Aluminum Roofing Fasteners: Concealed vs Exposed on a New England Roof

What is the difference between exposed and concealed fasteners?

Exposed fasteners are screws driven through the visible face of a roofing panel, while concealed fasteners are hidden beneath an overlapping panel, a lock or a clip. With exposed fastening, each screw has a rubber or neoprene washer under its head that seals the hole. With concealed fastening, the hole is covered by the next panel, so it never sees direct rain or sun. Exposed fasteners are typical of corrugated and ribbed sheet roofing on barns, sheds and some commercial buildings. Concealed fasteners are used on architectural systems such as interlocking shingle-style panels and standing seam. The difference sounds small, but on a full roof it adds up to hundreds of sealed holes versus none exposed to the weather. That number has a direct effect on how much maintenance the roof needs over its service life.

Why do exposed fasteners cause problems over time?

Exposed fasteners cause problems over time because their rubber washers wear out and the metal around each screw keeps moving. Sun and heat dry and harden the washers, and the panels expand and contract with every temperature change, slowly working each screw hole larger or backing screws out. Once a washer cracks or a screw loosens, water can get in around the shaft. On a sheet roof with hundreds of screws, it only takes a few failures to create leaks that are hard to trace. Installation matters too. Screws driven too tight crush the washer; screws driven too loose never seal; screws driven at an angle leave gaps. Owners of exposed-fastener roofs are usually advised to inspect and re-tighten or replace screws periodically. That is manageable on a shed, but it is real ongoing upkeep on a house.

How does New England weather affect roof fasteners?

New England weather is hard on exposed fasteners because of freeze–thaw, wind-driven rain and, on the coast, salt. In winter, meltwater can pool around a screw head and refreeze overnight, and ice expands in any small gap. Repeated cycles through a long winter work on every seal. Nor'easters bring rain driven sideways and upward by strong winds, testing openings that stay dry in a normal rain. Summer heat then dries and stiffens washers. Along Cape Cod, the Islands, the North Shore, the Rhode Island and Connecticut shoreline and the Maine coast, salt air adds corrosion risk for fasteners made of the wrong metal. A concealed-fastener system removes most of these exposures, because the fasteners sit under the panels instead of in the weather.

How does Interlock fasten its panels?

Interlock fastens its panels with concealed fasteners hidden beneath a four-sided interlock. Each panel locks into the panels above, below and beside it, and the fasteners that hold it to the deck are covered by the next panel. Nothing is driven through the exposed face of the roof. That design does two jobs. It keeps fastener holes out of the weather, and it lets each panel move slightly within its locks as aluminum expands and contracts. In a climate that swings from subzero nights to hot summer afternoons, that freedom keeps stress off the fasteners. The locked edges also help with wind. The system is rated for 120 mph winds when installed to manufacturer specification. Since 1999 we have installed 5,036 Interlock roofs across the six New England states using this method.

What fasteners should be used with aluminum roofing?

Aluminum roofing should be installed with fasteners and accessories that are compatible with aluminum, as specified by the system manufacturer. When two different metals touch in the presence of moisture, especially salty moisture, one can corrode faster than it would alone. This is called galvanic corrosion. That is why mixing random screws, nails or flashing metals with an aluminum roof can cause trouble, particularly near the ocean. Compatible stainless or coated fasteners and aluminum flashing and trim avoid the problem. For homeowners, the practical rule is simple. Use the components that come with the system, and make sure anyone who adds something later, such as a solar installer, an electrician running a mast or a satellite technician, uses compatible hardware and coordinates with the roofing company first.

Are there any downsides to concealed fasteners?

Concealed fastener systems have a few downsides: they usually cost more, they require trained installers, and repairs need someone who knows how the panels release and relock. Hidden fastening takes more engineering, more precise forming and more careful installation than screwing down sheets. That skill requirement means fewer contractors can install or repair these systems, and homeowners should check credentials. In Massachusetts, confirm Home Improvement Contractor registration; in Connecticut, check registration with the Department of Consumer Protection; in Rhode Island, check with the Contractors' Registration and Licensing Board. New Hampshire and Maine do not have a state roofing license, so ask for references and insurance there. For most New England homes, those trade-offs are worth it. An in-home roof consultation can show you how the fastening works on a sample panel.

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