What’s the Difference Between Structural and Architectural Roofing?

Published April 27, 2018 by Whirlwind Team

Difference Between Architectural and Structural RoofingDSC_0363_2

A standing seam roof is one in which the metal panels are interlocked at the edges to form a seal. The seal stands vertically, thus “standing seam.” The seams use concealed fasteners, with clips and fasteners hidden below the panel.

The seams can be single or double-rolled when mechanically seamed. Another style of standing seam panel can be snap-locked. The panel profile edges snap together and require no mechanical or hand seaming.

Standing seam metal roofs are divided into structural and architectural designs. Some architectural roofs are also structural, but most are non-structural. Each is engineered and installed to perform to different requirements.

The primary difference is in the structural supporting members. The design, slope of the roof and the use of the structure dictate which one is appropriate. The selection also determines whether specialized ventilation is required or if standard ventilation equipment can be installed.

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Architectural Standing Seam Roofing

Architectural roofing is also called hydro-kinetic roofing, meaning it sheds water. However, it is not watertight. It is most commonly installed for steep slope roof applications 3:12 and greater. Anything lower and you risk leaks.

The panels must be installed over a solid substrate and an underlayment to provide some weight-bearing capability. The support can be continuous or closely spaced solid decking or closely spaced sub-purlins. Sub-purlins and decking also eliminate the potential for oil-canning or waviness. The decking is typically plywood or oriented strand board.

The roofing is either mechanically seamed or snapped together depending on panel design. The seams are typically ½ inch to 1 ½ inch, shorter than structural panels and contributing to potential leakage. The low seam is one of the reasons why a substrate is required. The rooftop loads are passed to the decking instead of the frame.

Architectural standing seam roofing provides a host of benefits, although it is primarily installed on metal residential structures.

  • The roofing is meant to accentuate the surrounding architecture.
  • It permits the installation of standard attic ventilation fans.
  • It can be installed on very steep slopes, including nearly vertical slopes.
  • The roofing comes in a variety of shapes meant to resemble clay or tile shakes or other design options as well as a ribbed profile.

Architectural standing seam roofing meets building code wind uplift standards but is not meant for heavy loads or slopes below 3:12.

Structural Standing Seam Roofing

The term structural makes it sound as though the roof does not add curb appeal; however, structural roofing also comes in a variety of styles and shades. Structural roofing is also known as hydrostatic - it’s considered watertight.

Structural roofing does not require decking an underlayment although either can be installed. Instead, it is installed directly onto the purlins over an open frame and typically used for commercial, industrial or educational buildings.

  • Structural roofing is used for extremely low slopes, down to ¼ :12.
  • The seams are 1 ½ inch to 3 inches high, providing water tightness.
  • High seams also provide strength to withstand extreme wind uplift.
  • The panels can span open framing members.
  • The seams are either mechanically seamed or snapped together depending on the design.

The seam design and factory applied sealant prevent air and water infiltration, even when the roof is subjected to immersion. The height of the seams imparts added strength to withstand wind uplift and snow accumulation.

However, due to the method of panel attachment, specialized ventilation is required because the metal may be exposed to moisture from within the structure if there is no deck or underlayment.

Structural roofing passes weight directly to the frame and meets all requirements for wind uplift and weight-bearing capacity. For heavier loads, more purlins are placed in close proximity to each other instead of using thicker panels that could add weight to the roof. Structural roofs are designed to be weight-bearing, even in areas of the panels between the purlins.

The documentation should state the maximum vertical deflection limits, which can differ depending on the material used. Deflection refers to the amount of bending or deformity the panel can withstand before becoming unstable.

Structural roofing is designed to allow thermal movement. Panels expand and contract with temperature variations. This flexibility is especially important when longer panels are used. Clip design and other details make movement without damage possible.

Structural roofing is not appropriate for extreme designs with steep slopes.

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Hybrid Standing Seam Roofing

Some standing seam roofs combine the characteristics or both architectural and structural panels. The seams are about 1 ½ inch high and the panels are applied to slopes down to 1 ½ :12.

This roof requires a substrate, like architectural roofing, unless the slope is above 3:12. Once that slope is reached, no decking is required. A hybrid roof offers aesthetic appeal similar to architectural roofing but provides the strength of a structural roof.

The panels between the seams can have patterns or striations roll-formed into them, either to help with roof installation or simply for aesthetics. Rib rollers can be flat, ribbed, striated, corrugated or clip relief.

  • Flat rib rolling leaves no indentations between the seams.
  • Ribbed rollers can leave a V-shaped indent in the panel, a bead for longer, rectangular panel indents or circular panel indents that resemble pencil indentations.
  • A striated rib roller makes small, consistent lines indented into the panel and can reduce the appearance of oil-canning.
  • Corrugation makes a larger wave in the panel.
  • Clip relief refers to stiffening the rib adjacent to the seam that allows the space for a clip.

There are no strict rules governing which type of roof you choose as long as you meet the requirements of slope, seam height and underlayment for each roof type. If your design requires steeply sloped roofs, an architectural roof laid over decking and underlayment must be used, even if you are building a warehouse.

However, if you can stick to slopes below 3:12, you can attach the panels directly to the purlins. Be prepared to install specialized ventilation equipment, especially if you do not use decking, and remember that the seam height will be taller than an architectural roof.

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