carries its own weight, plus the weight of the ceiling materials? Height is a concern, it is a pretty low ceiling. I really only have about 9" (7 after the spray foam insulation) to work with since that is the top of the entrance/doorway.

I may need to hire out an engineer, but I figure its worth a shot asking here first for ideas.


  • Hello, and welcome to Home Improvement. How thick is the garage floor slab? (24' x 24' seems large for an unsupported garage floor.) And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Oct 26 '19 at 22:16
  • Unfortunately I don't know the actual thickness of the slab. I'm the 2nd owner of the home, and I don't have house plans or anything. – mrSpear Oct 27 '19 at 23:05
  • your question is answered here - diy.stackexchange.com/questions/84936/… and no you cannot glue everything up on concrete. That is just ridiculous. – DMoore Mar 26 at 18:36

I would consider a simpler, cheaper, easier solution--such as furring/ceiling tile, suspended ceiling tile, or furring/drywall. You don't have to drill. You can use a powder-load nailer to attach the furring. Much easier and will leave you more headroom.

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  • Ideally, I wouldn't do anything that would cause instability in the concrete slab so, firing nails into it would be out. Of course if that would be perfectly safe to do so, it would be a good route to go. I just don't know how to determine if that would work. Thanks for the idea, though. – mrSpear Oct 27 '19 at 23:06
  • How thick is the concrete? Does it have rebar and/or rewire? If so, and is >4" thick, I wouldn't hesitate with the nails. I'd check with a PE to be sure. If he says no nails, you may have a much bigger problem... (How would nails be any different than drilling all of those holes?) – peinal Oct 28 '19 at 0:03
  • I don't know the thickness of the ceiling itself. The walls supporting it are a good 6-8 inches wide. – mrSpear Oct 28 '19 at 20:36

Don’t forget that the minimum ceiling height is 7’, unless you have a sloped ceiling. (See ICC R305 of the code)

There is an exception for basements and it’s 6’-8” with beams that can be 6’-4” from the finish floor. (See ICC R305.4, exception 5.)

If you have a ceiling height issue, I’d recommend you glue (not nail or screw) the insulation and then the finish (wood or gypsum board) to the concrete ceiling.

Do not damage the rebar or the bonding of the concrete to the rebar, especially with a car overhead.

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  • Is there a glue or adhesive strong enough to hold both a insulation and drywall? without fearing it will eventually fall? – mrSpear Oct 28 '19 at 20:33

Rather than spray-on insulation, I would just glue dry-wall + insulation boards straight on to the concrete. Something like this.

You'll need a panel lifter to lift the panel into place.

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    Interesting. I've never seen a product like this. A 100% glue solution scares me, but I don't know what's commonly done or can be done. – mrSpear Oct 28 '19 at 20:34

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