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Let me try this a different way.

My question is about the 2x12 stringer and if there are certain dimensions I need to keep in mind to not undermine the structural integrity of the wood.

Using the sketch below with options A, B, & C. The original question was about (A). The goal is to attach the stringers to the wall, using the 2x10 cap as the last step of the stairs.

(A) If x is approximately 10", what is the minimum y can be? For argument's sake, if it were 1" would the stringers break? For my situation I would expect y to be about 6".

(B) If I just have regular stringers, not accommodating for anything, where x is always approx 10" - then I'd have a section of stringer that would need to be covered by a 2x step in front of the cap. Because the cap only extends about 6" from the wall.

(C) If I don't cut the stringer at the top tread (SteveSh idea), I think z has to be some minimum height in order to safely attach to the wall. What is a safe number for z in this scenario?

ABC-sketch

Original Question: I have a wood retaining wall with a cap, similar to the photo below. The cap is 2x10, the wall uses 3x12 boards, the posts are 6x6, the wall height is 48".

I would like to attach stairs to the wall to get from one terrace to the next (both terraces are flat, not sloped). I will be using (2) 2x6 for the new steps/treads. The plan is to use the cap as the last tread but since the wall is supporting part of the tread, I don't need the stringer to extend to the back of the tread.

Is there a minimum distance that a tread run needs to be on a stringer?

wood-retaining-wall

Unprofessional sketch. How much of the stringer can I cut before its unsafe? enter image description here

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4 Answers 4

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I interpret the question to be not about tread depth or riser height, but the remaining, load-bearing portion of the stringer (dimension Z), and the height and depth of the upper stringer heel (dimensions X and Y). Here are some general thoughts...

  • The remaining thickness (perpendicular to the bottom edge) generally shouldn't be much less than it is below one of the standard notches.
  • As the end portion doesn't carry as much bending force as the central area of the stringer, it could be somewhat thinner. See engineered floor and commercial steel roof trusses as an example of this--some hang solely on a top flange.
  • If you're using a steel hanger of some sort, it all becomes fairly moot. A flexible-angle stringer hanger could solve all your woes.
  • Another strategy is to line the stringers with a secondary member to the inside. In fact, my family did that for every stair set we built, for added stiffness.
  • Dimension Y doesn't really matter as long as there's sufficient material to fasten to from the end, even if only down low. It'll become prone to splitting on the wood grain, though, so consider piloting for screws.
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    I agree - all the sketches show adequate top tread size, but (a) and (c) in the extreme wouldn't be strong enough, if only attached to the wall/cap. If the top portion of the stringers can be screwed through the sides to a 6x6, that'd be strong even with smaller y/x.
    – tilde
    Commented Sep 14, 2021 at 19:48
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Is there a minimum distance that a tread run needs to be on a stringer?

No. Nobody cares if the tread is on a "stringer" or not as long as the tread is dimensioned properly and supported adequately. An existing board that is already there and is adequately supported by a structure other than your stringer is perfectly acceptable, as would be an existing concrete pad or other properly supported and adequately strong surface.

OR- cut the bottom of the stringer instead and make the top level with your 2 x 10 cap, as suggested by @SteveSh

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IRC says 10" minimum but there are likely exceptions.

I've heard that more important than meeting code-mandated minimums is consistency. Even with narrow stairs and short runs, the rise must be consistent from step-to-step (luckily, this is easy to calculate) because tripping is extremely likely with inconsistent rise over a span of stairs.

See the following illustration for code compliance, pulled from familyhandyman.com:enter image description here

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    I don't think this is what OP was asking, though to be fair, what OP was asking was not clear until the sketch was added.
    – Glen Yates
    Commented Sep 13, 2021 at 20:01
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  • OSHA 1910.25(c)(3) requires standard stairs to have a minimum tread depth of 9.5 inches (24 cm).

  • IBC 1011.5.2 requires that stair treads be 11 inches (279mm) minimum measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the treads leading edge.

  • IRC See Fredric Shope's answer.

enter image description here

OSHA and IBC determine the minimum tread depths and maximum riser heights based on research of stairways with various tread and riser dimensions. One important study by John Templer in 1976 showed that reducing tread depth to less than about 11 inches resulted in more missteps. Other studies have shown that as tread depth narrows missteps increase. Based on these results and others, the major building codes adopted standards for minimum tread depth and riser heights.

As this is not a major in-house stair, other than personal safety concerns, I assume you can use the minimum of the three.

Suggestion:

enter image description here

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