Framing plan for 24x36 1.5 story addition--any glaring mistakes?

[Ernest T. Bass]

Hey all,

This is actually going to be a very large addition, but it's being built as a free-standing 4-walled structure. 24x36, 4' knee walls for loft headroom. I'm thinking that this modified balloon framing is the way to go, but am very open to suggestions! Do you see anything wrong with this idea?



I haven't drawn every detail here yet.. Notable missing elements would be fire-blocking and gable framing.




This is something that has me a little concerned.. Are the cripples above the windows going to adequately support the 2x4 let-in ledger? I could make the headers full-width with foam insulation, but this would be easier.

I read through the whole thread about let-in ledgers being no more than 1 3/8'' thick.. But since this roof has a structural ridge, this wouldn't pose a problem, right?



The owners want a large 48'' window in the gable. I calculated that a 3x 2x12 header would handle the load, I figure gluing some layers of 1/2'' ply in there would be a good idea too. About transferring the load to the 1st floor studs.. I thought that notching a beefy header  into the stud under the point load would help spread the load on the foundation, and would be better insulating than a 3-stud post added to the wall. Will this work?



Front elevation.. 2x12 rafters, either a metal strap across the top or scabs above the ridge beam to tie them together (not drawn).



Partial cathedral ceiling. I figured a double 14'' lvl ridge beam with posts on 12' centers, but it will be spec'd by the lumber company when the time comes. The owners are going to be doing a good portion of the work, and wanted platform framing originally. From what I understand, this method will be more rigid and conservative of material. Any thoughts on that?

Thanks a bunch! I don't want anyone to spend a ridiculous amount of time re-running calcs (Don P... ), unless something about the design really bothers you.

[Don_P]

The one place you platform framed is the one place you can't. The cathedral section with a plate drawn across it has to be balloon framed... and at that height and supporting the ridgebeam it won't be with 2x6's. That area needs an engineer. The other gable should be balloon framed from the second floor up, or full height if you want, but again there is an unsupported hinge point across the gable wall.

If you're using single piece headers check not only their bending strength but also their side grain compression under the loads you have. Their bearing surfaces might be in crushing danger over the jacks and studs.

[Ernest T. Bass]

The cathedral section is being built up against the existing structure and will be tied in, so I don't think wind loads are a problem. Does this eliminate the need to balloon frame it?

And on the other gable, doesn't the floor platform brace the whole wall well enough? I thought about dropping the plate on that end to the floor height, mainly for a bigger window, but I was afraid that the structure wouldn't be tied together well enough without the lapped top plate corners. Could we use some steel strapping or other means to connect unevenly-heightened corners together?

For once I wish I could design a nice simple place with no complicated elements...

[Don_P]

Myself... it doesn't exist 
I'd think the cathedral wall is fine then. Strapping around the corner would be great on the other gable... I've always relied on the wall sheathing in that area to do the work of tieing it together.

[Ernest T. Bass]

So, do you think the plate should be lowered, then? Would I go underneath the joists and run the floor platform out between the walls, as in platform framing?

If the only purpose is to brace the gable wall, though, I'm not sure why it matters where the plate is, as long as that last joist is bolted to the studs and attached to the loft floor. I could also add a right-angle 2x8 or something to stiffen up that wall at the loft level, as I've seen you detail in tall, open gable walls.

[Don_P]

I'd frame it typically, the 1st wall under the joists and the loft wall on the floor above, that end is now firebreaked as well. The problem with putting a horizontal beam across a 24' wide space is it'll need to be pretty hefty. Mentally turn the wall on it's side and look at it like a floor, that's pretty close to what is going on when the wind hits it hard. The girder under that section spanning 24' would not be a double plate, or a 2x8, it would be a big LVL on well footed posts. The end connections of the horizontal beam to the side walls would need to be well though out. The short span here is from the loft floor plane to the roof plane if you can do it.

[Ernest T. Bass]

Sounds like the best way to go, and it opens up more options with the window height, too. I'm still not sure how a rim joist between the plates at that point is going to secure the wall any better than having the floor section bolted to the side of it, though.

Another thought is the central interior wall hitting that gable on the 1st floor. We could let-in some 1x4 braces into the wall, as you would an exterior wall to help further brace the gable.

They had been planning on just using t&g over the loft joists, but I can see that some sort of diaphragm-bracing sheathing for that floor would be in order to properly brace that gable wall, unless there was some well thought-out framing in place.

Okay, here's a few ideas I sketched up..



I thought it would be more simple and at least as strong to just drop the plate to the level of the joists and run the sheathing out to tie it all together. Any problems with that?



Hastily-drawn let-in brace on the interior wall.



Blocking between loft joists.

Looks to me that the sheathing is the strongest option, but do you think the let-in and the blocking could handle the bracing on their own?

Thanks so much once again, Don!

[Don_P]

I enjoy seeing what you are up to 

Sounds like the best way to go, and it opens up more options with the window height, too. I'm still not sure how a rim joist between the plates at that point is going to secure the wall any better than having the floor section bolted to the side of it, though.

What I was really trying to eliminate was the unbraced plate halfway up the loft wall.
The let in brace on the midwall is fine but probably overkill if the wall is sheathed, even in sheetrock I'd bet the sheetrock panels have more bracing capacity than the single let in brace. I believe you are required to have a brace wall every 25' so you are not required to have anything there.

The floor blocking would work if you nail the flooring to each diagonal block to keep it from sliding under load. Board sheathing was traditionally applied diagonally to triangularize the structure, you could do that with the T&G. Code minimum allows 1x T&G applied perpendicular to the joists with nothing else. Not saying that provides much diaphragm. The unbraced plate was the only real problem I saw.

[Ernest T. Bass]

Well, thanks again for all the help! Now and past.. I'm not really sure how you manage to help so many people here to such great lengths with nothing in return, but I can assure you that we sure appreciate what you do!

I'll keep all this in mind when we decide what materials we use. The interior wall covering we use may not offer too much bracing, so in the event that we lay the flooring perpendicular instead of diagonal it sounds like the let-in may still offer some additional support to that gable.