building a second floor floor system on top of existing attic floor system 2

[lubos1984]

[sub][/sub]
Good Morning.
I had a contractor reach out to me about building a second floor addition on top of the first. However, he would like to leave the existing attic joists as is to avoid removing drywall on the floor below.
I've attached a sketch of what he proposes. My question is does adding an additional floor system without removing the old create an additional hinge that needs to be examined ? If so how could this be remediated ? Maybe add thicker sheathing on the outside.
Thanks for your help guys.

 

[Eng16080]

I assume you're concerned with out-of-plane wind loads on the exterior wall and the connection between the top of the wall and the existing ceiling / new floor diaphragm. If that's the case, then if the existing connection between the wall and ceiling is adequate, I wouldn't mess with that. From there, assuming that you want the load in the ceiling diaphragm to transfer into the new floor diaphragm, I would look at providing connections between the ceiling joists and new floor joists.

This could be accomplished be aligning one side of the new I-joist with the side of an existing ceiling joist below and connecting the two with a plywood gusset plate. You probably don't need to do this at every ceiling joist, although that depends on your loads.

 

[SWComposites]

I would add blocking between all of the existing ceiling joists on top of the existing wall top plate.
And they need to remove the existing exterior wall sheathing and replace with sheathing that is continuous thru the old ceiling, new floor and the studs above and below.
Is the existing 2x4 exterior wall good for the added second story loads above?

 

[StrucPatholgst]

I had to unpack and recalculate one of these projects a few years ago. The owner wanted to live in the first floor while he added the second story. He ended up passing away before drywall and siding went up, and during the estate sale a buyer found out the original owner never pulled permits or got the foundation certified. Buyer hired me to check the whole thing out. I looked at every fastener, every stud, every joist. Dug up and measured the footing. The prior owner basically built a code compliant second floor platform, attic, and roof structure on top of a 1940's era first floor and foundation. I think I recommended a couple of different beams in certain spots, a couple shear panels on one of the long walls, blocking at the transition, a boatload of hold-down straps and ties, but if I remember correctly all of the fastener schemes and joist spans were on the money. The old man did a decent job.

 

[jerseyshore]

I get this situation a lot around here and this is my answer to everyone: It sounds good in theory, but don't bet on the drywall actually staying up there in good condition.

I tried it with my own house. One day of rain and that drywall is coming down. Just guys working and walking around up there is going to mess up the drywall on the ceiling.

Some people say, okay I'll put a tarp on the house every day. Sure can, then you don't need the existing ceiling.

Others say, I want to minimize the dust on the first floor. Well who doesn't, but that is only going to barely help (if the drywall actually stays in place). It's going to be dusty and dirty no matter what you do.

You're not saving that much in the end. A drywall crew can knock out a whole ceiling in a couple of hours.

If the existing walls are pretty level, then I usually just recommend sistering with new TJI's. Around here most people don't have that extra height to put joists above joists.

If the walls are uneven and they want to level out the 2nd floor, I do a similar detail to what you posted in the OP. They can rip that plate to be level if necessary. Always cut down the existing sheathing and lap it over the entire joint.

 

[LittleInch]

What does this look like in plan and view from the left?

Is spacing of the I joist and 2x 4s exactly the same?

What is at the end of the existing ceiling joist as it only seem to bear on half of the wall?

but agree with jersey - the reality is very different from theory and that existing attic space won't really be usable / livable when the building works start. Especially once someone drops a hammer through the ceiling.... Or a foot.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KootK]

My question is does adding an additional floor system without removing the old create an additional hinge that needs to be examined ?

Yes, it does. And it's rather keen of you to pick up on that. Per the model below, you would be forever relying on the ceiling sheathing to act as a diaphragm to stabilize the system under gravity loads. Maybe you're cool with that and maybe you're not. It's certainly been done, for better or worse. It doesn't take much to brace something like this for gravity loads.

If so how could this be remediated ?

I imagine that you would have to turn the ceiling joist system into a diaphragm that you do have confidence in. Some options:

1) Sheath the top of it with wood panel sheathing entirely.
2) Block and sheath the top of it with wood panel sheathing in a doughnut shaped ring around the perimeter.
3) Install in plane diaphragm trussing in the form of 2x diagonals attached to the top of ceiling joists.
4) Install some substantial LVL on the flat around the perimeter to be the perimeter diaphragm.

Obviously, all of these solutions have the potential to mess with the economy that your contractor hopes to achieve with this. My guess is that #3 would be the preferred solution with respect to cost.

 

[XR250]

I'm gonna have to disagree with KootK.

The 2x4 plate between the old and new will transfer the shear load up into the new floor diaphragm. Regardless, we count on sheetrock ceiling diaphragms for restraint in basically every house with a gable roof or where the floor joists run parallel to the exterior wall.

 

[KootK]

The 2x4 plate between the old and new will transfer the shear load up into the new floor diaphragm.

Unfortunately, that does nothing to stabilize the low hinge in my sketch above which, I believe, is the core issue for OP here.

Regardless, we count on sheetrock ceiling diaphragms for restraint in basically every house with a gable roof or where the floor joists run parallel to the exterior wall.

I agree that is often done but it's sketchy and there are other ways:

1) For a gable roof, you've got the ugly diagonal bracing from the top of the wall to the roof sheathing.

2) Where floor joists run parallel to exterior walls, there should be blocking to connect the tops of the walls to floor sheathing.

 

[KootK]

The 2x4 plate between the old and new will transfer the shear load up into the new floor diaphragm.

Actually, you're right. I see it now. That's actually what I was getting at with the two fasteners that I showed in my sketch but, somehow, I got turned around.

That mechanism applies a moment to the end of the joists where there compression flange would be unbraced but that's probably not much of a concern. Maybe just throw a line of blocking a few feet in from each end of the ceiling joists.

Thanks for the correction.

 

[XR250]

I agree that is often done but it's sketchy and there are other ways:

1) For a gable roof, you've got the ugly diagonal bracing from the top of the wall to the roof sheathing.

2) Where floor joists run parallel to exterior walls, there should be blocking to connect the tops of the walls to floor sheathing.

I agree, but I rarely see it.

 

[KootK]

I agree, but I rarely see it.

Not even the floor joist blocking? It seems nuts not to toss blocking in for a couple of joists at that condition.

 

[XR250]

Not even the floor joist blocking? It seems nuts not to toss blocking in for a couple of joists at that condition.

Right? Especially with trusses or I-joists.
I also rarely see it in similar basement conditions. Welcome to the South!

 

[jerseyshore]

Another thing I almost always specify when extending a wall up like this is to have them install new wall studs full height, usually at 48" o/c max, maybe 32" o/c if it's tall. I always try to minimize any problems that hinge might have. There's nothing better than a full height continuous stud.

 

[LittleInch]

Why can't you just lay the new engineered joists in between the existing joists? Or maybe add 1 inch x 4 inch plate to create a small gap bergen drybiard and new joist?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.