Metal on the coast, and where it actually fails

Why metal suits this coast at all

Two reasons, and neither is the one people give.

The first is that metal does not rely on an adhesive seal between overlapping pieces to stay down in wind. A shingle roof holds together because each course is glued to the one below by a strip that softened in the sun and set. That works, and when it lets go at an edge the whole course lifts at once. A properly fixed metal roof is mechanically attached along its length, which is a different and better relationship with uplift.

The second is that metal does not shed a mineral surface. The thing that ends most shingle roofs here is the slow loss of the granules that keep ultraviolet off the asphalt. Metal has no equivalent failure.

What metal does have is a corrosion problem, and on this coast that is the entire design question.

The two systems, and why the difference matters

Standing seam runs the panels up the slope and joins them at a raised seam, with the fixings underneath it, clipped rather than driven through the face. There is no hole exposed to the weather anywhere on the field of the roof. That is the whole advantage and it is a large one.

Exposed fastener panel screws straight through the face into the structure below, with a rubber washer under each head to seal the hole. It is cheaper to buy and quicker to fit, which is why it is on almost every shed, shop and cover on this coast.

The trade is that the sealing element is now a small rubber ring in full sun and salt air, and there are hundreds of them. They harden, they shrink, they stop sitting flat, and after that every fixing is a way in. The panel above them can be in perfect condition while the building leaks in twenty places.

The remedy is a rescrew: one screw size up so the new thread bites clean metal, new washers across the entire roof rather than only at the stains. It is a day's work on an outbuilding and it is routine rather than remarkable.

The parts the quote does not mention

Coatings. A galvanised finish protects by sacrificing itself, which is genuinely clever and strictly finite. Salt air spends that protection much faster than inland air, so a galvanised component near the water is on a shorter clock than the brochure figure suggests. Ask what the finish is on the fasteners and the trims, not just on the panels.

Dissimilar metals. Two different metals in contact, with salt water bridging them, corrode faster than either would alone. It turns up as fasteners chewing themselves out of panels, or trims failing where they meet something else. It is avoidable entirely by specifying compatible materials, and it is routinely not specified at all.

Trims and end laps. The long unbroken runs of panel on an outbuilding lift at the ends and at the gable trims first, usually where something was fastened lightly. On a coastal site that is where the attention belongs.

What to ask for, in one paragraph

On the water, ask specifically about the fasteners and the small metal parts rather than the panel. Ask what the finish is and whether stainless is an option for the fixings. Ask how dissimilar metal contact is being avoided. Ask what the fastening pattern is at the perimeter and the corners rather than in the field. And ask what the maintenance expectation is, because an exposed fastener roof has one and a standing seam roof largely does not.

If the roof on the house is shingle and the question is really about the covering as a whole, what a replacement involves is the page for that. If something is leaking now and the panels look fine, the leaks and flashing page starts in the right place.