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I'm not even sure how to describe what has happened to the wooden stairs in my basement, but have included a link to a picture. One stair tread has come out of its slot(?) And one of the support boards has separated from the tread. Not sure where to start on this one, primarily concerned about structural integrity and safety.

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    You need to get behind and under the stairs and see what has failed. Some wood rotted. Maybe termites. Perhaps there was an anchor that failed. The entire staircase may not have been built well. We cannot say without seeing the structural side.
    – RMDman
    Commented Feb 18 at 14:29
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    Fair point, the underside is accessible with some difficulty. I'll give it a shot Commented Feb 18 at 15:08
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    I added two more pictures showing directly under the right tread, and width-wise along the underside where you can see where the tread and support have come out of the spaces carved for them. I get the feeling wood isn't supposed to be "missing" around the bottom of the support board as you can see in image 2? Do these photos help? Commented Feb 18 at 17:16
  • These stairs appear to have risers which are angled back to give an extra inch or so of tread width. Is that correct? Together with the nosing of close to an inch this ought to make these stairs easy to use. Are they? (Some basement stairs are pretty treacherous.) What is the rise and what is the run of these stairs? The rise of course is the vertical distance from the top of one tread to the to top of the next. I think the run is the horizontal distance from the edge of one nosing to the edge of the next. Commented Feb 19 at 2:31
  • Can you get between the side of the stairs and the wall on both sides? Threaded rod, washer and nut outside, turnbuckle in the middle. Commented Feb 19 at 13:17

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Notwithstanding other opinions, here, this seems to be only faintly bad. (Not "rebuild this whole staircase!" Not in a million years.)

Yes, the tread has lost its wedge and fallen out of the housing on the stringer. Don't step on it.

A perfectly adequate fix would be to push the tread up from underneath, and firmly attach a 2x4 to the stringer to support it. (4-3" screws, predrilled should be sufficient.)

Opinions from afar are worth approximately what you've paid for them. If you're uncertain, get a pro in to look at it and see what they say.

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    That's exactly what I ended up doing. Looking at other ways to reinforce the structure, maybe stand some 2x4s vertically to support right where the treads meet the risers? I'd need to properly fasten the wood to the concrete floor though Commented Feb 19 at 3:32
  • Vertical supports would clutter and obstruct the space. You might put a supporting cleat on the other side in case the work done already would pull the tread out of the slot on that side. Commented Feb 20 at 0:45
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There’s nothing much holding that stair up! I wouldn’t put any weight on it, if I could help it, until the entire staircase is repaired or replaced. It’s already fallen quite a bit, and is about to collapse.

On modern residential wooden stairs there is a notched board underneath both edges of a stairwell called a stringer, which the treads and risers rest upon — nails are then used to fasten them in place. The stringers rest on the floor (or a piece of PT lumber, if on concrete), and are nailed to the bottom of the frame at the top of the stairs.

I thought the original builder was trying to make a housed stringer — a stringer board with essentially dado joints joined with the risers and treads — but like you identified, that joint has failed.

Well, actually, that joint was probably always bad. It wasn’t very deep, and the walls have spread apart slightly as the house has moved over the decades. It was never even close to wide enough to support the full tread, either. It honestly seems like the slot was an “oh s*%t” fix by a lazy/DIY carpenter to make the treads fit into a skirtboard, which is supposed to be decorative, not structural!

To repair, you have a number of options:

  • Rebuild the staircase, improving the existing stringers. Move the stringers closer to the treads so they won't pull out easily. Use deeper grooves for the joints.
  • Cheaper and easier, demolish it and replace with new materials. At least then you'll know it's built to your expectations. But, that old wood isn't rotted, and it is much denser than the lumber we can get at the big box stores these days, so you might like to keep it around.
  • Use cleats or stair-angles to extend the sides of the existing joint. Don't forget that the whole staircase was built by the same person, so you should do this to both sides of every stair, not just the one that happened to fail first. They're all suspects in this slow-motion murder scene.
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    For what it's worth I'm pretty sure they have been this way for many years so giving some reinforcement might buy some time. Appreciate it, I'll take a look at what I have to use. Commented Feb 18 at 18:44
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    You could also use either timber cleats or stair angles, to better hold the pseudo-stringer to the risers and treads. Commented Feb 18 at 18:50
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    The terms "pseudo-stringer" and "decorative skirtboard" seem hyper critical. What do you think the thickness is? If it is well secured it should be strong enough. The slots are not deep enough. Could these stairs be made sound by putting in cleats below (assuming there is access)? Seems to me a lot of videos show overbuilt stairs in the US. Taking wide 2x lumber and cutting out triangles is wasteful. Procedure in some other countries is to make slots in the stringers, at least for the treads. Commented Feb 18 at 19:22
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    The truth might be somewhere in between here. The stringer/skirtboard is about an inch thick I believe. The primary issue seemed to be that the the fallen step is not wide enough to actually reach the notch (dado joint?) that's carved into the stringer (bottom arrow in my first photo). As a temporary fix, I screwed a wood block to the stringer underneath the tread (I have some access underneath) at the proper height. I also screwed the tread itself to the stringer for good measure. Doesn't feel any less secure than before the issue, so hoping it holds for a while. Commented Feb 18 at 21:37
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    Certainly well over the top to reconstruct the whole staircase. Not necessary at all.
    – Tim
    Commented Feb 19 at 13:34
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Use of a car jack, or acro-prop, depending how high the step is. Jack up the offending tread, level with the routed slot. Drill through the stringer, with holes wider than the screws you'll screw through into the tread. Using minimum 3 screws with hex heads will mean larger ones can be used, as you'll be screwing into end-grain, but that can't be helped. 4"-6" will do a good job. They should pull the stringer and tread together.

Then use a strip of, say, 1"x1" underneath the tread, screwed into the stringer, like a bracket, to keep it perpendicular to the stringer, as a safety measure. Glue is an optional extra.

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  • This is pretty much what I ended up doing and it seems stable. Thinking I should do that for all the treads at this point for good measure. People have also mentioned cleats, how would that work? Commented Feb 20 at 12:42
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Here is a video of a Japanese carpenter using slots for the treads (and the risers). Astounding confluence of expertise, tools, wood. This may make sense in a country with such high magnitude earthquakes, but . . .

The late American carpenter Larry Haun (The Very Efficient Carpenter) had the mission of producing good quality, affordable housing for the average home buyer. He used wide 2x lumber for stringers and cut triangles out of them. And Haun made significant overcuts on the stringers to avoid having to finish with a hand saw. Most American carpenters finish with hand saw to avoid weakening the stringers.

In the one example of his method (go to 48 min 46 s) that I have found, the risers were angled back at the bottom to give an extra inch of tread width, treads were plywood with no nosing, covered with carpeting. But of course basement stairs could have rubber overtreads giving just the right resilience and grip. Larry Haun's stairs are crude compared to dramatic high style ones but would seem to be OK for a basement.

The problem with this OP's stairs could be remedied by supporting the treads with cleats or metal brackets attached to the stringers below the treads.

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