Roof array dead load and attachment count

Dead load per attachment, a lag-screw check against an allowable load you enter, and the largest attachment spacing that passes it.

Illustration only. NOT a structural calculation and not a substitute for a licensed engineer (PE). It uses one uniform net uplift pressure and ignores edge and corner zones, snow drift, seismic, rail and clamp limits, and your AHJ's adopted code. Take the real pressure, allowable load and maximum span from the racking manufacturer's certification letter or design tool.

Examples:

Array and roof

Modules in each row Number of rows (2 rails per row) Module short side (in), along the rails (portrait) Module long side (in) Module weight (lb) Rack and hardware weight (lb per ft2 of module area), placeholder Rafter spacing on centre (in); first rafter assumed within 40% of spacing of the rail end Attachment spacing along each rail (in)

Limits you must take from your own documents

Net uplift pressure (psf), illustrative 50; use the engineer's letter or tool Allowable uplift per attachment (lb) Largest spacing the AHJ or letter allows (in) Module pull design load (psf), datasheet

Flat roof ballast (optional)

Net uplift pressure for ballast (psf), illustrative 10; uses the rack placeholder above Ballast factor (the page's source used 1.5)

Item	Result	How
Array area	-	modules x short side x long side
Dead load, total	-	-
Dead load, per ft2	-	total / area
Attachments at your spacing	-	-
Dead load per attachment, mean	-	total / attachments
Dead load, interior attachment	-	-
Uplift on interior attachment	-	-
Share of allowable used	-	-
Largest spacing that passes	-	-
Attachments at that spacing	-	-
Ballast per module (flat roof)	-	-

Sources and assumptions are on the glossary page. Rail overhang follows IronRidge's 40% of span rule (Tech Tip v1.01, 2022). Ballast follows the City of Chicago's 2013 guideline method for flat roofs and is not an ASCE 7 design.