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My current stairs are built inside the stringers, I want to cut the stringers as shown in the picture on both sides in order to lay treads and have them overhang. Can I do this? Or will it weaken the stairs?

Current Stairs

Red lines indicate where I want to cut. I want to cut at every stair and on both sides.

Current Stairs

Goal

Goal

  • The addition of a wall under the outer stringer is the crucial detail here, you're not JUST cutting the stringer. – batsplatsterson Nov 1 '18 at 10:04
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Cutting your stringers that way will absolutely weaken them. Your "After" diagram shows the space under the stairs filled in with something; if that "something" does a good job in supporting the newly-weakened stringers, then you may be OK.

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    Agreed. Normally a stair set that size would have at least three notched stringers, and they'd be sistered on all interior sides with additional lumber. Now, if you were to add suitable lumber inside each of the two stringers, you might be ok. – isherwood Nov 1 '18 at 2:26
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No, changing (notching) the “skirting” into “stringers” will not affect the stability of the stairs, provided that 1) the new stringer against the wall is nailed to the studs in the wall, 2) the stringer at the “open” side of the stairs is supported by a wall, as shown in your “Goal” picture, and 3) riser boards are at least 1x and installed as shown.

The load of a stair is carried by the risers spanning from side to side of the stair. It is not carried by the balance of the notched stair stringers. If you look closely at a notched 2x12 stringer, there is only a 2x6 left. That 2x6 will not support much of a load when it spans 12’ or so...from top of stairs to bottom of stairs.

However, a 1x6 (or so) riser can easily span 3’ - 4’ and support a live load of 40 psf as required by code.

Often an additional stringer is installed at the center of the stair. This stringer is not to help the other stringers span from top to bottom of stairs. Rather, it’s to help keep the riser “square”, plumb and straight and to help the tread from bowing down along the back edge.

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    I have to disagree with your final paragraph. The center stringer absolutely does strengthen the lengthwise span, and many modern stair sets with their long spans would be dangerous without it. Taller walls and floor systems result in 3-5' longer spans than in days of yore. Even with stringers cut from 1-1/4" OSB, stiffness and overall strength would not be adequate with just two stringers. – isherwood Nov 1 '18 at 15:16
  • @isherwood Taller walls create longer spans and that center stringer does less and less. The reason the center stringer is important is because it keeps the riser board “in-line” and “working” properly as a support “beam”. When there’s no support on the sides of the stairs (or no riser boards) then an intermediate wall (or two) is installed to reduce the span of the stringers....just adding more stringers won’t help much. – Lee Sam Nov 1 '18 at 16:36
  • Erm, tell a bridge builder that. :P – isherwood Nov 1 '18 at 16:43
  • @isherwood Bridges are built differently. First of all, they are designed using dynamic loads not static loads. Second, bridges are designed using chords in a triangular pattern. They don’t use members that have been notched...notched in half like stringers. – Lee Sam Nov 1 '18 at 17:04
  • Hooboy. I won't go much further with this, but the notches aren't part of the equation. The uninterrupted lower portion of the stringer is essentially a beam. More beams equal more strength and stiffness. I'm not sure how that's even debatable. (So I won't. :) ) – isherwood Nov 1 '18 at 17:08

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