We are trying to finish an above garage attic space as a playroom (since the ceiling will be slanted - 7' to 4' headroom). The "room" is 25' x 8' and currently has 2x4s that are 16" apart.

My current place is to double up on the 2x4s by sistering them before putting a 23/32" plywood subfloor, topped with a light laminate or vinyl floor.

Is that a good plan? We don't intend to put any significant amount of furniture in here.

  • 1
    Not a good plan unless you get adequate engineering. Those are almost certainly rafter ties, not floor joists, and so are not meant for any loads. Nov 23, 2015 at 5:02

3 Answers 3


I don't think those rafter ties will work for support. The width of the room isn't what's important so much as the span from support to support. I can't imagine a garage layout that would have a span short enough to make with 2x4's, even sistered. You'd also have to have the attachments to the top plate of the walls adequate to support the weight of the floor. If one end of the span is supported by a beam up the middle of the garage, that probably wasn't sized to support a floor either.

It's doable (almost everything is) but it isn't as simple as doubling up the 2x4's.

  • Thanks. I already bought all the lumber (2x4s), but I'll replace it with 2x6 and sister those to the existing 2x4s. We'll lose a couple of inches, but the ceiling height in this "room" is already so little that you can barely stand straight on one side... so it won't make a big difference.
    – 0pt1m1z3
    Nov 23, 2015 at 13:46

You need to do the math.

Check out this free calculator or consult floor joist span tables for what you are doing. This calculator shows 2x4's limited to 5'8" span for eastern white pine. Also the type of lumber is important. http://www.awc.org/codes-standards/calculators-software/spancalc

What you plan to do with the room is not relevant. If you build something, someday you will be gone and someone else will use it. That is why we have codes and engineering standards so it is built safe the first time. You don't want it to collapse with your family in there. They are worth the extra time and money.

Span tables or the referenced calculator include standard dead load and live load weights per square foot and standard deflections for floor joists to keep the floor from flexing too much as you walk across it.

Good luck and Happy Monday!

  • 1
    Span tables aren't the whole story. You need to consider how the rafter ties are connected and supported. Nov 23, 2015 at 12:05
  • 1
    Thanks ArchonOSX. After this thread and my research I'm gonna go with 2x6 sisters instead. Based on the calculations those should be more than sufficient.
    – 0pt1m1z3
    Nov 23, 2015 at 13:49
  • 2x6 might be adequate but you need to support the ends of the joists. Simpson might make a specialty joist hanger, or you can build a wall underneath, or you can create a custom header. Not trivial, but not hard either.
    – User95050
    Nov 23, 2015 at 15:08
  • The ends of the joists will sit on top of the same top plates that the 2x4s sit at both sides.
    – 0pt1m1z3
    Nov 23, 2015 at 16:47
  • Usually when you are building headers or joists you want grade 2 lumber not the standard stuff that you can buy at Home Depot. It costs a little more but is denser and usually better quality. southernpine.com/grade-descriptions
    – ArchonOSX
    Nov 23, 2015 at 16:54

I built a porch with a 2x4 floor with 3/4" OSB for flooring. 24" OC. I criss crossed 2x4s underneath as "beams" in which I placed concrete blocks underneath. It held up my massively heavy wood stove and a stack of wood logs for two years. Add the fact that a lot of traffic went through there. When I took down the porch, I checked to see if there was any drop. The porch had not moved a mm. I believe 2x4 construction is under estimated because it isn't practical in terms of traditional construction codes. But with proper bracing and support, you would be surprised at what can be done. Thats why they invented I Joists. Less wood can do the job if done right. And you don't need an engineer.

  • When you took down the porch --- meaning you disassembled it? And then checked for sag after the load was removed? Doesn't sound like much of a test. Aug 10, 2018 at 17:35

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