Skip to content
PropertyRoofs

Commercial metal roofing

Commercial metal roofing is steel panels on a sloped roof, either joined at raised standing seams over hidden clips or lapped and fastened straight through to the structure. It is not a flat membrane: the International Building Code minimum for standing seam is 1/4:12 (a quarter inch of rise per foot), and many panels need 1/2:12 or more. It's used on new commercial and industrial buildings, and to re-roof flat warehouses, plants and schools on new framing set over the old roof.

Warranty available
Weathertightness up to 25 years (some makers)
Common gauges
24 or 22; 26 on some exposed-fastener panels
Seams
Machine-seamed, snap-together, or lapped
Surface
PVDF or SMP paint, or acrylic-coated Galvalume
Attachment
Concealed clips or exposed fasteners

How it's installed

What sits under the panels decides the method. Structural standing seam spans from purlin to purlin with no deck; architectural panels need continuous support and a steeper slope. On an existing building, the new roof can go over the old one.

Standing seam on clips

Clips fastened to the purlins or deck hold each panel at its edge, and the seam is then closed with a seaming machine or snapped together. The clips let the panels slide as they expand and contract with temperature.

Exposed fastener

Panels overlap at the ribs and ends and are screwed through into the purlins. Code allows 1/2:12 with sealant in the laps and requires 3:12 without it.

New slope over a flat roof

Light-gauge steel framing is set on the existing structure to form a sloped plane, and standing seam panels go on top. The existing structure must be inspected and engineered for it first.

Metal over metal

New panels are screwed through the old panels' ribs into the existing purlins, or set on sub-framing above them, so the old roof stays in place.

Pros and cons

Where metal wins

  • Standing seam has no screws through the field of the roof, only at the edges, end laps and penetrations.
  • Single-source leak warranties keep the panel maker responsible for the full term, even if the installer goes out of business.
  • PVDF paint carries its own warranty against cracking and peeling, separate from the leak warranty: 25 to 45 years, depending on the maker.
  • A flat or worn-out roof can be covered without a tear-off, which lowers the chance of water getting in during the work.

Where it falls short

  • It needs slope. Minimums run from 1/4:12 to 3:12 depending on the panel, so a flat roof needs new framing first.
  • Oil canning, a visible waviness in the flat of the panel, can't be fully prevented. Heavier gauge and embossing make it less noticeable.
  • Copper, lead and AC condensate draining onto the panels corrode them, and flashings at edges and penetrations are often where problems start.
  • Exposed-fastener roofs have a hole at every screw that needs checking, and they carry shorter leak warranties, if any.

Metal roofs and hail

Many standing seam and exposed-fastener steel panels are listed as UL 2218 Class 4, the top impact class, but that test looks for cracks and openings, not dents. FM Approvals rates panel roofs under FM 4471 for moderate (MH) or severe (SH) hail, with steel-ball impacts on the seams as well as the flat; a listing with no hail letter, such as Class 1-120, means SH. FM Global's Data Sheet 1-34 maps moderate, severe and very severe hail zones, so an FM-insured building should confirm which rating its zone calls for.

Insurers have been writing exclusions that pay only for functional hail damage, not dents that just change the look, so read the policy before choosing a panel. Sheffield Metals notes that heavier gauge, a solid deck instead of open framing, and striations in the flat of the panel all reduce denting or hide it. An IIBEC study found hail dents in Galvalume panels did not corrode faster than the rest of the panel. After a storm, check for punctures, damaged trim and loose fasteners.

Back to all roof systems

Sources

  1. USACE / NAVFAC / AFCEC (Unified Facilities Guide Specifications, via WBDG), UFGS 07 61 14 Steel Standing Seam Roofing (August 2025)
  2. MBCI, PBR Exposed Fastener Panels
  3. Butler Manufacturing (BlueScope Buildings North America), Butlerib II Roof System product sheet
  4. MBCI, Selecting Metal Panels Based on Roof Slope
  5. MBCI, SuperLok Standing Seam Roof Panels
  6. McElroy Metal, Metal Roofing Warranty: Weathertightness
  7. McElroy Metal, 138T and 238T Standing Seam Roofing Systems
  8. McElroy Metal, Retrofit and Recover
  9. MBCI, How to Retrofit Existing Roofs with New Metal Roofing Systems
  10. Butler Manufacturing (BlueScope Buildings North America), MR-24 Roof System product sheet
  11. MBCI, Warranties
  12. IIBEC Interface, A Deep Dive into Hail-Caused Dents: A Study of Corrosion Resistance within Dents in Galvalume-Coated Steel Roof Panels (Fall 2025)
  13. MBCI, The Color of Success: MBCI's Durable Coating and Finish Options
  14. McElroy Metal, Metal Over Low Slope Retrofit
  15. MBCI, Weathertightness Warranties (brochure)
  16. Butler Manufacturing (BlueScope Buildings North America), Butler-Cote Finish System
  17. McElroy Metal, Fluropon Kynar 500 PVDF with Galvalume Substrates Warranty
  18. MBCI (NCI Group, Inc.), Precoated Signature 300 Panel: 70% Fluoropolymer Panel Limited Pass-Through Warranty
  19. Metal Construction Association, Oil Canning in Metal Roof and Metal Wall Systems (technical bulletin)
  20. MBCI, Building Maintenance Manual
  21. McElroy Metal, Technical Bulletin 04-267-06: UL 2218 Impact Resistance Class 4
  22. Central States Manufacturing, Dent Protection Explained
  23. FM Approvals, Examination Standard for Class 1 Panel Roofs, Class Number 4471 (March 2025)
  24. Professional Roofing (NRCA), Designing for hail resistance (May 2018)
  25. Sheffield Metals, Metal Roofing and Hail

Updated October 2026.