PV Flat-Roof Ballast Weight and Roof PSF Screen
Dead-load screen for a ballasted flat-roof PV array: total = modules x (module weight + ballast) + racking; added pressure = total / array area, compared to the allowable. 30 modules at 50 lb plus 40 lb of ballast each on a 150 lb rack is 2,850 lb over 630 sf -- 4.5 psf, under a 5 psf allowable; push the ballast to 60 lb and it is 5.48 psf, over the line. A SCREEN, not a design -- the PE-stamped ballast plan and the structural engineer govern; wind uplift and the roof structure govern the design.
Formula and source
total_wt_lb = modules x (module_wt_lb + ballast_per_module_lb) + racking_wt_lb; added_psf = total_wt_lb / array_area_sf; pass = added_psf <= allowable_psf.
PV ballast load-screen identity by name (total dead load over the array footprint, screened against the allowable); first-principles arithmetic.
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