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New Mech, Units on Existing Joist Roof

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I have a condition where an Owner wants to add new mechanical units onto an existing roof constructed from 18H5 joists, 1.50” roof deck, 1” rigid insulation, ballasted roofing.  The roof joists are at 5’-6” on center.  It is a former super market building.

 

I still have to check the joists for the additional load.  My thought is to use a treated 4x4 curb on top of the roof deck to support the unit and span between joists.  My concern is with the deck.  The holes for the ducts will create a 66 inch wide unsupported deck edge.  The deck spans into the hole so it will cantilever form the last bar joist some distance.  If this were new construction I would have a steel angle supporting the edge of deck.  Since this is existing construction it will be difficult to add an edge support angle to the deck.  I guess one question is “do I really need to support the deck under the unit, adjacent to the hole”.  Out of habit I say yes, but does it really need it if the hole is covered by the unit? 

 

The next question would be how to support the edge of deck if I choose to do so.  Again, if this were new work I would have an angle frame fabricated that would be supported by the top chord, sitting directly on top of it, before the deck is placed.  Since this is existing construction I can’t do that. 

 

I recall hearing that it is recommended to not weld to an existing bar joist that is under load.  The angles are thin and the heat of the weld can cause more damage than is worth the effort.  I was thinking about using light gage steel, metal stud framing, to support the deck.  I could fasten the light gage steel to the deck and to the joist chord with TEK screws.  This would eliminate welding.  Has anyone ever done this before?  Any suggestions on the best way to detail this?

 

I would appreciate all insight from anyone who has already tackled this subject.

 

Thanks.

 

Rich