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1970's Panelized Roof - Question

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I have client who is adding 10 A/C units to the top of an existing building constructed in the 70’s. The roof is a panelized roof:

2x4 @ 16” o.c. DF #1 Sub-Purlins – 8’-0” maximum span

4x16 @ 8’ o.c. DF #1 Purlins – 18’-9” maximum span

6x24” DF/DF GLB (exact size not yet verified) Beam at 18’-8” apart up to 47’-6” span.

7x26” DF/DF GLB (exact size not yet verified) Girders at 45’ Average spacing and 37’-6” maximum span


The problem is that I can’t get the GLB’s to work based on a 12-PSF dead load and reducible 20-psf live load (16-psf on the Beam and 12-psf on the Girder). I still have to add the A/C units to the Girders.


What have others done to calc out an old roof for additional mechanical loads on an older Masonry bldg w/ panelized roof? The building was originally a Thrifty Drug Store and is being remodeled into two stores?


Dennis S. Wish, PE

California Professional Engineer

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