How a Metal Roof Gets Installed, Step by Step

Most metal roofs that fail did not fail because of the metal. They failed because of how they were put on. Knowing the sequence is the easiest way for a homeowner to tell a thorough quote from a thin one, so here is how a roof goes on, in the order it happens.

Tear-off and the deck

The old roof comes off down to the decking. This is the only moment anyone will see your deck, and it is when problems get found: rot at the eaves from old ice damming, delaminated OSB around a chimney, plank decking with gaps too wide to fasten into.

A quote that promises no deck repair is not being careful. It is guessing. Deck replacement should show up as a clearly priced unit rate so you know what a surprise costs before it happens. Installing over existing shingles is possible when there is one layer and the deck is sound, and we inspect the roof and attic before agreeing to it. I still prefer to see the wood.

Ventilation

This gets skipped constantly, and it shortens the life of everything above it. A roof needs balanced intake at the soffit and exhaust at the ridge. If the intake is blocked with insulation or was never adequate, the assembly cannot dry, and moisture works on the deck from below no matter how good the roof is. If a contractor never asks about your attic, they are quoting a product rather than a roof.

Underlayment

High-temperature synthetic across the field, self-adhered membrane at the eaves past the interior wall line, in the valleys, and around every penetration. The temperature rating matters. Standard shingle underlayment is not rated for the heat under a metal panel. There is more on this in the underlayment article.

Flashing and trim

This is where the money and the leaks both are. Valleys, sidewalls, chimneys, skylights, and every wall-to-roof intersection get formed metal flashing. Where a wall is involved, that means counterflashing, metal let into or lapped behind the wall covering, not sealant smeared over a joint.

Sealant is a maintenance item with a service life. Formed metal is a detail. Any quote that solves a sidewall with caulk is deferring a leak, not preventing one.

Panels

For standing seam, panels are roll-formed to the exact length of the run, on site for anything long. That is what removes horizontal seams from the roof. They are held with concealed clips that fasten to the deck and let the panel move.

That movement is not optional. A 30-foot steel panel changes length by roughly a quarter inch across a Midwest temperature swing. Clips that allow for it are why concealed-fastener systems last. Pinning a long panel rigidly at both ends is how you get oil canning, fastener fatigue, and eventually a tear.

Seams are then folded shut with a seaming machine, a single or double fold depending on profile and slope. Snap-lock profiles skip the machine and are fine on steeper slopes. Mechanically seamed is what you want on low slope or an exposed site.

Exposed-fastener panels screw through the face into the deck, with a rubber washer under each head sealing the hole. It is quicker and cheaper. Every one of those washers is a wear item, and they need attention somewhere around year ten to twelve.

Ridge, trim, and closures

Ridge cap over a vented ridge, foam or metal closures where the panel profile meets trim, and gable and eave trim to finish the edges. Closures are small and cheap, and skipping them is how wind-driven rain and wasps both get in.

Where installs go wrong

• Flashing solved with sealant instead of formed metal. The most common cause of a leaking metal roof.
• Fasteners driven wrong. Overdriven crushes the washer, underdriven leaves a gap. Both leak, on a schedule.
• No allowance for expansion on long panels.
• Cutting panels with an abrasive saw. The hot filings land on the finish and rust in place. Nearly every "my metal roof is rusting" call I have taken traced back to this. Panels get cut with shears or a nibbler.
• Ventilation left as it was found.

The one question worth asking

Manufacturer certification is not a badge. It is what lets a manufacturer stand behind workmanship as well as material. They will only back an install done to their spec by a crew they have trained. A roof put on by an uncertified crew may carry a material warranty, which covers the metal and nothing about how it was attached. Every failure in the list above is workmanship.

If you ask one thing about installation, ask who certifies the crew and what the warranty covers when the problem turns out to be the install rather than the panel.

Now you know the sequence, and you can hold any quote up against it.