PV Flat-Roof Ballast Weight and Roof PSF Screen
Dead-load screen for a ballasted flat-roof PV array.
Example
You enter
- Module count 30
- Module weight (lb) 50
- Ballast per module (lb) 40
- Total racking weight (lb) 150
- Array footprint area (ft²) 630
- Allowable added pressure (psf) 5
You get
- Total added weight 2850 lb
- Added roof pressure 4.52 psf
- Screen PASS (at or under the allowable)
Details, formula, and sources
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.
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.
The load arithmetic is public first-principles; the ballast quantity and allowable pressure come from the PE-stamped ballast plan and the structural engineer.
Estimate. AHJ and licensed professional govern.
Field names used by the API: modules, module_wt_lb, ballast_per_module_lb, racking_wt_lb, array_area_sf, allowable_psf, total_wt_lb, added_psf, pass
- Ballast quantity per module from the PE-stamped ballast plan (entered); not sized herePE ballast plan
- Allowable pressure the allowable added dead load from the structural engineer (entered)structural engineer