By The Roofing Brief Team. Last reviewed: October 2026.
The core metal roof components are the metal panels, the deck and underlayment beneath them, the fasteners or clips that hold the panels down, and the trim and flashing that seal every edge and penetration. The table below names 15 parts of a residential metal roofing system, with the IRC section or maker’s manual behind each one. Which parts you need depends on one decision: an exposed-fastener panel or a standing seam panel.
Many contractor and manufacturer guides list these parts in a flat pile, as if every metal roof uses all of them. It does not. Slope sets the first fork: IRC Section R905.10.2 (2024 text, as adopted in the 2025 California Residential Code) allows lapped exposed-fastener panels down to 3:12 without lap sealant and 1/2:12 with it, and standing seam down to 1/4:12. The leak-critical parts are the small ones: closures, butyl tape and trim screws.
The system type decides which components you need
Before naming parts, split metal roofs into two families, because the fastening components differ completely. Exposed-fastener panels (corrugated, R-panel, 5V-crimp) screw directly through the face of the panel into the deck or purlins; Union Corrugating’s MasterRib installation manual uses #9 x 1-1/2 in hex-head wood screws and caps purlin spacing at 2 feet. Standing seam panels hide their fasteners under a raised vertical seam and float on concealed clips.
Exposed-fastener systems use gasketed screws and no clips. Standing seam systems use clips, no face screws, and a mechanical or snap seam; Metal Sales’ Magna-Loc guide, for example, requires a mechanical seamer and lists a 1/2:12 minimum slope. Everything else (deck, underlayment, trim, closures, and ridge cap) appears on both, but the sizing and detail change. For how the two profiles compare on cost and lifespan, see our guide to metal roofing types.
Metal roof components at a glance
This table names every part of a residential metal roof, what it does, which system uses it, and the spec or rule that governs it. Each rule comes from the IRC text or the named maker’s manual; check the manual for the exact panel you buy, since panel makers set their own fastener and lap details. For installed pricing, see our metal roof cost guides rather than a parts list.
| Component | What it does | Exposed-fastener | Standing seam | Spec or rule | Source |
|---|---|---|---|---|---|
| Metal panels | Primary weather surface | Yes | Yes | Galvalume to ASTM A792 AZ50 or galvanized to ASTM A653 G90 minimum | IRC Table R905.10.3(2) |
| Roof deck (sheathing) | Structural base for panels | Often (or purlins) | Usually solid deck | Wood structural panels, solid lumber or spaced sheathing; purlins at 2 ft max for MasterRib | IRC R905.10.1; Union MasterRib manual |
| Underlayment | Secondary moisture barrier | Yes | Yes (high-temp) | ASTM D226, D4869, D1970 or D8257 types | IRC Table R905.1.1(1) |
| Gasketed screws | Fasten panel through face | Yes | No | #9 x 1-1/2 in hex head into wood; about 80 per square | Union MasterRib manual; Western States |
| Concealed clips | Hold seam, allow movement | No | Yes | Fixed clip No. 22 MSG steel, 3-1/2 in wide; floating clip base No. 16 MSG | Metal Sales Magna-Loc guide |
| Eave (drip) trim | Sheds water off the edge | Yes | Yes | Panels overhang the eave at least 1 in | Union MasterRib and Advantage-Lok manuals |
| Rake (gable) trim | Caps the sloped side edge | Yes | Yes | Lap trim ends 4 in; pancake screws 12 to 24 in apart by trim piece | Union MasterRib manual |
| Ridge cap | Covers the peak seam | Yes | Yes | Set over outside closures; one lap screw per lineal foot of trim | Union MasterRib manual; Western States |
| Valley flashing | Channels water where planes meet | Yes | Yes | Lap valley trim ends 4 in; ice barrier under it in cold climates | Union MasterRib and Advantage-Lok manuals |
| Sidewall / headwall flashing | Seals panel-to-wall junctions | Yes | Yes | Side wall and end wall trim profiles | Union MasterRib manual |
| Z bar / transition trim | Bridges pitch changes and ridge gaps | Yes | Yes | Z-closure under open hems at ridge and transitions | Union Advantage-Lok manual |
| Foam / metal closures | Seal panel-profile gaps at ends | Yes | Yes | Inside closure at the eave over butyl tape | Union MasterRib manual |
| Butyl tape / sealant | Weatherproofs laps and trim | Yes | Yes | 3/8 in butyl on lap ribs below 3:12; lap sealant required to go below 3:12 | Union MasterRib manual; IRC R905.10.2 |
| Pipe boots / penetration flashing | Seals vents and pipes | Yes | Yes | Listed in maker’s accessory line | Union MasterRib manual |
| Ridge vent | Exhausts attic air at the peak | Yes | Yes | Net free vent area 1/150 of the vented space (1/300 with conditions) | IRC R806.2 |
The layers under the panels: deck, underlayment, and insulation
Below the visible metal sit three build-up layers that decide how long the roof lasts. The roof deck is the structural base, underlayment is the backup water barrier, and insulation controls heat and condensation. IRC R905.10.1 lets metal panels fasten to wood structural panels, solid lumber or spaced lumber sheathing, and on open-frame buildings like barns and pole barns purlins (horizontal steel or wood strips) can replace a solid deck, with the panels spanning between them.
Roof deck (sheathing)
The deck is the plywood, OSB, or plank base the whole roof mounts to. A 7/16 in (24/16 span rating) panel covers rafters at up to 24 inches on center under IRC Table R503.2.1.1(1), and a 15/32 in (32/16) panel covers up to 28 inches without edge support. A solid deck is standard under standing seam and under any high-value coastal or high-wind roof, because it gives underlayment and clips a continuous surface. Union’s MasterRib manual allows panels directly over 2×4 purlins at 2 feet maximum but warns against that option over heated spaces unless insulation and a moisture barrier are added.
Underlayment
Underlayment is the sheet membrane between the deck and the panels that catches any water that gets past the metal. Metal roofs run hot, so high-temperature membranes are the usual spec: the Grace Ice & Water Shield HT data sheet rates that 40 mil ASTM D1970 membrane up to 260°F (127°C). Union’s Advantage-Lok manual recommends its REPEL synthetic underlayment in warm climates because asphalt felt can stick to the panels and tear, and ice and water shield at the eave and valley in cold climates. See our metal roof underlayment guide for the high-temp, synthetic, and felt comparison.
Insulation and vapor control
Insulation minimizes heat transfer and, paired with ventilation, controls the condensation metal panels are prone to. On conditioned homes it usually sits in the attic or between rafters, and IRC R806.3 requires at least a 1-inch air space between insulation and the roof sheathing at eave vents. On metal buildings, a faced fiberglass blanket or rigid board often goes directly under the panels. Union’s MasterRib manual recommends ventilation and vapor barrier protection whenever panels go over a heated space, because condensation drips can look like roof leaks.
How the panels attach: fasteners versus clips
The attachment method is the single biggest difference between metal roof systems. Exposed-fastener panels are screwed through the metal face; standing seam panels are locked to hidden clips. Union’s Advantage-Lok manual draws the line at length: on continuous runs over 25 feet, its concealed fastener method is required so the panel can move with temperature.
Gasketed screws (exposed-fastener systems)
Gasketed screws fasten corrugated and R-panel roofs directly through the panel into the deck or purlin, sealed by an EPDM washer. Western States Metal Roofing’s screw guide estimates 80 screws per square (100 sq ft) and one lap screw per lineal foot of trim, and Union’s MasterRib manual adds 7/8 in lap tek screws at 24 inches on center along side laps. The washer is the weak point: Home Genius Exteriors’ maintenance guide calls for inspecting every exposed fastener annually and budgeting for washer replacement every 10 to 15 years. Placement matters, and our guide on where to screw metal roofing covers rib versus flat and spacing.
Concealed clips (standing seam systems)
Clips are small metal brackets screwed to the deck that grip the panel seam while letting the panel slide as it heats and cools. In Metal Sales’ Magna-Loc guide, the fixed clip is one piece of No. 22 MSG steel, 3-1/2 inches wide, fastened with pancake-head screws, and the floating clip is a two-piece assembly on a No. 16 MSG base that lets the panel move. Snap-lock panels with a nail flange work differently: Union’s Advantage-Lok manual places 1-inch pancake-head wood screws in 5/8 in slots at 24 inches on center maximum, and lists 45 to 90 fasteners per square for a 16 in panel depending on spacing. Where the fixed point goes on each panel is covered in our guide to standing seam metal roof installation.
Trim and flashing: the leak-critical metal
Trim and flashing are the bent metal pieces that close every edge, corner, and penetration. A complete roof needs eave and rake trim around the perimeter, ridge cap at the peak, and flashing anywhere the roof meets a wall, valley, chimney, or pipe. Union’s MasterRib manual laps rake and valley trim 4 inches at the ends and fastens trim with pancake screws 12 to 24 inches apart, depending on the trim piece. For the full flashing family, see our roof flashing types guide.
- Eave (drip) trim: lines the bottom edge and directs runoff into the gutter, protecting the fascia; panels overhang it by at least 1 inch in Union’s manuals.
- Rake (gable) trim: caps the sloped side edges and blocks wind-driven rain from lifting panels.
- Ridge cap: covers the seam where two roof planes meet at the peak, usually paired with a vented closure.
- Valley flashing: a wide metal channel under the panels where two slopes meet, carrying the heaviest water volume on the roof.
- Sidewall and headwall flashing: seals the junction where panels run into or up against a vertical wall.
- Transition trim: bridges a pitch change, such as a steep roof meeting a low-slope porch.
- Pipe boots: flexible or metal collars that seal around plumbing vents and other penetrations.
Closures, Z bars, and sealants: the parts quotes skip
Closures, Z bars, and sealants are the small components that make the difference between a dry roof and a callback, and they are the parts a bare panel quote can leave out. A closure is a foam or metal strip shaped to the panel profile that plugs the open ribs at the eave and ridge; Union’s MasterRib manual sets the inside closure at the eave over a bead of butyl tape. A Z bar is a Z-shaped flashing that fills the gap between ribs at the ridge, and Union’s Advantage-Lok trim is designed so its open hems lock onto the Z-closure.
Foam closures block pests, wind-driven rain, and blowing snow at panel ends; Union’s MasterRib manual says the eave closure prevents water infiltration and insect or bird infestation. Metal closures do the same job with more durability at the ridge. Butyl tape and color-matched sealant weatherproof every lap and trim overlap, and on slopes below 3:12 Union calls for 3/8 in butyl tape along the top of every lap rib, which matches the IRC rule that lapped panels need applied lap sealant to go below 3:12.
Ventilation components
Ventilation parts move air through the roof assembly to prevent condensation. The main component is a ridge vent at the peak, paired with intake at the eaves or soffit. IRC R806.2 sets the net free vent area at 1/150 of the vented space, or 1/300 when 40 to 50 percent of it sits in the upper attic within 3 feet of the ridge and the rest in the bottom third (plus a Class I or II vapor retarder in Climate Zones 6 to 8).
On a standing seam or exposed-fastener roof, a profiled ridge vent sits under the ridge cap and lets air escape while keeping water out; Union lists a vented closure strip and a Versavent universal vented closure for that job. Off-ridge vents, gable vents, and powered vents are alternatives on complex roofs. Size the exhaust from the R806.2 ratio: a 1,500 sq ft attic needs 10 sq ft (1,440 sq in) of net free area at 1/150, or 5 sq ft (720 sq in) at 1/300.
Which components fail first
The parts that fail first are almost never the panels. On exposed-fastener roofs, the screw washers compress and back out with thermal cycling; the Home Genius Exteriors guide budgets washer replacement every 10 to 15 years, while no code or panel standard sets a washer service life. Sealant at trim laps and pipe boots is the other routine service item, so check both on the same annual inspection.
This is why the component list matters more than the panel brand. A premium standing seam panel over missing closures and unsealed laps still leaks. When you plan a takeoff, count the small parts first: closures for every panel end, boots for every penetration, butyl tape for every lap, and one lap screw per lineal foot of trim per Western States’ rule. Our metal roofing calculator covers panel counts and the hidden trim list that estimates often miss.
Metal roof parts list: a takeoff worked example
A metal roof material list for a 30-square (3,000 sq ft) exposed-fastener roof, using the published counting rules above, looks like this. Linear footages for trim come from your roof plan; the counts below show only the rules.
| Part | Counting rule | 30-square example | Source |
|---|---|---|---|
| Panel screws | 80 per square | 30 x 80 = 2,400 screws | Western States |
| Side-lap screws | 7/8 in lap tek at 24 in o.c. along each side lap | One per 2 ft of lap | Union MasterRib manual |
| Trim screws | One lap screw per lineal foot of trim | 200 LF of trim = 200 screws | Western States |
| Concealed fasteners (snap-lock alternative) | 45 to 90 per square at 24 to 12 in spacing | 1,350 to 2,700 | Union Advantage-Lok manual |
| Ventilation | Net free area 1/150 of attic floor area | 1,500 sq ft attic = 1,440 sq in | IRC R806.2 |
Which metal roof parts to use: decision table
| If your roof | Use | Threshold | Source |
|---|---|---|---|
| Slopes below 3:12 | Lap sealant on every lapped panel, or standing seam | Lapped panels: 3:12 without sealant, 1/2:12 with; standing seam: 1/4:12 | IRC R905.10.2 |
| Has panel runs over 25 ft | Concealed clips or slotted fasteners | Union requires its concealed fastener method over 25 ft | Union Advantage-Lok manual |
| Is in an ice-dam climate | Ice and water shield at eaves and valleys | Union recommends it at eave and valley in cold climates | Union Advantage-Lok manual |
| Runs hot (dark color, hot climate) | High-temp underlayment | Grace Ice & Water Shield HT rated to 260°F | GCP data sheet |
| Sits over a heated space on purlins | Solid deck or insulation plus moisture barrier | Union warns against bare purlins over heated spaces | Union MasterRib manual |
| Is in a wind design region | Wind-tested panel system | FM 4474, UL 580, UL 1897 or ASTM E1592 test | IRC R905.10.5 |
Frequently asked questions
What are the main components of a metal roof?
A metal roof has four component groups: the metal panels, the layers beneath them (deck, underlayment, and insulation), the fasteners or clips that hold the panels, and the trim and flashing that seal the edges and penetrations. Our parts table names 15 distinct parts, from the ridge cap down to the eave drip trim and closures, and the IRC covers the panels themselves in Section R905.10.
What is the difference between exposed-fastener and standing seam components?
Exposed-fastener systems use gasketed screws driven through the panel face and no clips. Standing seam systems use concealed clips under a raised seam and no face screws. Both share the same deck, underlayment, trim, closures, and ridge cap, but standing seam adds clips and drops the field screws. Under IRC R905.10.2, standing seam can also go down to a 1/4:12 slope, while lapped panels need 3:12, or 1/2:12 with lap sealant.
What are Z bars and closures on a metal roof?
A Z bar is a Z-shaped metal flashing that fills the gap between panel ribs at the ridge, blocking water that gets under the ridge cap. Closures are foam or metal strips shaped to the panel profile that plug the open ribs at the eave and ridge. Both seal the profile gaps that would otherwise let in wind-driven rain, snow, and pests; Union Corrugating designs the open hems on its Advantage-Lok trim to lock onto its Z-closure.
Do you need underlayment under a metal roof?
Yes, in nearly all residential applications. The 2024 IRC points metal roof panels to the underlayment rules in Section R905.1.1, which list ASTM D226, D4869, D1970 and D8257 products for metal panels on solid sheathing. Metal roofs run hot, so a high-temp membrane such as Grace Ice & Water Shield HT, rated to 260°F, or a synthetic underlayment is the usual spec instead of standard felt.
What holds a metal roof down?
It depends on the system. Exposed-fastener panels are held by gasketed screws driven through the panel face into the deck or purlins, about 80 screws per square per Western States Metal Roofing. Standing seam panels are held by concealed clips screwed to the deck that grip the seam and let the panel move with temperature. Perimeter trim and closures lock the edges against wind uplift on both systems.
What trim does a metal roof need?
Every metal roof needs eave (drip) trim along the bottom edge, rake trim on the sloped sides, and ridge cap at the peak. Roofs with valleys, walls, or penetrations also need valley flashing, sidewall and headwall flashing, transition trim at pitch changes, and pipe boots. Union Corrugating’s MasterRib manual laps rake and valley trim 4 inches and fastens trim every 12 to 24 inches.