RTU support on wood truss top chord

[vandede427]

Does anybody have a suggested detail of how to support a roof top mech unit on a relatively flat wood truss top chord? Wood trusses @ 24" o.c. with flat bot chord, top chord slope 1/4":12".

 

[jike]

Use standard wood curbing to set unit on and flash to.

http://www.ldiline.com/showpdf.cfm?proj=6269&file=004MEC_M0.2.pdf&dir=MEC

Even thought this details are for a metal roof, it might help you get the idea.

One thing that is often overlooked is the anchorage of the unit to resist wind and seismic forces.

http://www.daretoprepare.org/building/rooftopmech.html

 

[StructuralEd]

Are the trusses designed for the equipment?

 

[Ron]

As StructuralEd noted, make sure the trusses are designed for the extra load. Considering that, the curb then gets dictated to some degree by the type of roof membrane system that you will have. Most membrane system manufacturers have curb details that they would prefer to use. This has to do with minimum curb heights, nailing patterns for the membrane termination, type of cant strip, type of termination to be used (for instance, single ply terminations are different than built-up roof or modified bitumen roof terminations).

Make sure you accommodate the amount of insulation expected in your curb height. All too often we see curbs that are too low after the insulation is put in and the membrane put on. Most manufacturers and the National Roofing Contractor's Association recommend a minimum height of 8 inches above the FINISHED ROOF SURFACE for the curb. Further, the upslope side of the curb should have a saddle installed to divert water to the sides. For your slope, that will take away another inch or so of height, assuming your curb is no wider than 24 inches.

You can use pre-fabricated metal curbs, but watch out for condensation issues. You'd be surprised how much water you can get from condensation, that will then collect on your trusses, causing deterioration. Stay with a wood curb if you can.

 

[dik]

The truss manufacturing market is highly competitive and there is generally little latitude for added loadings. Also, snow accumulation around the unit can cause greater loading than the unit itself, if this is an issue.

 

[vandede427]

here's the detail I came up with.