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I have a 12' span with a floor joist that is currently 2 2x12's. It was originally a 16'+ span with an attic opening that was 3' wide thus the reason for the double 2x12's. I'm installing an L staircase to the attic (live load, but no bathroom or kitchen will be up there) and the headroom at the point of the double floor joist is not tall enough to meet code. If I was able to cut the double 2x12's down to 2x6 size, which I think is roughly comparable to a single 2x8 (a 2x8 spanning 12' meets code for a floor joist 16 OC where I am), then the headroom would meet code.

You can see in the attached drawing what I'm trying to do. Can I notch a double 2x12 like this where they're still 2x12 depth at the bearing point, reduced to 2x6 depth for the 3 feet of stairs and then back to 2x12 for the remainder? Again, I only need a single 2x8 for this span so my thinking is double 2x6 depth for that 3 feet of 2x12 will suffice.

enter image description here

newbie
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1 Answers1

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It’s not clear the spacing of the 2x12’s nor how many 2x12’s you’d like to notch. However, I’ll make a few assumptions and you can let me know if I’m wrong.

The Code requires a Live Load of 20 psf for uninhabited attics and 30 psf for habitable attics. (See ICC Table 1607.1) Normal living spaces are required to have 40 psf.

I’ll assume 20 psf Live Load (furniture, people, etc.) plus 10 psf for Dead Load (subfloor, joists, ceiling below, etc.) for a total load of 30 psf.

I’ll also assume your 2x12 joists are 16” on center spanning 12’ and notched as shown in your sketch.

Depending on the species and grade of lumber, a 2x6 at 16” on center spanning 12’ (the notch essentially reduced the 2x12’s to 2x6’s) can support about 37 psf (live and dead load).

Therefore, your double 2x6’s at 16” on center spanning 12’ can easily support a Code required 30 psf (live and dead load).

Bearing is adequate with the joists resting on the 2x4 top plate.

I’d be very careful of your vertical cut. Do not over cut and make a notch into the remainder of the joist.

The weak link in this design is the bearing end. If the notched end piece can be “racked” it could split off and become detached. I’d insure that these joists do not move laterally by carefully extending the subfloor to the adjacent walls so it can’t move.

If you are going to use the space for living space or if you’re going to store heavy items (like stacks of books, etc.) then my analysis is not correct. (I’d put my Christmas decorations in that corner.)

Lee Sam
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  • All your assumptions are correct and this is exactly the info I was needing. If I decide to do it I will make sure not to over cut and will extend the floor and add some blocking to make sure it can’t be racked (not sure if that’s a technical term but understand what you mean). Oh also it’s just the one double 2x12 that I’d notch. Rest of 2x12’s will be whole and need to be bc some still span 16 feet. – newbie Feb 23 '20 at 16:22