How to Add Support to Floor Joists

LESLIE: Alright. Now we’ve got Kurt in North Carolina on the line who’s working on a restoration. Tell us about the project.

KURT: So I’ve got 2×6 floor joists spanning 15 feet. And I’d like to know if I rip some ¾-inch plywood and sister it up against the 2x6s and glue and screw it, if that would be sufficient. My crawlspace has six vents under the floor and I want to seal them up. I read it doesn’t need cross-ventilation. It’s kind of old-school. And I put six-mil poly on the ground. Your thoughts, please.

TOM: Alright. Well, first of all, in terms of beefing up the floor joists, sistering the floor joists by doubling them – I don’t necessarily think I would use plywood on them; I would double them. Well, I mean it may not be flimsy but the thing is, if you want to sister a floor joist and help support it, you need to go from bearing point to bearing point. So if it’s going from a girder to an exterior wall, the sister beam has to go the same length.

KURT: Yeah.

TOM: You know, another thing that you could do, Kurt, is you could run another girder, at the midpoint of that 15 feet, from end to end. Now, it doesn’t necessarily have to be – has to be as strong as the main girder for the house, because you’re really just taking the flex out of it. So if you poured a small footing underneath it and just got something in there to kind of stiffen the floor, that would take the bounce out.

KURT: Right. Yeah, I thought about that on the main floor but my second story, I didn’t want to – if I put a glulam in, I only have 7 feet, 5 inches to ceiling height.

TOM: I understand. So, doubling them is a solution, as well as using a mid-span girder.