Metal Roof Repair: Screws, Seams, Trim and the Mistakes That Make Leaks Worse

Exposed-fastener roofs use ribbed or corrugated panels screwed straight through the face into the framing or deck, with a rubber washer under each screw head. They are common on barns, garages, outbuildings and some newer farmhouse-style homes. Standing seam roofs use panels that lock together at raised seams, held down by concealed clips inside the seam, so nothing pierces the flat of the panel.

You can usually tell from the ground: rows of screw heads across the panel mean exposed-fastener, tall clean seams with no visible screws mean standing seam. Metal shingles, stamped to look like shingles or shake, are a third type with their own interlocking edges.

The difference matters because almost every repair decision follows from it. Exposed-fastener roofs leak mostly at screws and laps. Standing seam roofs leak mostly at trim, transitions and penetrations, and occasionally at a seam that has opened.

Where two exposed-fastener panels overlap, the lap is sealed with tape sealant and stitch screws. Metal Sales' guide calls for tape sealant at end laps and under closures so the fasteners pass through the sealant. When those sealants age or were left out, water wicks into the lap, especially on low slopes and when snow melts slowly on the roof. The repair is to open the lap, clean it, and re-seal it with the manufacturer's tape sealant and new fasteners, not to run a bead of caulk along the outside.

Slope matters here. The Michigan Residential Code, following the 2015 IRC, sets a 3:12 minimum slope for lapped, non-soldered metal panels without lap sealant, 1/2:12 when lap sealant is applied, and 1/4:12 for standing seam. A screwed-down panel on a porch or addition below those slopes will keep leaking at the laps no matter how often it is patched.

Trim is the other common leak point: ridge caps without closures, wall flashings that rely on a single bead of sealant, rake trim lifted by wind, and pipe flashings. A metal roof's pipe boots are still rubber or neoprene and wear out like any other. Counter flashing at walls and chimneys still depends on sealant somewhere. Those parts need periodic attention even on a roof whose panels will outlast you.

Most residential metal panels are steel coated with zinc or an aluminum-zinc alloy (Galvalume), then painted. The Metal Construction Association's technical bulletin on dissimilar metal corrosion explains that when two different metals are joined by moisture, the less noble one corrodes faster. Its examples matter for repairs: copper has a severe corrosive effect on steel, aluminum and zinc, and runoff from a copper roof or pipe onto a Galvalume surface can corrode it even without direct contact.

Metal Sales' guide says the same in plain words: Galvalume panels should not be in contact with, or subject to runoff from, copper, lead or uncoated steel, and air conditioner condensate, which usually contains dissolved copper, should be piped off the roof. The MCA bulletin adds that preservative-treated lumber should not be used on metal roofs and that graphite pencil marks are extremely corrosive to aluminum and aluminum-zinc coatings.

Paint scratches are another trap. Touch-up paint from the panel maker protects a small nick, but Metal Sales notes it does not have the chalk and fade resistance of the factory finish and will discolor faster, so large areas of touch-up stand out within a few years.

Metal roofs move more than any other common roof covering, and West Michigan gives them a lot of range to move through. In NOAA's daily records for the Grand Rapids airport, the hottest reading of each year from 1991 to 2020 averaged about 94 degrees and the coldest averaged about 6 below zero, a swing of roughly 100 degrees in air temperature alone. The record extremes since 1991 are 104 degrees in 2012 and 22 below zero in 1994, and single days have swung as much as 45 degrees from low to high. A dark panel in full sun runs hotter than the air, and a clear winter night pulls it colder. Every one of those swings works the screw holes, laps and trim that this article describes.

The 2015 Michigan Residential Code has a few rules for metal roof panels that matter on a repair. Section R905.10.4 says panels shall be secured to the supports under the code and the manufacturer's installation instructions, and that in the absence of those instructions, galvanized fasteners are used for steel roofs and stainless steel fasteners are acceptable for metal roofs. That supports the advice above: match the panel maker's fastener, not whatever screw is in the truck.

Underlayment is another. Michigan's version of the code's climate table, adopted as Mich. Admin. Code R 408.30516, answers yes to ice barrier statewide, but Section R905.1.2 lists the coverings that need one: asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate, and wood shingles and shakes. Metal roof panels are not on that list; Section R905.10.5 sends panel underlayment to the general underlayment section instead. Panel makers' instructions can ask for more than the code minimum, such as a self-adhered membrane at the eaves, so a repair at the eave should follow the instructions for your panel.

Snow is the last Michigan factor. Metal sheds snow suddenly, and a slab sliding off a warm roof can tear off gutters, crush a lower roof or land on a walkway. Nothing in the metal panel sections cited here requires snow guards, but on roofs over entries, decks or lower roofs they are worth discussing during any repair. Ask your building department if a permit question comes up.