best way to constuct knee wall to support rafters...

[jimtheengineer10]

A client wants to install a new roof on a trailer. He want to increase the ceiling height by about a foot. The lumber supplier told him to build a one foot tall wood stud knee wall on top of the existing wood stud wall and then have the rafters attach to the top of the knee wall. My initial thought was this creates a hinge point type connecting between the proposed knee wall and the existing wall. Am I overthinking this or will this work if the knee wall sill plate is fastened to the existing wall top plate with bolts every 16" o.c. or so?

One other thought I had was to cut thru the top plate next to each stud and sister a 4' long wood stud to the existing stud (3' sistered to the existing stud and 1' of the new stud extending up ). See the attached pdf.

Thanks.

 

[ztengguy]

I would go with your second option. Will a 1'0" height increase cause the existing studs to fail, probably not, but something else to check.

 

[AELLC]

I don't understand how you sketched that detail.
Isn't the situation usually like this?

It is only a foot- maybe just sheathing the ext w/ plywood would work.

I get a lot of these, but much more than 1 foot. The Builder always always claims there is no need to add anything - grrrr.

 

[jayrod12]

Instead of rafters you could do roof trusses and they could engineer the bearing to provide that 12" of additional clearance. Probably more material cost than rafters and a site framed knee wall but less labour and less fighting about detailing.

 

[AELLC]

jayrod, good idea, those are called "truss legs".

 

[jayrod12]

Learn something new every day. Thanks AELLC.

 

[jimtheengineer10]

AELLC, my sketch was of the option to sister new studs to the existing studs. Your sketch was what the lumber supplier suggested but I wasn't sure if that would create a hinge connection.

I had thought of the truss leg option and will discuss it with my client but he wants to do everything as cheap as possible and I see him trying to do this himself so the labor won't cost him anything.

So if he doesn't want to go with the truss leg option is a 1' knee wall acceptable (bolt plates together every 16"???) or should I go with the 2nd option of sistering the new studs to the existing studs?

 

[jayrod12]

I get the "free" labour. But his time must be worth something, when I'm doing my own home renovations I estimate my costs applying an hourly wage to myself (Obviously I work for myself for an extremely low hourly rate. I do this because although I could save money on labour or materials sometimes it makes sense to either pay someone else to do it, or pay slightly more for the proper materials for the job which make it go by quicker. If he could install the trusses in one day compared to taking a week to install the knee wall and then rafters with all the connections etc, in my mind the added material cost would be worth it.

 

[jdgengineer]

In situations like this in the past we have aligned the new studs with the existing studs and applied a CS16 strap or similar across the splice on the inside. This way, the plywood provides the continuity on the outside and the strap provides the continuity on the inside. With a 1'-0" height increase the moment at the hinge should be relatively small as it is close to the support.

Depending on the span rating of the plywood, you could potentially do this at every other stud.

If you have a very deflection sensitive wall finish I would probably add in new full-height studs as the strap is somewhat gimmicky.

 

[msquared48]

"a new roof on a trailer"...

Better check the floor framing of the trailer to see if it can handle the additional load. The floor joists are usually cantilevered out from the chassis of the trailer. You may need to install new bearing walls at the exterior of the trailer under the joists.

Also, I do not like the idea of cutting the top plate to install the new vertical member. The existing top plate resists the chord force for the existing roof diaphragm, and unless you are going to abandon this as a diaphragm, I would rethink this detail real hard.

Mike McCann
MMC Engineering

 

[AELLC]

I just thought of this: Usually the studs on prefab homes are only 2x3. ugh.

 

[msquared48]

That's right. Wood or metal studs, depending on the age and model.

Mike McCann
MMC Engineering

 

[AELLC]

The bolts thru the top plate wouldn't amount any benefit.

 

[CELinOttawa]

I've seen smaller than 2x3... They were actually closer to 1-1/4 by 2-1/4 than 1-5/8 by 2-5/8. *sigh*

I also don't want to seem like an elitist; But I've been stiffed for payment on more than one "cheap" job, including two trailer mods. If the client is starting with that cheap a base home, there is a higher chance that they will fail to be able to pay you when anything goes wrong and they are faced with the "Finish the job or pay the Engineer" question...

 

[AELLC]

CEL

GMTO. I did a very intensive project on one of these years ago, got paid bupkus, therefore no more trailer jobs for me.

 

[CELinOttawa]

@AELLC: They are horrible, horrible fauxbuildings... The fact that someone takes the wheels off at all should be enough to get them fined. Trailers are under designed in all respects, supposedly because one can get in and drive away from bad weather. They are also designed to a North American "medium-high" for each condition, but basically everywhere you go will have a "Very high" condition of one sort or another (snow, EQ, Wind, etc).

Garbage.

 

[AELLC]

I don't advertise, but I get a lot of referrals - usually to check the tie down systems on them.

They always, always result in a black hole of hours expended, why should I take them, it will only increase my yearly gross by 1% but give me a colossal headache?

I always say, "Sorry, I no longer deal that type of construction..." and tell the owner he can find another engineer by checking into our State Registration Board, on-line.

 

[CELinOttawa]

@AELLC: You're seeming wiser by the minute, despite your UNITary flaws.... ;0)

Oh, and TERRIBLE pun intended, should that not have been obvious.

 

[CELinOttawa]

And far be it for me to bring this discussion back to the topic at hand, but you can't just sheath this new wall to the old wall and call it a day... You'd turn your spindly little sh*ts of studs (at some crazy random high-efficiency spacing that doesn't often work with a 1220x2440 sheet) into swiss cheese. Carefully check your into stud fastener penetration and you'll find that both sides need to be penetrating about 40mm (about 1-1/2"). But your stud is (at best!) a 2-5/8" thickness in the direction in question. When you then check the nearly never needed spacing requirements for opposing fasteners (which I understand some codes don't even bother including as per a talk by Professor Quenneville - formerly RMC now Auckland), you fail.

Bad job. Bad, bad jobs.

 

[AELLC]

I am not getting wiser, it is just I have had very little billable work to do these past 2 weeks, and I have been unusually verbose after that sagging truss debacle got my juices flowing.

I am gathering my talking points about Imperial vs Metric, so look out.