Guidance on Stair Stringers

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Old 08-08-19, 01:31 PM
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Guidance on Stair Stringers

Hello,

I am looking for a bit of help on the rules around notched stair stringers. Basically I just want to ensure that the thickness remaining after cutting out the notch is adequate to support the stairs as well as the maximum allowed length of the stringers and I can't seem to find any code on this.

My stairs are planned to be 3 feet wide with 3 stringers and the total length would be just under 15 feet. Would 2x12's or 2x14's be sufficient in this case? Is there any resources you can point me to with rules related to this?

Thanks,

Dave
 
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Old 08-08-19, 03:40 PM
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I think 5" is the minimum thickness of wood you want remaining. I use 2x12's for stringers. For a run that long I don't like to rely on just the 5" of wood remaining so after cutting out the stairs I sister a 2x4 for added strength.
 
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Old 08-08-19, 03:44 PM
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R301.5 states that stairs must be able to hold 40lbs per sq ft or as the footnote reads,
  1. Individual stair treads shall be designed for the uniformly distributed live load or a 300-pound concentrated load acting over an area of 4 square inches, whichever produces the greater stresses.

R311.7.3 Vertical rise. A flight of stairs shall not have a vertical rise larger than 147 inches (3734 mm) between floor levels or landings.

Since stair designs differ depending on a variety of factors that is about as specific as the codes get.

The best way to strengthen a staircase is to add additional stringers. And LVL's are always preferred over dimensional lumber for interior stairs.
 
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Old 08-09-19, 06:19 AM
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Thanks Guys, the total rise is about 10 feet so I should be good on R311.7.3

So if I go with LVL's and add a fourth stringer that should be plenty good for strength? Almost sounds like overkill but I'd prefer to err on the side of caution of course.
 
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Old 08-09-19, 07:02 AM
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We don't know your rise or run so it's hard to rubber stamp a plan sight unseen. I assume your rise and run does not exceed what's normal, so you should be good. But I imagine you could drive a truck down those stairs if you use LVL and 4 stringers. IMO you should stick with 3.

One thing that is harder than you think is to cut 3 stringers that are EXACTLY the same. My technique is to do the math and figure out exactly what the hypotenuse of each triangle will be. So for instance if your rise is 7 and your run is 11, the hypotenuse is 13.04. So I would mark that as a running total on the top of the stringer... (as measured from the nose of your first tread) 13.04, 26.08, 39.12 etc.

Then set up your framing square with a pair of brass stair gauges (stops) set to your rise and run, and try to accurately align your framing square on the marks you just made to achieve your rise and run. Doing that you "should" be able to mark out very accurate matching stringers.

Then it is just a matter of cutting accurately. If you are a newbie you may want to clamp on a guide for your saw so as to make perfect cuts. Always remember which side of the line to cut on... your blade should always be on the waste side of your cut. Generally it is fastest to cut all your tread cuts first (left side of the blade) then go back and cut all your riser cuts (on the right side of the blade) or vis versa. Do not over cut the corners as that weakens the stringer immensely. You will have to finish your cuts with a hand saw or jig saw.

After you get all 3 cut, I like to gang them all together and clamp them together. Then you can visually see if they all match. If any adjustments need to be made, I like to do that with a grinder and flap sanding disk while they are all together. In the end they should match exactly. This makes less trouble with any rocking or bowing of the treads as you install them. And you should use construction adhesive as you as assemble your treads and risers. Fastening the tread and riser together where they meet (from the back side) where possible.

Probably too much info.
 
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Old 08-09-19, 08:04 AM
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No that's all good stuff to know, thanks!
 
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