How to finish concrete ceiling - beam span calculation

Home Improvement Asked on January 3, 2022

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.


3 Answers

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.

Answered by Martin Bonner supports Monica on January 3, 2022

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.

Answered by Lee Sam on January 3, 2022

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.

Answered by peinal on January 3, 2022

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