Vertical Home Addition

[Ben29]

I have a tiny house that is getting a huge vertical addition. The existing exterior walls are comprised of 2x4 studs at 16"o/c. The owner wants to keep the existing roof framing and wall framing in tact. Note in the drawings below how they want to add a tiny knee wall on top of the existing 2x4 stud wall, and then add a new floor and 2x6 stud bearing wall above. Of course I will check to see if the existing wall works for the additional load, but furthermore I am concerned about this "kneewall situation" - should I be?

Also, is there anything that I need to be careful of when installing a new 2x6 stud bearing wall above an existing 2x4 stud bearing wall? I read online that this is a common practice for renovations, but I personally have never done that before.

PS: KootK, if you are reading this, congrats on being engineer of the week!

 

[phamENG]

I think you're on the right track. I wouldn't be concerned with the knee wall. The bottom is braced by the existing roof diaphragm that is staying in place, and the top will be braced by the new floor diaphragm above. It's not unlike a split level framing arrangement where a 3' wall goes up to catch the split level above and then restarts to to the roof.

For the 2x6 on 2x4, just look at the statics in the intersection of walls and floor. There will be some minor eccentricities. These are often ignored, but as it's your first time it would be good to run through the exercise and get a feel for what they are, where the come from, etc.

 

[KootK]

KootK, if you are reading this, congrats on being engineer of the week!

Thanks Ben29. I'd love to help you here but, unfortunately, you can't be the grand puba two weeks in a row. So there's really nothing in it for me...

I'd love to see that existing roof gone. Why does the owner want to retain it? To save the cost of refinishing the space below?

 

[Enable]

Agree with phamEng: eccentric loading condition created from mismatched sills of 2” is no big deal in platform framing. I may have a marginal concern with offset studs; would probably double up the sill plate on the 2x6 to better distribute over the sheathing.

Also agree with KootK: seems like install of joists for the room above the roof with necessitate removal of the roof sheathing. This will significantly reduce the integrity of the diaphragm. If this is required, you may want to consider additional blocking or partial re-sheathing to offset.

Stud wall is not a concern by itself. You can hurricane strap them if you really want. Though, I would be concerned about integrity of the existing rafter ends. You would want to check them for rot, general decay, and/or carpenter ants that may have significantly reduced their capacity.

You didn’t ask but since it’s on the Drawing, the proposed CMU wall to pick up the office / living room might be able to be swapped out for a bolted angle iron into the adjacent foundation wall (Hilti HY-200/HY-70 + 3/4" threaded rod are plenty strong). You would not need to build a new footing / easy for labour to do vs cost of a mason. I know you said you had an aversion to a ledger…but how about a steel ledger?!

 

[Ben29]

Highlighted below is the existing portion of house to remain (elevation view). The house was built in 1956. The monoslope roof is sloped 1" per foot. I don't think I can install a wall on top of a raked wall, can I? I think I need to demo the existing exterior side wall and rebuild it in order to provide a flat surface to build a new wall on top of. Right?

 

[Ben29]

I'd love to see that existing roof gone. Why does the owner want to retain it? To save the cost of refinishing the space below?

They want to live in the house (in the basement) while the addition is installed!!

 

[Ben29]

You didn’t ask but since it’s on the Drawing, the proposed CMU wall to pick up the office / living room might be able to be swapped out for a bolted angle iron into the adjacent foundation wall (Hilti HY-200/HY-70 + 3/4" threaded rod are plenty strong). You would not need to build a new footing / easy for labour to do vs cost of a mason. I know you said you had an aversion to a ledger…but how about a steel ledger?!

Thank you for the suggestion. I suppose a 6x4x3/8 (LLV) galvanized angle would do the trick? I will need to assume something for the existing footing and check to see if it can take the additional load.

 

[Enable]

They want to live in the house (in the basement) while the addition is installed!!

LOL been there and done that. Never again. People don’t realize how annoying construction can be until they are in the thick of it. AND with them home all the time, everything will be inspected by their uncritical yet wandering eye and compared to what they found online (EDIT - it is the constant answering of these questions that is so annoying that I could not bear to do this again. Never in a million years).

Thank you for the suggestion. I suppose a 6x4x3/8 (LLV) galvanized angle would do the trick? I will need to assume something for the existing footing and check to see if it can take the additional load.

I would think so. Could also put some stiffeners on there just because.

I don't think I can install a wall on top of a raked wall, can I?

You can absolutely build a wall on a rake. Why not? Your new wall will be plumb so your load coming down will decompose into a force along the rake and one perpendicular to it. The perpendicular load just needs a rigid enough sill that it'll distribute nicely to the plumb studs below. And the force along the plane of the rake that will be resisted by a combination of friction (which you will neglect) and fastener shear strength. If the loads are high and fastening it adequately is a problem, that’s one thing. But in principle? Not a problem.

But if you felt uneasy what you could do is cut the existing wall down to the size of the smallest stud (far left on your elevation view). Make level all the way across, cap with a double top plate, and build off of that. No need to demo the entire wall then just the top little bit.

 

[Ben29]

@Enable, you don't know the half of it. They have 4 kids under 9 y/o.

 

[Ben29]

you could do is cut the existing wall down to the size of the smallest stud (far left on your elevation view). Make level all the way across, cap with a double top plate, and build off of that. No need to demo the entire wall then just the top little bit.

Wouldn't this create the dreadful "hinge"? If I did this then the new top plate would be 18" below my joist bearing elevation. Doesn't that feel like too much? Or does it not matter too much because it is a side wall (not a bearing wall)?

 

[IRstuff]

Could you stair-step the slope before adding the wall above?

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[Enable]

Wouldn't this create the dreadful "hinge"?

I've never heard of a "hinge" effect but I assume you mean that creating a pin in an assembly that can rotate at said pin (such as the exterior wall of a house) would be a bad idea? We agree on that (if not accounted for)!

But a couple things with this. 1) you would connect the plates such that it had some rotational capacity. I might use 1/2" anchor rods w/steel plate washers top and bottom in the stud cavity. Plus nail the living hell out of everything and use SS ties for below / above double sill (could even change this to a nice solid piece of timber instead of 2x plates). 2) you would place the splice at a point of low moment, which by the geometry I am pretty sure it already is. And 3) if still concerned you could open up a handful of areas vertically from the inside (cut out the drywall back to the studs) and secure some full height studs. They would extend from the existing sill to the underside of your new joists. These full height studs will sandwich the other new walls that will be built on top of the old one and thus force rotation closer to grade.

I would be more than fine with the above; that said, your uneasiness may be more than mine. I have investigated and repaired 100s of structures that should not be standing, and yet when I get to them they are still there (and I have to work under them)...so perhaps I am a little more cavalier. If you are uneasy though, don't do it (not worth the lost sleep). Best advice I was ever given was to make a design you can go to bed with!

The safe bet here is to bring your new studs back to grade. Though, if you do go that route at least try to keep the existing wall face standing! Perhaps remove the exterior cladding / sheathing and place new full length studs into the cavity from the exterior. That way you at the very least damages to the interior finished side is minimized. Or do the opposite (save the outside) depending on which is more valuable / makes most sense based on finish schedule.

If I did this then the new top plate would be 18" below my joist bearing elevation. Doesn't that feel like too much? Or does it not matter too much because it is a side wall (not a bearing wall)?

See above. Especially by bringing a few studs down to grade this won't be an issue. Though just so I know where your head is at can you explicate your concern? For me: bearing is not an issue (eccentricities assumed small -> little delta effects), full face wind loading not an issue (see discussion above about making the connection rigid), lateral restraint is not an issue (plywood sheathing to stagger the connection), and that's about all I got for concerns.

EDIT - Here is an image of something you could do. Though I would very likely add through bolts and steel (maybe angle iron in this case at stud/timber interface) and not just rely on the SS splice connection. It would probably work as is though.