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Maximum stringer width on basement stairs


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02-01-15, 10:13 AM   #1  
Maximum stringer width on basement stairs

Hopefully this is the best Forum to post this question. We are moving the treads on our basement stairs to allow for more room to frame an adjacent wall.

The treads are 36" and 2" thick. We moved one of the stringers a bit to level the stairs. So there are currently two stringers (and there has always been only two stringers) supporting these stairs and the space between the stringers is 30". My question is about the stringer width...

I am having trouble finding any information about what the code calls for concerning width between stringers. Is that 30" space allowable or do I NEED a center stringer?

 
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02-01-15, 10:42 AM   #2  
I'm not a carpenter so I can't offer you any help with the code but if you have ability to add a center stringer.... I'd install it.

This forum is fine for your question and the guys will be stopping thru later.


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02-01-15, 11:16 AM   #3  
Stringers

Maximum stringer spacing is 16 inches.

 
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02-01-15, 11:24 AM   #4  
Probably when your original stringers were installed, the thickness of the treads was taken into consideration as far as weight distribution went. Newer treads are only 1" thick and will require more support and to help in keeping flexing/squeaking to a minimum. Are your existing stringers made from 2x12's? Many lumber yards are getting away with selling stringers made from 2x10's and they are absolutely not adequate. SO, if you have to replace them all, then cutting all three can make them identical. Also take note of your rise and run. There should not be more than 3/8" difference in any riser , and they should be around 7" in rise.

 
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02-01-15, 12:01 PM   #5  
Larry, correct me if I'm wrong, but AFAIK there isn't any code for stringer spacing per se. R301.5 states the deflection limits for the completed stairs. This means that you could possibly have a staircase with stringers 30" apart, and if the treads or treads+risers do not have deflection that exceeds the limits imposed by R301.5, it's all good.

The OP stated the treads are 2" thick. (didn't state if there were risers or not... I assume not.) If these are open stairs, I believe that would allow for a deflection of about 1/8" @ 300 lbs.

See table 6 at http://www.awc.org/publications/DCA/DCA6/DCA6-09.pdf where it states that a 2x6 SYP tread meets the L/288 and 300 lb concentrated load requirement. I would assume the OPs 2" tread meets or exceeds that.

You obviously could not do this with 5/4 deck boards, which is why deck manuals usually state that stringers should be 16" on center, or maximum of 18". (this assumes no risers.) I see no reason why the OP would need to add a stringer in this case.

 
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02-01-15, 12:32 PM   #6  
There are risers (although not pretty ones). I didn't realize that would factor in.

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02-01-15, 12:41 PM   #7  
Not sure if you already did it or not, but assuming you have access to the back side of the staircase, risers should be glued and screwed to the back side of the tread. I'd suggest you use 3/4" plywood risers rather than 7/16" OSB.

 
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02-01-15, 12:52 PM   #8  
Thank you very much for your help. The plan is to put the screws in once I know for sure if I NEED another stringer. Ultimately, I need to pass inspections as I finish my basement and my reasoning behind the OSB was simply because we are going to have these carpeted and they will have padding placed over them.

Ideally, I would like to be finished with this part of the project. However, I am certainly more than willing to hear suggestions. By your estimation these stairs would not NEED the additional stringer, correct?

Is the plywood a necessity? Why would you use this vs the OSB?

 
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02-01-15, 01:02 PM   #9  
Yeah, Brant, I know of no "code" requirement except for deck stairs using 5/4" lumber for treads. Since the OP is using 1 1/2" lumber, the two 30" apart should be sufficient with risers. What you have for risers is sufficient. I would have split the difference between the treads and wall so I could have installed a kick board on each side. Camera optics cause parallax views, so make sure your treads and risers are close to the same measurement all the way up.

 
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02-01-15, 01:12 PM   #10  
Necessity? no. Stronger? yes. And that's kind of what your question revolves around.... the strength of the staircase as is, without adding another stringer. If you want to do all you can to strengthen it without adding another stringer, you would use plywood risers and glue and screw. When gluing OSB the glue really only sticks to the outer layer of plies, and the chipped OSB layers aren't glued as well as plywood plies are. If that 300 lbs of pressure was all put on the back of the tread (where the riser is screwed into the back of the tread) a screw would tearout the side of the OSB far easier than plywood.

But if you want to leave it as is, I'm sure it would be fine. If I was an inspector I don't think I'd like seeing OSB though. Just personal preference.

It is 7/16" OSB, isn't it? Kind of thin.

 
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02-01-15, 01:20 PM   #11  
Just kicking more sand......knowing the OSB is only 1/2", I'd prefer to see 3/4" dimension lumber there if I were inspecting. I'd wait to see if he kicks it before I did anything. It will be fine otherwise.

 
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02-01-15, 02:44 PM   #12  
The OSB is 7/16". Thanks to everyone for all the helpful info.

 
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