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I have read that cut stair stringers longer than 6 feet in length require mid-span support. Solid stringers seem to be able to get to about 13 feet without support. So how do you combine these two ideas when you are building 4 foot wide stairs and the two outside stringers are solid and the two inside stringers are cut?

Can that span the entire length unsupported, or does the presence of the two cut stringers bring you back to a maximum of 6 feet unsupported? In my case, the total rise will be about 8 feet or so, for stringers somewhere around the 13 feet mark.

Or is this simply a better-safe-than-sorry thing and you just support it always?

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    I'd add to the answers below that it was common practice for my company to add 2x4 or 2x6 stiffeners to the sides of stringers (where they're not conspicuous). Why you may not need them to meet code requirements. They can greatly improve the feeling of stoutness in a staircase.
    – isherwood
    Mar 5 at 20:48
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It is not possible to use a solid stringer mid-tread, so where the code addresses increased span allowance for use of solid stringers, there is a presumption that the middle stringers (if any) are cut. Your 48" width will need two cut stringers, because anything exceeding 36" width need max stringer spacing of 18"OC. There is no support or span requirement that is dependent on stringer depth. Max span 13' 3" (or 16' 6" for southern pine). All stringers shall be minimum 2 x 12.

Your 4' wide 13' span stair will have plenty of support with the 4 required stringers...

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  • Why do you say, “max stringer spacing is 18” oc”? It’s not in the Code. Also, why do you use “Max span is 13’ 3” ? What load, spacing and span does that represent? (Btw, you know the span is the horizontal distance, not the length of the stringers. Think of it as a ladder leaning up against a wall. The “span” is determined by how far out the bottom of the ladder is from the wall, not the length of the ladder. The span would maybe be about 2’ ...not 14’ or so.)
    – Lee Sam
    Mar 5 at 1:35
  • 2013 AWC Prescriptive Residential Wood Deck Construction Guide - Design For Code Compliance Mar 5 at 2:36
  • I see recommended maximum span (horizontally) but nothing on maximum or minimum number of stringers on center. Fig 28 and 29. AND as I noted, it’s horizontal distance not length of stringer.
    – Lee Sam
    Mar 5 at 2:59
  • OK @Lee Sam, but I didn't say anything about length of stringer in my answer; obviously it's on your mind though. "For stairs greater than 36" in width, a combination of cut and solid stringers can be used, but shall be placed at a maximum spacing of 18" on center (see Figure 29)" page 17. Dude Lee, I respect your input on this forum so if you have better guidance please post your own better answer... Mar 5 at 6:24
  • Actually you do refer to the length of the stringer several times. I guess my point is the design of the stairs should be based on total structural design. Rather than pick one item (like stringer spacing) and saying this is the correct spacing, etc. (18” max) the entire design should be examined. The Prescriptive Guide you refer to is old and out of date and has errors for current construction (like size of holes for safety..it says 4 3/8” diameter and the current code is 4”) The op referred to 13’ span so our references were for 13’. Obviously there are options for longer spans.
    – Lee Sam
    Mar 5 at 17:33
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The Code requires a stair to be designed to 40 psf. (See ICC Table R301.1)

The net dimension of a stringer after the riser and tread have been removed (depending on the exact rise and run) is about 5” - 5 1/2”. This is about the size of a 2x6 and a 2x6 can span about 13’ and support about 45 psf.

Therefore, you need a minimum of 3 stringers (about 12” on center) plus the two outer stringers spanning no more than 13’ (horizontally) to comply with the Code. (If you need to span further, you could use more stringers...say 8” on center plus the two outer stringers.)

The length of the stringers may be 16’ or so, but the exact horizontal dimension should not exceed 13’ to comply with the Code. The stringers can be SPF and have a grade of No.1 or better.

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  • "and a 2x6 can span about 13’ and support about 45 psf.", why is this "psf"? Wouldn't it be pounds per linear foot? Or is that 45 psf at 16"OC?
    – P2000
    Mar 5 at 6:12
  • @P2000 The Code requires 40 psf live load...plus there is a dead load (treads, stringers, etc.) The OP is asking about what to use for a 13’ span. Therefore, stringers must be 12” on center to span 13’.
    – Lee Sam
    Mar 5 at 6:25
  • ICC is the council, IRC is the code. Mar 5 at 6:33

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