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I'm considering finishing a large attic space and have a question about the floor joists. I recently finished the area over my garage and the floor joists (garage ceiling) were already up to code for floor live loads (2x14 engineered beams IIRC). However this new space only has 2x8s, which I'm not certain are rated for live loads over their current span.

So my question is, can I fur them up to meet code? i.e. by stacking another 2x6 or 2x8 edge-on-edge, across the entire span, producing a thicker beam? I know that furring is allowed to make room for insulation, but I can't find specifics as to whether the furring constitutes a single board, and would meet load requirement code, or whether the load needs to be supported by a single, solid, board (which would requirement to replace them all).

(My gut tells me it would be allowed, maybe with a larger safety factor)

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    Knowing the open span of the 2x8s will help. 8ft is very good, 10ft good, going over 10ft is getting iffy/needs more details.
    – crip659
    Commented Aug 12 at 14:19
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    Is "furring" the right word here? To me its synonymous with packing, perhaps to level an otherwise wobbly backing.
    – Criggie
    Commented Aug 13 at 2:23
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    Are you talking about "sistering" the beams, ie side by side, flat edge to flat edge?
    – MikeB
    Commented Aug 13 at 8:18

2 Answers 2

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Probably not. Simply stacking boards does not increase resistance to flexing as much as a thicker board would. You'd be better off sistering the joists, I believe.

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  • Makes sense. I suppose at the end of the day there's really not a ton of cost difference between just buying the "proper" sized joist (probably 2x10) and sistering an entire new floor, vs furring / stacking.
    – The Shoe Shiner
    Commented Aug 12 at 14:23
  • An engineer would possibly allow it if the 2 members would be fully gusseted together on each side, with full length plywood, glue and fasteners. A lot to do, and I am not an engineer.
    – Jack
    Commented Aug 12 at 14:53
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    True, but by that time you might as well just sister it.
    – keshlam
    Commented Aug 12 at 15:10
  • Yeah, I was gonna write a "sister" answer but saw this one. IMO that's the way to go—don't need to even fasten the ceiling below to the new joists. And hopefully there aren't too many penetrations to deal with!
    – Huesmann
    Commented Aug 12 at 17:13
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For stacked wood to be stronger than the parts it has to be laminated so that the parts can't slide past each other

So if you want to make a 2x14 out of a 2x6 and a 2x8, you'll be needing some special glue and clamps etc...

It's probably much easier to just nail a 2x14 to the side of the 2x8 - this is sistering.

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    I'm ignoring the fact that mutiplication is not commutative over addition in the field of dimensional lumber.
    – Jasen
    Commented Aug 13 at 8:52
  • Speaking as a mathematician, I think you must mean distributive. Which is strangely apposite in the context of the OP's question!
    – TonyK
    Commented Aug 13 at 16:14
  • distributive it is.
    – Jasen
    Commented Aug 13 at 19:58

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