Interlocking Aluminum Shingles: How the Four-Sided Lock Works

What does “interlocking” mean on an aluminum shingle?

Interlocking means each shingle has formed edges that hook mechanically into the shingles around it. The top edge is fastened to the deck, the bottom edge locks over the course below, and the side edges engage the panels to the left and right. Once a row is down, no edge of any panel is left loose. This is different from a conventional shingle, which simply lies on top of the one below and depends on nails, sealant strips and gravity. With an interlocking system the panels hold one another in place, and the roof behaves as one connected surface rather than hundreds of separate pieces. Interlock® stamps this four-sided design into its Slate, Cedar Shingle, Shake and Mediterranean Tile profiles, so the look changes while the locking principle stays the same.

Why does a four-sided lock matter in high wind?

A four-sided lock matters because wind damage usually starts at a free edge. Gusts get under a loose tab, lift it, and the damage spreads up the slope. When every edge is engaged with a neighbor, there is no tab for the wind to grab. That is useful on exposed New England sites: hilltop farmhouses, homes on open coastline from Narragansett Bay to Down East Maine, and ski-country houses in Vermont and New Hampshire. Interlock® aluminum shingles are rated for winds up to 120 mph when installed to manufacturer specification. The qualifier is important. The rating assumes the right fastening pattern, starter and edge details, and a sound deck, which is why the installer's training matters as much as the panel design.

How do hidden fasteners help keep water out?

Hidden fasteners help because a hole that is never exposed to rain cannot become a leak path. On interlocking aluminum shingles the screws or nails go through the top flange of each panel, and the next course covers them completely. Roof systems with exposed fasteners rely on rubber washers to seal every penetration, and those washers are exposed to sun, cold and movement for as long as the roof is in place. Concealed fastening avoids that maintenance item altogether. The interlocking joints also break up wind-driven rain, so water that is pushed sideways in a nor'easter meets a formed metal channel instead of an open seam. Underneath, an underlayment over the deck adds a second line of protection.

How does the interlock handle New England temperature swings?

The interlock handles temperature swings by letting each panel move slightly within its joints. Aluminum expands when it warms and contracts when it cools. On a New England roof that can mean a panel baking in July sun and sitting under snow at single-digit temperatures in January. If metal is pinned rigidly with fasteners through its face, that movement works at the holes over the years. In an interlocking system, the panels are fastened along one edge and float within the locks on the others, so the movement is absorbed in the joints. That is one reason the fasteners stay tight and the joints stay closed through decades of seasonal cycling.

Can a single interlocking shingle be repaired or replaced?

Yes, a damaged interlocking shingle can usually be replaced without stripping a large section of roof. Because the panels lock into one another, a trained crew can release the surrounding pieces, remove the damaged panel and fit a new one in its place. It is not a job for a general handyman. Forcing the locks or walking on the panels in the wrong spots can bend the edges and create new problems. If a falling limb or other impact damages part of your roof, call the installer that put it on. Homeowners who bought an Interlock® roof from us should contact our service team so the repair is done the way the system was designed.

What else should you know before choosing interlocking shingles?

Interlocking aluminum shingles need a roof pitch of at least 3/12, so very low porches or additions may need a different material. They are light, roughly 0.4 to 0.7 lb per square foot, which suits older New England framing on Colonials, Capes and triple-deckers. Where a house has cedar shingles, cedar shakes, concrete tile or natural slate, those are removed first. A single layer of asphalt may be left in place only if an inspection finds it suitable. If your home is in a historic district, check with your local historic district commission before choosing a profile and color. In a Massachusetts condominium, the trustees or association under Chapter 183A will usually need to approve exterior roofing changes.

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