How metal roofs are installed, step by step

Tear off, or go over the shingles?

Yes, metal roofing can go over shingles, and it is one of the few coverings the code goes out of its way to allow there. The 2021 IRC (§R908.3.1) permits a re-cover where the new covering is installed to the manufacturer’s approved instructions, and it says plainly that complete and separate systems, such as standing-seam metal roof systems, that carry the roof loads straight to the structure and do not rely on the old roof for support do not require the old covering to come off. The commercial code (IBC §1512.2.1) uses the same words.

The permission ends in three places. Section R908.3.1.1 bars a re-cover when:

• the existing roof is water-soaked or too deteriorated to be a base for new roofing;
• the existing covering is slate, clay, cement or asbestos-cement tile;
• the roof already has two or more layers of any covering.

Over wood shakes, metal is allowed, but if the new layer creates a combustible concealed space the whole surface gets covered first with gypsum board, mineral fiber, glass fiber or another approved material (R908.4). Old flashings, edgings and vents are replaced where rusted, damaged or deteriorated (R908.5).

Deck and underlayment

The IRC lets metal panels go on solid or spaced sheathing (R905.10.1). The IBC asks for a solid or closely fitted deck unless the panel is designed for spaced supports (§1507.4.1). On a house, that means sound plywood or OSB with no soft spots. Panels show every hump in the deck, and a flat pan over an uneven deck oil-cans. Budget replacement sheets the way the decking cost guide does.

The code leaves underlayment details on metal to the panel maker. Table 1507.1.1(2) says to apply it “in accordance with the manufacturer’s installation instructions.” The ice barrier rule (R905.1.2) lists asphalt shingles, metal roof shingles, slate and wood. It does not list metal panels. The manufacturer usually asks for more than the code does here. McElroy’s Medallion-Lok details call for an approved high-temperature peel-and-stick underlayment for the first 3′-0″ up from the eave, lapped over the eave trim. The same goes under the valley trim and 3′-0″ out on each side, with #30 felt over the rest. Its approved-underlayment bulletin has two lists, nail-down and peel-and-stick. The peel-and-stick list includes CertainTeed MetaLayment, Henry Blueskin High Temp and Polyglass Polystick MTS Plus.

Most of those membranes carry “HT” or “High Temp” in the name for a reason: the membrane sits right under the metal. Use what the panel maker approves, not what is left over from the last shingle job. The underlayment types guide compares the sheets themselves.

Eave trim, rakes and valleys

The code’s drip edge rule (R905.2.8.5) is written for shingle roofs, so the edge metal on a panel roof comes from the panel system. The shingle order is in the drip edge guide. On Medallion-Lok the eave detail (C-1) is built up in layers:

1. A perimeter cleat goes on the fascia, fastened at 1′-0″ on center.
2. The eave trim hooks onto that cleat and is fastened at 5′-0″ on center, high on its flange.
3. A continuous 3/32″ × 1″ tape sealant runs along the trim flange.
4. An offset cleat goes over the sealant, fastened at 1′-0″ on center through or just above the sealant line.
5. Each panel end is notched with snips and its tab folded down 180° (detail B-2), so it hooks the cleat.

The eave end of the panel is hooked on, not screwed. That is the first sign of a system built to let the panel move. Valleys work the same way (C-17). The valley trim is fastened at 5′-0″ on center, a sealant line runs about 2¾″ in from its edge, an offset cleat is set in that sealant, and the panels hook onto the cleat. The open rib ends get color-matched sealant. A valley here is a metal channel, not the shingle details in the valley types guide.

Panel layout and squaring off the eave

McElroy’s detail notes keep saying the same thing: “align your panels and square to your eave.” The first panel sets the angle for every panel after it. If it starts even slightly out of square, the error grows across the roof and the last panel meets the rake at an angle.

So the layout is done before the first panel goes up. Measure the eave and both diagonals of the plane, because real roofs are rarely square. Snap a line square to the eave and start the first panel on that line, not on the rake board. Then divide the eave by the panel’s cover width (12, 16 or 18 inches on Medallion-Lok) to see where the last seam lands. The manual has separate “off module” rake details for a run that ends on a cut panel. Plan that cut at the ground so you do not end on a narrow sliver at the far rake.

Clips, and how far apart they go

On a standing seam roof, the clip holds the panel down. The panel itself is never screwed. Each Medallion-Lok UL-90 clip hooks over the male leg of the panel just laid and takes two #10-12 × 1″ pancake-head screws into the deck. The next panel’s female leg then snaps down over the leg and the clip together.

There is no single right clip spacing. It is a wind number. McElroy tested Medallion-Lok to ASTM E1592 and published how much uplift the panel resists at each clip span. Closer clips resist more:

Medallion-Lok 18″, 24 ga. Design load = ultimate ÷ 1.67, the AISI safety factor the bulletin applies.

Moving the clips from 5 feet to 1 foot nearly triples the design uplift. That is why the corners and edges, where wind pull is highest, get tighter spacing than the middle of the roof. Read the spacing for your wind speed and roof zone from the manufacturer’s load tables or the approved product listing, and write it on the job sheet. At the ridge, the manual puts a clip about 1′-0″ downhill on each side. The standing seam vs exposed fastener comparison covers fixed and floating clips.

Screw pattern on exposed-fastener panels

R-panel, 5V and ribbed panels are screwed straight through the face. McElroy’s Max-Rib 26-gauge fastening table has one screw next to each major rib across the panel, 9″ on center. The spacing along the panel then depends on wind speed and roof zone. Over 1/2″ plywood with #9-15 × 1½″ Woodgrip screws:

Bulletin 99-187-04, from ASCE 7-05. Buildings no taller than 40 ft at the peak; roof pitch 1.5:12 to 6.1:12.

Edge and corner zones need roughly two and a half times as many screws as the field. A crew that screws the whole roof at the field spacing has under-fastened the parts that fail first. Count screws per zone, not per panel, when you price an exposed-fastener job in the metal roof cost calculator.

Ridge closure and the vented ridge

A closed ridge (detail C-13) is sealed in layers. Tape sealant goes between the panel legs about 2″ down from the panel end. A field-cut zee closure is fastened through its flange, the sealant and the panel into the deck at 4″ on center. Urethane sealant runs up each seam leg against the closure. The ridge trim is then pop-riveted to the zee at 6″ on center.

The vented ridge (C-14) swaps the zee for a profile vent rolled out into a sealant line. A spacer trim goes on top, fastened through pre-drilled holes at 4″ on center at most, with no fastener closer than 2″ to a seam. The ridge trim is riveted to the spacer. Size the opening with the attic ventilation calculator before you choose between a ridge and box vents.

Pipe boots and other penetrations

A plumbing vent through a metal roof gets a flexible boot made for metal. The manual details a Dektite. Detail C-18 is clear about the order:

• cut the hole in the panel 1″ larger than the pipe;
• over a solid deck, over-cut the hole in the deck too, so no fastener pins the panel to the structure;
• the boot must sit in the flat pan of the panel and is never fastened to a standing seam;
• set the base in butyl and fasten it to the panel at 2″ on center at most; a stainless drawband at the top is recommended.

Minimum slope: code, then the panel

The code minimums for metal panels are low. The IRC (R905.10.2) and the IBC (§1507.4.2) use the same three lines:

The panel’s own rating then sets the real floor, and it is usually higher. Medallion-Lok is a standing seam panel, so the code would let it go down to 1/4:12. McElroy’s data sheet sets its minimum at 3:12. Max-Rib, an exposed-fastener panel, is rated down to 1:12, but from 3:12 down to 1:12 it needs tape sealant in the side laps. That matches the code’s lap-sealant line. Use whichever number is higher. Check the pitch with the roof pitch calculator before you order panels.

Thermal movement

Steel grows in heat and shrinks in cold. Every detail above leaves room for that movement or controls it. McElroy’s technical bulletin on steel panels sets three rules. There is no national standard for the temperature swing, so ask the local building department. About half the total movement is expansion and half is contraction. And the length that counts runs from the panel’s fixed point to its floating end, whatever the panel length or end laps.

Its worked example is a 130-foot run at a 150°F swing, which moves about 1.5″ in total. Movement grows in step with length, so a 20-foot eave-to-ridge panel on a house moves about 0.23″ over the same swing. That is about a quarter inch, but repeated every day. That small movement explains the details you just read:

• the eave hooks a cleat instead of being screwed;
• the pipe hole is cut 1″ oversize;
• the deck hole under a boot is over-cut so nothing pins the panel;
• the manual offers floating rake and ridge details.

A screw driven through a standing seam pan “to be safe” works against all of those details.

What the installation costs

The sequence above explains most of the price gap between the two main systems. Standing seam means clips at engineered spacing, hooked eaves and sealed closures. Exposed fastener means screws through the face. Installed, per square:

The full breakdown is in the metal roof cost guide and the standing seam cost guide. If you are still choosing a material, the shingles vs metal comparison puts the two side by side. The shingle installation guide covers the same steps for a shingle roof.