A metal roof is not just panels. By the time we finish a standing seam job in Naperville or a corrugated barn-style roof out past Elgin, we’ve installed eight distinct layers and parts that all have to work together. If any one of them is skipped or cheapened, the roof leaks — usually at a seam, a penetration, or a wall, not out in the middle of a panel.
Here’s what actually goes on your house, from the deck up.
1. The Roof Decking
Everything starts with the wood. On most Chicagoland homes, that’s 1/2″ or 5/8″ plywood or OSB. On older housing stock — Berwyn bungalows, Oak Park foursquares, farmhouses in Kane County — you’ll often find 1×6 or 1×8 plank decking with gaps between the boards.
Metal panels need a flat, solid, dry surface. Standing seam is especially unforgiving; it telegraphs every dip, hump, and delaminated sheet of OSB. If your decking has soft spots from a previous leak, or planking with 1″+ gaps, that gets addressed before a single panel goes on. On plank decking we typically overlay with plywood so the fasteners and clips have consistent bite and the panels lay flat.
You won’t know the condition of your deck until the old roof comes off. Any honest estimate includes a per-sheet price for replacement so there’s no surprise mid-job.
2. Underlayment
Metal doesn’t get installed directly on wood. The underlayment is your secondary water barrier — it’s what handles the water that gets past the metal during wind-driven rain or when snow melts and refreezes under the panels.
Two things matter here in our climate:
High-temperature ice and water shield. Standard peel-and-stick ice barrier can soften and lose adhesion under metal, which gets significantly hotter than asphalt in July. We use a high-temp membrane rated for the temperatures metal roofs actually reach. It goes in the valleys, around penetrations, along the eaves, and up any sidewalls.
Eave coverage that meets code and reality. Illinois requires ice barrier from the eave edge to at least 24″ inside the exterior wall line. On low-slope sections, on north-facing planes that never see sun, and on homes with a history of ice dams, we go further up. The extra roll is cheap compared to a ceiling repair in February.
The rest of the deck gets a synthetic underlayment — a woven, high-tear-strength sheet, not felt paper. Felt wrinkles, tears, and can trap moisture against the deck.
3. The Metal Panels
This is the part homeowners are actually shopping for. The three variables:
Profile. Standing seam has hidden fasteners and raised, mechanically seamed or snap-locked ribs. Exposed fastener panels (R-panel, corrugated, ag-panel) screw through the face of the metal with a rubber washer. Standing seam costs more and lasts longer because nothing penetrates the water plane. Exposed fastener is a legitimate choice for garages, additions, porches, and outbuildings — just know the washers are a maintenance item every 10–20 years.
Metal type. Most residential work around here is Galvalume-coated steel, 24 or 26 gauge. Aluminum is a good choice near water or in areas with salt exposure, and it won’t rust, but it dents easier. Copper and zinc are premium options you’ll see on historic homes in Evanston or Hinsdale — beautiful, and priced accordingly.
Gauge and finish. 24 gauge is thicker and more resistant to oil canning and hail dents than 26 gauge. For the finish, a Kynar 500 (PVDF) paint system holds color far longer than a cheaper SMP coating. If a bid doesn’t specify gauge and paint system, ask.
4. Fasteners and Clips
Standing seam panels attach with clips — either fixed or floating — that screw to the deck and hold the panel legs. Floating clips let the panel expand and contract as temperatures swing. That matters here. A 20-foot steel panel moves close to a quarter inch between a January morning and an August afternoon. Pin a panel down at both ends with fixed fasteners and you get buckling, or you get elongated screw holes that start leaking.
Exposed fastener systems use screws with EPDM washers driven through the panel into the deck or purlins. The screws must land in the flat of the panel, driven straight, and torqued so the washer compresses without squishing out. Over-driven screws are the number one reason exposed fastener roofs leak, and it’s entirely an installation issue.
Fastener metal should match panel metal. Steel screws in an aluminum panel will corrode.
5. Flashing
Flashing is bent metal that directs water away from every place the roof meets something else. On a metal roof it’s fabricated to match the panel color and profile, and it’s where most of the skill in the job shows up.
- Valley flashing — a formed W-valley or open valley with a center rib, sized for the water volume coming off both planes.
- Sidewall and headwall flashing — where the roof runs into a wall, a dormer, or a chimney. On brick, this means counterflashing set into a cut reglet in the mortar joint, not caulk smeared on the face of the brick.
- Chimney flashing — apron, steps, and a cricket on the uphill side if the chimney is wider than 30″. Chicago has a lot of masonry chimneys and a lot of chimney leaks that get blamed on the roof.
- Pipe boots — plumbing vents get a high-temp silicone or EPDM boot, sealed to the panel with butyl and mechanically fastened. Cheap boots crack in about 8 years; good ones last much longer.
Prefabricated, one-size flashing rarely fits a metal roof correctly. Most of what we install is bent on a brake to the specific pitch and panel we’re working with.
6. Trim
Trim is the finish metal at the edges — functional, not decorative.
- Eave trim / drip edge kicks water off the roof into the gutter instead of behind it.
- Rake or gable trim covers the panel edge at the gable end and locks it down against wind uplift. This matters on exposed lots in the western suburbs where straight-line winds come through in summer.
- Ridge cap covers the peak where the two planes meet.
- Hip caps and transition trim handle changes in pitch and roof-to-roof intersections.
Panel ends should be hemmed — folded back on themselves — at the eave so they hook the drip edge and won’t lift or cut anyone working on the roof later.
7. Sealants, Butyl Tape, and Closure Strips
Metal panels have profiles with gaps. Where the ribs meet the ridge or the eave, those gaps need to be filled or you get blown snow, rain, wasps, and birds under the roof.
Closure strips are foam or metal pieces cut to the exact panel profile, installed at the ridge and eave. Foam is common; metal closures last longer.
Butyl tape is a non-hardening sealant used inside seams, under trim laps, and at panel endlaps. It stays flexible through freeze-thaw cycles and doesn’t dry out the way most caulk does.
Sealant should be a metal-compatible urethane or high-grade polyurethane, not general-purpose silicone. And it should be used as a backup inside a properly designed lap — not as the thing holding water out. Anyone relying on a bead of caulk to stop a leak is buying you a few years, not fixing it.
8. Ventilation
Metal roofs need intake and exhaust just like asphalt. Warm, moist air rising out of your living space has to leave the attic, or it condenses on the underside of cold metal and drips back onto your insulation. And an overheated attic in winter melts snow on the roof, which refreezes at the cold eave and builds an ice dam.
The typical setup:
- Intake at the soffits — continuous vented soffit or individual vents, with baffles keeping insulation from blocking the airflow.
- Exhaust at the ridge — a vented ridge cap designed for the panel profile, with a filter media to keep out driven snow.
The ratio matters: roughly 1 square foot of net free vent area per 150 square feet of attic floor, split close to evenly between intake and exhaust, with slightly more intake than exhaust. A lot of Chicagoland homes have a ridge vent and almost no soffit intake, which means the ridge is pulling air from the house instead of from outside. That’s a ventilation system working against you.
If you’re replacing a roof, this is the one and only easy time to fix the ventilation.
A Ninth Thing to Ask About: Snow Retention
Not a core component, but worth knowing. Metal sheds snow. A full roof-load of snow releasing at once over a front door, a walkway, a deck, or a driveway is a real hazard, and it will take your gutters with it. If you have any of those below a metal roof plane, budget for snow guards or a snow retention bar system. They mount to the seams (on standing seam) without penetrating the panel.
What This Means for Your Estimate
When you compare metal roofing bids, the panel price is the easy part to compare and the least likely place a contractor cuts corners. The differences show up in the underlayment spec, the gauge and paint system, whether flashing is fabricated on site or pulled off a shelf, whether the clips are floating, and whether ventilation is being corrected or just carried over.
Ask for those items in writing. A metal roof installed correctly should outlast two or three asphalt roofs. Installed poorly, it fails in the same places every roof fails — and it costs a lot more to redo.
If you’re weighing metal for a home in the Chicago area and want someone to walk the roof, look at the decking, check the attic, and tell you what the job actually involves,
Contact AST Contracting for all your roofing needs.