[ksprul]

Hi,

I’m looking at adding AC to a 20x40 garage which I’m using as a workshop. The garage has a boarded, ply floor attic above. The option I like the best from an efficiency and aesthetics standpoint is to use a split system with in-ceiling cassette air handlers.

The cassettes are 21” square and I’d like to install two, into the ceiling which is built from 12” I-joists for the 20ft, shorter ceiling span, spaced 12” OC.

My plan is to install headers and trimmers to create a 36x24 box for each cassette. My thoughts for each box is to purchase a 9-1/2deep 1-3/4wide LVL truss and cut it up to provide headers and trimmers for the box and to sister the cut joists where they meet the box.

The thought process being i can rip the LVL down to 9" and glue and screw it to the webbing between the i-joist flanges

I hope I used the right terms, Most of my carpentry experience is from the UK where everything is called subtly different names.

Would really appreciate your thoughts, and if i'm over or under engineering.
Karl

 

[Nealtw]

That wouldn't pass.
You will need 1 full length LVL sistered to a joist on both sides and the a header at each end of the hole with LVL all with hangers.

 

[Nealtw]

much like this.

 

[ksprul]

Thanks for the quick reply! By full length do you mean the entire 20ft span? I'm just cutting and impairing a single i-joist for each opening. if I understand your diagram you're showing 3 cut i-joists.

If I do have to sister the entire length not sure how I can get a 20ft lvl up into the finished attic. Any other suggestions?.

one final question your diagram shows the sistering on the inside of the box. I would need to sister on the outside to allow the cassette to fit. assume that would not work as I would not be using hangers?

 

[Nealtw]

Thanks for the quick reply! By full length do you mean the entire 20ft span? I'm just cutting and impairing a single i-joist for each opening. if I understand your diagram you're showing 3 cut i-joists.

If I do have to sister the entire length not sure how I can get a 20ft lvl up into the finished attic. Any other suggestions?.

one final question your diagram shows the sistering on the inside of the box. I would need to sister on the outside to allow the cassette to fit. assume that would not work as I would not be using hangers?

Sorry it should have been two cut. The joists that you put in to support the cut ones have to hang from the beam, so the beam has to be on the opening side. then you put one back in to make the hole the right size

You cut and remove plywood above so you can work there.

They have special hangers for single LVLs and TGIs.