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I am trying to relocate the stairs on an existing deck. The problem is that there is a concrete slab under the deck so I can't just easily cut a hole through it to put in footers to support the stair landing.

I have an idea which would allow it to be supported by a beam attached under the bottom of the existing beam and joists. Seems like structurally it should be fine, but I don't know if it's considered acceptable practice.

In the image below, the lighter colored structure is the existing deck. The darker structure is my proposed addition. The blue is the back of the house it is attached to.

The existing joists are 2x8 spaced 16". The span of the shorter ones on the existing deck is about 8'. The longest ones at the ends are about 10'. I would hang the new double 2x8 beam under the existing joists using 4x4's with through bolts. The back is obviously supported by the house, so this is just to hold up the front of a roughly 3'x4' landing and the staircase itself. (stairs are just shown roughly to illustrate the direction they will run)

enter image description here

Are there any reasons why this would be a bad idea?

isherwood
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Steve In CO
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  • Forgot to mention I am in Colorado where we have to be concerned about freezing. The existing deck supports are on footers which go below frost line. Isn't there a concern with seasonable movement of the slab putting stress where it's attached to the house? – Steve In CO Apr 04 '22 at 15:57
  • @Ecnerwal, any deck attached to a structure must be properly supported. A standard slab doesn't qualify. – isherwood Apr 04 '22 at 20:01

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Your idea is clever, but it has a couple weak points.

  • Deck joist cantilever is usually limited to 2 or 3 times the joist height, depending on local code. Beams tend to be even more restricted. You probably won't get to the 42" or so you need for a proper landing. Even a snug 36" likely wouldn't pass inspection (hypothetical or real).
  • You can't rely on fasteners in shear to carry load. In your plan, the support blocks would presumably be attached with screws. That's not usually allowed, especially with beams. Even through-bolts may not be allowed. You'd need rated structural steel hangers, particularly at the rim joist near the landing. That's where the highest load occurs.
  • The cantilever compounds load at that rim. You'll need to be sure it's up to the task. Your drawing appears to show a tripled rim joist, which may be adequate, but you'll want to do the math.

If you can overcome those issues I think it's a good idea. Otherwise, you only need one post (at the outside). The inside can be carried on the rim joist. It's not terribly difficult to saw a square through a slab, and a post-hole digger will get you to footing depth.

isherwood
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  • Re fasteners carrying load: Other than the rim joist wouldn't the force be upwards against the joists? – jay613 Apr 04 '22 at 20:56
  • @isherwood I fear you may be right, and it's probably best to just bite the bullet and cut a hole. Given today's lumber prices it's probably the cheaper option too, even factoring in the cost to rent a saw. – Steve In CO Apr 04 '22 at 20:59
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    @jay613, it's complicated. If you consider the rim connection to be fixed, yeah, the other points are in compression. However, if the rim connection sags or slips, the next one is in tension, and so on. I'm not sure how the calculations are done. – isherwood Apr 04 '22 at 21:14