What underlayment, trim, vents and flashing does a metal roof need to last?

A high-temperature synthetic underlayment, because the underside of a metal panel runs considerably hotter than the underside of an asphalt shingle and standard felt or ordinary synthetic can soften and fail. Ice-and-water membrane still goes at the eaves and in the valleys and should also be a high-temperature grade, since standard membrane can release its adhesive under a hot panel. This is one of the places where a metal roof quote that looks cheap usually is cheap.

On an exposed-fastener panel, the neoprene washer under the screw head is the service item and it typically needs attention somewhere between fifteen and twenty-five years, sooner on a south-facing elevation. Washers harden, the screw backs out slightly with thermal movement, and each one becomes a small hole. A standing-seam roof avoids the question entirely because the fasteners are concealed clips under the seam, which is a large part of why standing seam costs more and lasts longer.

Yes, the ventilation requirement is the same as for any other roof, and it is met with a purpose-made metal ridge vent and profile closures that seal the flutes of the panel while allowing air out. The closure is the part that matters: without it the ridge is open to wind-driven rain and to insects, and with the wrong closure the vent is blocked and the attic is unventilated behind a vent that looks correct from the ground.

Yes, and it must be a purpose-made high-temperature boot designed to conform to the panel profile, fixed and sealed at its base into the pan of the panel rather than across a rib. A standard shingle boot on a metal roof hardens faster because of the heat and does not seal to a ribbed surface. Pipe boots are the single most common leak point on metal roofs and they are a maintenance item rather than a lifetime one.

That is oil-canning, and it is a visible waviness in the flat part of the panel. It is largely cosmetic and it is caused by some combination of an uneven deck, stress in the steel coil, and thermal expansion in a panel that is fixed too rigidly to move. It is reduced by a flat deck, striated or pencil-ribbed panel profiles, lighter colours and matt finishes, and by clips that let the panel float. Most manufacturers exclude oil-canning from warranty because it does not affect performance.

On many Maryland roofs, yes, and it is often forgotten. A metal roof sheds accumulated snow in a single slide rather than melting it away gradually, and a full roof of snow releasing at once will take a gutter with it and is genuinely dangerous over a door, a walkway or a driveway. Snow retention devices are designed against the roof area, the pitch and the expected load, and they are fitted with the roof rather than added later.