How a Standing Seam Metal Roof Works

You have probably noticed them: metal roofs with long, flat panels and narrow raised ribs running straight down from the ridge, with no rows of screws showing. They show up on new houses, church additions, barndominiums and storefronts across Anderson and the Upstate. If you are weighing one for your own home or building, it helps to know what is under those ribs before you compare quotes.

This guide explains the parts of a standing seam roof, why the panels are allowed to move, how the seams close, and where the building code does and does not allow this kind of roof. It is a plain description, not a sales pitch. When you are ready to talk about your own roof, the residential metal roofing and commercial metal roofing pages are where Anderson Metal Roofing and Shingles picks up the conversation.

A standing seam roof is a system, and each piece has one job. The Insurance Institute for Business and Home Safety (IBHS) groups standing seam and snap-lock panels together as concealed-fastener roofs, because the screws that hold the roof down are hidden under the seams instead of driven through the panel face. Here is what is up there, from the deck up.

• Deck and underlayment. The panels sit on solid or spaced sheathing (the 2021 International Residential Code, Section R905.10.1, allows either, as long as the panel is designed for it) with an underlayment that meets Section R905.1.1. The Metal Roofing Alliance notes that a smooth synthetic underlayment is often recommended under metal so the panels can slide during temperature swings.
• Panels. Long sheets of coated steel, aluminum or another metal, roll-formed so each long edge turns up into a vertical leg. The flat area between the legs is called the pan.
• Seams. Where two panels meet, the raised legs overlap or interlock. This joint stands above the water line, which is where the name comes from.
• Clips. Small metal pieces that hook over or into the panel edge and are screwed to the deck. The IBHS low-slope metal guide describes them as concealed hold-down clips, and says sliding clips let the roof expand and contract.
• Fasteners. The screws go through the clip or a hidden flange, not through the exposed pan. The only exposed fasteners on a well-detailed standing seam roof are usually at trim, flashing and some penetrations.
• Trim and flashing. Ridge caps, eave and gable trim, valley metal and the flashings around pipes, chimneys and walls. The IRC (Section R903.2) requires flashing wherever a roof meets a wall, changes slope or has an opening.

Metal grows when it warms and shrinks when it cools. A long panel running the full length of a roof slope moves a measurable amount between a frosty morning and a hot July afternoon. The National Park Service, writing about historic sheet metal roofs, points out that wear and metal failure at joints can come from the metal's repeated movement with thermal changes. That is the problem standing seam clips are designed to solve.

Most clips let the panel slide while the clip stays put, which is why installers talk about fixed versus floating clips. The company's post on standing seam metal roof clips walks through that choice and why the substrate and insulation thickness matter. The short version: the clip has to match the panel system, and the installer should follow the panel manufacturer's instructions on clip type and spacing.

Movement also matters at every place the roof is pinned. The IBHS low-slope metal guide says accessories should not pin the panels and recommends approved seam clamps for standing seam. Sealants are chosen for the same reason: the Metal Roofing Alliance lists thermal movement, expansion and contraction as the most common reasons substrate-compatible sealants are used, and says to use the sealants the panel manufacturer approves.

There are two broad ways a standing seam joint is locked. In a snap-lock panel, one leg is shaped to snap over the other during installation, with the clip trapped inside. In a mechanically seamed panel, the legs are laid side by side and then folded over each other after the panels are in place, generally with a seaming tool that runs down the seam. IBHS lists both families, standing seam and snap lock, as concealed-fastener systems.

Which one you get depends on the panel profile, the roof slope and the manufacturer's approval for your roof. The Metal Roofing Alliance buyer's guide suggests asking whether the panels lock together or overlap, and whether the roof is approved by its manufacturer for the pitch and geometry of your roof. Those two questions will tell you a lot about what is being proposed.

Either way, the seam itself is not where the roof is attached to the building. The clips are. A tight seam keeps water out; the clips and their screws keep the roof on in a windstorm, which is why fastener pattern and clip spacing belong in writing on your estimate.

IBHS describes two kinds of metal roofing: hydrokinetic roofs that shed water, and hydrostatic roofs built to stay watertight even when water sits on them. IBHS describes steep-slope metal roofing as the hydrokinetic kind. Water lands in the pan and runs downhill to the eave, while the seams stand above the flow. The steeper the roof, the faster that happens.

Slope is where the code draws its lines. In the 2021 International Residential Code, Section R905.10.2, the minimum slope for standing-seam roof systems is one-quarter unit vertical in 12 units horizontal (a 2 percent slope). Lapped, nonsoldered-seam metal roofs without lap sealant need 3 units in 12 (25 percent), and with approved lap sealant they can go down to one-half unit in 12 (4 percent). South Carolina's Building Codes Council adopted the 2021 codes effective January 1, 2023, with state modifications, and lists the 2024 codes as taking effect January 1, 2027. Ask the Anderson County or City of Anderson building department what applies to your address and whether a permit is needed.

The code minimum is a floor, not a recommendation. The IBHS steep-slope guide says hydrokinetic metal roofing is normally installed at slopes of 2 in 12 or greater, and the Metal Roofing Alliance glossary says most metal roofs are installed on pitches of 4 in 12 or greater, though many systems can go shallower. The panel manufacturer's installation guide sets its own minimum, and that is the number your installer should be quoting from. Read the gauge and coating lines on that same spec sheet; the guide on what metal roof gauge means explains how to read them.

Standing seam is not the answer for every roof. On a pole barn, equipment shed or hobby shop, an exposed-fastener panel screwed straight to purlins is the normal choice and is generally less labor to install. The guide comparing standing seam and exposed-fastener metal roofs lays out that decision. A roof that is close to flat may be better served by a membrane system; see the flat roof systems page.

If you have penetrations planned, such as vents, satellite dishes, solar racking or snow guards, plan them before the panels go on. The company's post on installing accessories on a metal roof covers why dissimilar metals and the wrong cutting tools cause trouble. For solar, the solar panels page describes clamping the racking to the raised seam rather than drilling the panel.

Next step: get the panel profile, seam type, clip type, gauge, coating and underlayment in writing from any installer you talk to. Anderson Metal Roofing and Shingles gives free quotes, offers virtual consultations by Zoom or FaceTime, and notes that measurements are verified on site, which can adjust the price. Start at the residential metal or commercial metal page, or request a quote.