RTU on Deck 2

[Briansch]

We are installing a rooftop AC on an existing roof. We will be installing angle framework to support the unit and curb between joists which are 5' o.c. I intend to build the curb with treated wood either on top of the deck directly over the angle framework and build it up directly from the angle framework. Are there any guidelines as to when it is OK to build on top of the deck so the deck is not "squashed" between the curb and support frame? Maybe an allowable plf of curb perimeter? Most details I have seen show the curb on top of the deck. A Vulcraft deck manual was no help. An NRCA manual detail shows it both placed on the deck, and directly on the support "for heavy units". Deck is 22 ga Type B. The unit has not been specified yet but I'm guessing it will be around a 1200 lb.

Thanks in advance!

Briansch

 

[Ron]

The loads you have should not be significant enough to affect a 22 ga. deck. Assuming a 16-foot perimeter for the angle frame and nailer, that would yield a load on the deck of 75 lbs/ft, or less than would be experienced with typical foot traffic on the deck.

As an example of a higher load, a "hot" lugger for roofing with 15 gallons of asphalt weighs about 175 lbs, and with two wheels, exerts a load of almost 90 lbs and a contact area of about 4 square inches.

 

[Briansch]

Thanks for the response, Ron.

Briansch

 

The load contributed to the roof deck is likely going to be 300-400# concetrated load at each corner of the rtu. You might want to check with the cut sheet of the rtu. The cut sheet will show you the distributed load or the manufacturer should be able to tell you the likely load distribution.

When supporting a rtu on metal deck, what I normally do is providing steel channels under the unit. But, for lighter units, i.e. 150#-350#, I would provide 2x's inside the deck flutes to keep the deck from crushing.

SDI/steel deck manuf. normally recommends a 250# max. concentrated load on the deck.

 

[JAE]

Check out:

http://www.njb-united.com/usd/ddds/data37R.pdf

 

[Ron]

JAE...good link, thanks! Consistent with my experience.... not an issue.

I'm starting to worry about you...you come up with some of the most obscure, yet relevant links. How do you do it?

 

[Briansch]

Thanks, vbridge and JAE. JAE, that was a very good link! It was exactly the information I was looking for.

Briansch

 

[bridgebuster]

try

http://www.njb-united.com

They have several on-line design aids. Recently, in Modern Steel Construction they printed one for crushing of decking.