How to replace deck joist hanger in place?

Question

I have a deck joist hanger or two that I was considering replacing in-place if possible. The issue is this deck is 20+' in the air on uneven ground so supporting the joist from underneath before removing the hanger isn't that feasible. However looking at weight distribution, all the newer deck boards secured to it, spreading the weight all over the deck, would it be an issue to remove in-place and install a new one, or could that cause an issue with the joist sinking and making a mess of things? I also could probably use ratcheting straps to the next joists on either side to support from underneath I suppose if needed to distribute the weight.

I've also seen hangers that can slide over the old one and the mounting holes are in a different location, so if the experts here think that's better let me know (need the part number or link).

So is it possible to replace a joist hanger in place on 1 end without supporting the joist from below or is it not possible?

EDIT Here is the solution I ended up using which was a combination of all the advice given below. It uses (2) 4x4s as the posts, and (2) 2x4s for lateral supports that butt up against the deck boards on the backside. The rod is a solid 3/4" steel bar. The ratchet strap I'm using is good for 400lbs, but I have 1000lbs if needed. The ratchet will allow precise adjustment to be level once putting in the new bracket. The thing I like about this rig is it still allows me to work around and under it with clearance. If anyone reads this in the future there are probably a dozen good ways to do this and all the information provided useful.

Answer 1

You can indeed replace the hanger with only limited support.

It's not a beam. The dead load (weight of structure & flooring without people on deck) is perhaps only 1/5 of the designed live load (people, barbecue with meat, structure & flooring), and currently holds just a portion of the weight of the floor boards.

So as long as you can stay off the pertinent joist while working, you should be fine. For this, lay some 2x lumber flat across the neighbouring joists. Your existing floor boards may already provide enough bracing to provide you with a safe working surface, so it depends on where the boards are removed and where you are working.

Then you'll need to support the joist while replacing the hanger. The temporary support must enable you to re-align the joist to its original location, so that the member is flush with the beam it attaches to. For this you have a few options:

1. a 20-ton jack with a long 20ft post, or just a long 20ft post which you jam between joist and a pad on the dirt.

Best to cut the post to size, plus some extra, so that it's a very tight fit and requires a (sledge) hammer to put in place. It's ok if you need to add a shim or another board on the pad to provide extra lift or adjustment.

20ton is what I have at hand, and probably overkill for a (deck) joist: they're used for holding up beams and headers. A 2-ton would do as well.

The post in question can be two 10ft posts joined butt-ended with wood or metal straps bracing the joint. Yes there is generally a buckling hazard at such a joint, but this is temporary, your load is not high, and so this should do.

2. Ratchet strap the joist to neighbouring joists by running the strap under the joist under repair and over the neighbour joists on either side. The "v" shape thus formed by the strap should have a sharp angle not an obtuse (blunt/flat) angle. If the angle is too blunt, the tension on the straps may not suffice to hold up the joist and keep it aligned. The height of the joists might suffice to get a sharp angle, but if not, you can add a block of wood between joist and strap to sharpen the strap angle.

3. Brace the joist under repair by a few pieces of 2x lumber from underneath. Fasten them flat to the underside of neighbouring joists, one joist on either side of the one under repair. This is temporary so the joist under repair can rest on the brace. The brace may buckle a bit, and you can make up for it by inserting shims between brace and joist, until the joist is aligned again.

A further option, based on the idea in the answer by Greg Hill, is a lever:

4. As a variant of #2 and #3, apply a lever resting on two neighbouring joists as in the drawing. Use shims for adjustment, placed between the red bar and one or both of the blue blocks. This requires only one person and is entirely installed from above. No notching of the wood is required if the slope of the lever is minimal due to use of two blue blocks.

You can dispense with the second block if the first one is shallow, thus keeping the lever horizontal enough so that the green strap does not slide off. You can then shim the lever on the non-block side. The blocks can, but do not have to, partially rest on the beam.

Answer 2

With all the furniture and people moved off that area of the deck it'll take little force to lift the joist tail into position. A person might be able to do it manually, but it can easily be accomplished with a lever (ie a 2x4). If you use the end of the lever as the fulcrum then you can lift on the lever; if you place a fulcrum in the middle of the lever then you can push downward on the lever to lift the joist. I'd choose the former because I can stick a block, a bucket, or some other object under the lever to hold it up without the help of an assistant.

Use chain, rope, ratchet strap, etc to connect the lever to the joist. Arrange things so that the lever is at a low angle. This will minimize the chance of the chain/rope/strap sliding down the lever.

Answer 3

When you remove the old joist hangar, the joist is going to drop down a bit. I don't think it would be much, 1/4" to 1/2" since the joist will be supported by (hanging from) the deck boards.

But you need to find a way to bring the joist back up to be even with your beam before you attach the new joist hangar.

You may be able to pry it up using the adjacent joists as the fulcrum point.

Do you have an automotive floor jack, or can you borrow one? If so, you could run that out to the beam and use a sling under the joist and over top of the jack lift and use the jack to bring the joist back up to level with the beam. Most floor jacks have something like a 4,000 lb lifting capacity, which should be more than ebough here.