Three kinds of metal roof
- Standing seam: long panels joined by raised seams, fastened with hidden clips.
- Exposed-fastener panels: corrugated or ribbed panels screwed through the face.
- Metal shingles: pressed to look like shingles, slate or shakes, installed course by course.
How long metal lasts
HUD's useful-life table for capital needs assessments plans a metal roof at 50 years, against 20 for asphalt shingles. On exposed-fastener panels, ask what the contractor's work warranty says about the fasteners and seals, and for how long (roof warranties).
A single leak on a roof with years of life left is usually a repair.
Over the old roof or after a tear-off
The residential code's general rule is a tear-off to the deck, with a recover allowed over one layer in good condition (IRC R908.3 and R908.3.1.1). Metal has one exception of its own: a complete and separate roofing system, such as a standing-seam panel system designed to carry the roof loads straight to the structure without relying on the old roof for support, does not require removing the existing covering (IRC R908.3.1).
That exception is about the system, not about metal in general. Panels screwed to the old shingles do rely on them. Ask the contractor which case the quote is, and whether your building department agrees.
Ice barrier and snow
The code's ice barrier rule (IRC R905.1.2) names metal roof shingles among the coverings that get one where the climate table requires it: every town in Massachusetts, Connecticut and Rhode Island, Sussex County in New Jersey, and wherever a Pennsylvania municipality's table says so. See ice barrier by state.
Ask where snow coming off the new roof will land. Massachusetts publishes a ground snow load for every town, from 25 to 60 psf (780 CMR 51.00, Table R301.2(2)); in Connecticut it is 30 to 40 psf by town. Ask whether snow guards above doors, walkways and lower roofs are part of the quote.
Florida
Every roofing product used in Florida needs a product approval, and in Broward County, which is in the High-Velocity Hurricane Zone, the code says "All roofing components, roofing systems and roofing assemblies shall have a valid and current product approval" (FBC Building § 1512.2.1). Ask for the approval number of the panel and its fastening. See replacing a roof in Florida.
Metal and solar
A roof that will carry solar panels for 25 years or more should outlast them. That is one reason homeowners choose metal before panels go on; see replacing your roof before solar.
What changes the price
Standing seam against exposed fastener against metal shingles; panel gauge and finish; tear-off or not; snow guards; the number of valleys, dormers and penetrations; and in Florida the approved assembly. See what sets the price.
Questions
How long does a metal roof last?
HUD’s useful-life table for capital needs assessments plans a metal roof at 50 years, against 20 years for asphalt shingles.
Can a metal roof go over existing shingles?
Only as a complete and separate system, such as standing-seam panels designed to carry the roof loads straight to the structure without relying on the old roof (IRC R908.3.1). Panels screwed to the old shingles do rely on them, so the general rule applies: a recover over at most one layer in good condition.
What types of metal roofing are there?
Standing seam uses long panels joined by raised seams and fastened with hidden clips. Exposed-fastener panels are corrugated or ribbed and screwed through the face. Metal shingles are pressed to look like shingles, slate or shakes and installed course by course.
Does a metal roof need snow guards?
Ask where snow coming off the new roof will land, and whether snow guards above doors, walkways and lower roofs are in the quote. Massachusetts publishes a ground snow load for every town, from 25 to 60 psf; in Connecticut it is 30 to 40 psf.