Upside-down Joist Hanger: Deck Over Concrete Stairs

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Old 06-08-14, 10:13 AM
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Upside-down Joist Hanger: Deck Over Concrete Stairs

I'm building a small, low deck over our concrete front stairs. Because the threshold of the door is about 3" above the concrete stair, I am not able to put joists there and will be supporting the decking there with 2x4s laid on their wider sides.

The rest of the deck will be supported by posts. Since the part of the deck closest to the house would be above disturbed soil (next to the foundation) I am wanting to use the concrete stairs as an obvious primary support for that end of the deck. The first step down gives room for a 2x8 joist.

See photos.

So...

Looking in the attached pictures, the longest frame members (running out from the house, alongside the stairs) would be supported by the 2/8 resting on the stairs. Joist hangers are designed for situations where the "butt end" joist is supported by the "through" joist/beam - but in this situation it is the opposite case. My idea is to mount the joist hanger upside down so that the "through" joist hangs off of the stair joist.

Has anyone used this technique before? Any alternate suggestions?

I'm not wanting the stair joist to stretch full width as that would structurally split the deck into two parts (front/back) which could sag/shift independently of each other, and it leaves the back half needing much support from the next-to-house posts instead of benefiting from the cantilever effect of the longest joists extending away from the house.

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Old 06-08-14, 11:48 AM
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Welcome to the forums. Hopefully the steps have a good footing. You won't turn a joist hanger upside down at any time. You'll need skewed hangers for the angle joints. Where you have joints that you can't use joist hangers, you can use angle bracing on the exposed inside angle.,
 
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Old 06-08-14, 03:19 PM
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I've seen upside down hangers advised (in a carpentry book) for cantilever applications - when the net force is an uplift force a normally mounted hanger is upside down in terms of direction of force. Consider a rim joist attached to joists cantilevered over a beam. What is hanging off of what in this situation?

The stairs should provide plenty of stable support given that they have held their own weight (which dwarfs that of the entire deck) over years of freeze-thaw cycles with no movement relative to the house.

Are there any other suggestions for supporting off of the stairs?
 
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Old 06-08-14, 03:51 PM
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IF, you had a cantilever, yes, but you technically don't. Your outer posts should likewise be sitting on post bases on 12" footers to the frost depth of your area. This would keep a cantilevering from occurring.
 
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Old 06-08-14, 06:21 PM
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I know it's not cantilevered - my point was that the forces are acting the same way. I might not be clear about which attachment point I am talking about.

In the image below, the red circle is the attachment point. Because the "butt end" joist is resting directly on the stair, there is no possible downward force caused by this joist. The "through" joist, which the "butt end" joist is to be attached to at the red circle, does apply a downward force - and only ever a downward force due to it's own weight and any load carried.

Using a joist hanger right side up makes no sense here, as the "hung" joist would be pushing upwards and out of the hanger as the joist it is attached to pulls downwards. In other words, it is not hanging from the other joist - it is holding it up .

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Frost depth is up to 3.5 meters (12 feet) here in northern Canada. We know all about what that entails (there are some roads that look like rollercoasters) and we do what we can. The soil beneath our house is sandy glacial riverbed and drains well. Frost heave is minimal.

I am interested in hearing alternate solutions. I'm not mounting this - as currently designed - with a right-side-up hanger though.
 
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Old 06-08-14, 06:45 PM
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If the steps are not experiencing any lift from heave, you won't experience any upward forces calling for a joist hanger upside down. I would suggest an inside corner brace as well as face nailing the through joist to the cross joist. I am assuming you don't have any room between the concrete and cross joist to install a joist hanger anyway. Is that right?
 
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Old 06-09-14, 07:53 AM
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It's not lift from heave, its the reaction force balancing the weight. Newton's third law. The cross joist on the stair pushes up on the other joist simply because the other pushes down on the stair joist. The forces are balanced, so nothing moves. If the forces were unbalanced then movement would occur - like gravity pulling on an object with nothing under it pushing back up (holding it up).

There is space for hardware since the stair joist must be held off of the stair with PT shims to bring it up to height.

The inside corner brace is an idea - though it still leaves the weight supported by nails instead of the "shoe" of the hanger.
 
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Old 06-09-14, 01:47 PM
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You don't have enough of a lever (Archimedes) to cause uplift. Your off hanging joists are too close to the steps to lift easily, and by the time you get all the flooring it will become a monolith, almost. If you have the room and feel warm and fuzzy, install the upside down joist hanger. You still won't have any means of preventing downward force.
 
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Old 06-09-14, 06:38 PM
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I appreciate your time.

To be clear I am not talking about uplift but reaction (normal) force. Nothing is lifting.

There seems to be a fundamental misunderstanding either of what I am planning to do, or of the physics involved. I am not seeking verification of how forces are distributed here - I have a firm grasp on that.

I AM interested in hearing alternate solutions for supporting the deck - simpler or cheaper if possible. Maybe someone has done something similar. Or, input from people who have worked with upside down hangers... like "chisel out a sliver of joist so the hanger sits flush on top."

Again, I am not wanting to argue about the direction of forces at the hanger. Having completed the first half of a mechanical engineering degree, and finished off majoring in math and physics, I am not in doubt about this and do not need to discuss it further. I appreciate the dedication to keeping me from making a perceived mistake, however. Again, thank you for your time.
 
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Old 06-09-14, 07:23 PM
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Just don't let your physics booklearning get in the way of building your deck. I work with architects who never set foot on a jobsite and get held up while they argue with me on why it will work, and I am secure in my argument why it won't work. I always win, but I waste a lot of time.

Not intending this as a demeaning statement by any means. I am secure in the fact that a corner brace and end nailing will give adequate support in both directions.
 
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