Unique design criteria and code provisions apply to each type of metal panel roof system

by Kurt Fester, BECxP, CxA+BE

As metal panel roof systems continue to rise in popularity, you need to be aware of the requirements and recommendations related to their installation. But first, it is important to understand how these systems differ.

NRCA distinguishes between two primary categories of metal panel roof systems: structural and architectural. Metal panels used in structural metal panel roof assemblies have the strength and capability of spanning structural members, such as joists or purlins, without being supported by a continuous or closely spaced roof deck. Structural metal panel roof assemblies typically are weatherproof. They are designed to resist the passage of water at joints, laps and junctures under minimal hydrostatic pressure. In most configurations, structural metal panel roof systems do not require underlayments.

Architectural metal panel roof systems require continuous or closely spaced substrates. Architectural metal panel roof systems typically are designed to shed water rapidly over the panels’ surface, so the seams are not necessarily watertight.

It’s worth noting metal shingle roof systems are considered water-shedding roof systems, as well. Metal roof shingles differ from metal roof panels in that shingles are smaller elements. The International Building Code,® 2021 Edition provides the following definitions for a metal roof panel and metal roof shingle: “Metal Roof Panel: An interlocking metal sheet having a minimum installed weather exposure of 3 square feet (0.279 m²) per sheet. Metal Roof Shingle: An interlocking metal sheet having an installed weather exposure less than 3 square feet (0.279 m²) per sheet.”

Metal shingles can take the shape of individual or multiple asphalt, tile, slate or wood shingle configurations. They also can be formed in various shapes and in single or multiple courses.

Metal panel roof systems can be further categorized into either flat or standing seam. For flat seams, the edges of two adjacent sides of each rectangular sheet are folded over a minimum of ¾ of an inch; the edges of the two other adjacent sides are folded under a minimum of ¾ of an inch. Flat-seam metal panel roof systems with nonsoldered seams are intended to shed water whereas when the joints are soldered, they become weatherproof.

The term “standing seam” refers to almost any kind of metal roof panel with a raised vertical seam. However, strictly speaking, it refers to those metal panels that interlock or are seamed together vertically above the panel’s pan. Standing seams can be used as architectural elements to highlight aesthetics, accommodate thermal movement, facilitate panel interlocking and seaming, and add structural characteristics to metal panels. The seaming process varies depending on the seam profile, but the most common standing seams have a male leg and a female leg. Seams may be formed by hand or mechanical seamers, and seams may be single- or double-locked. Other types are snapped or rolled and locked together, which joins together friction-fit components.

The model building codes have different requirements for different types of metal panel roof systems.

The 2021 IBC requires the following:

The First: 1507.4.1 Deck requirements. Metal roof panel roof coverings shall be applied to a solid or closely fitted deck except where the roof covering is specifically      designed to be applied to spaced supports.

The Second:1507.4.2 Deck slope. Minimum slopes for metal roof panels shall comply with the following:

1.The minimum slope for lapped, nonsoldered seam metal roof panels without applied lap sealant shall be three units vertical in 12 units horizontal (25-percent slope).

2.The minimum slope for lapped, nonsoldered seam metal roof panels with applied lap sealant shall  be one-half unit vertical in 12 units horizontal (4-percent slope). Lap alants shall be applied in accordance with the approved manufacturer’s installation instructions.

3.The minimum slope for standing-seam metal roof panel systems shall be one-quarter unit vertical in 12 units horizontal (2-percent slope).”

NRCA recommends ½ of an inch per foot as the minimum slope for structural metal panel roof systems even though numerous manufacturers and IBC allow structural metal panel roof systems to be installed on slopes as low as ¼ of an inch per foot.

NRCA’s slope recommendation is based on concerns for watertightness at end laps, transverse seams, panel ends, junctions of metal, and where deflections may be present at penetrations and curbs.

Designers also should consider valleys included in roof system design when determining minimum slope because valley slope is less than panel slope. When a structural metal panel roof system design includes valleys, NRCA recommends the panels be sloped to provide a minimum valley slope of ½ of an inch per foot.

Architectural metal panel roof systems, on the other hand, perform well on slopes of 3:12 or greater. For lower slopes, climatic conditions must be carefully considered, particularly the amount of expected ice, snow and debris accumulation.

To be continued.