What Is an Interlocking Roof?

What is an interlocking roof?

An interlocking roof is a roof covering made of pieces that hook or lock into each other. Instead of depending only on gravity and a nail at the top of each shingle, every piece is held by the ones around it, so the finished roof behaves like one connected skin. The idea is old. Clay and concrete tiles have used interlocking edges for generations. Interlock® applies it to light aluminum panels shaped as slate, cedar shingle, shake or tile. Each panel locks to the course below, the course above and the panels beside it. The fasteners go through the top edge and are covered by the next row, so they are never exposed to rain, sun or snow. That is the basic definition you will see used across the industry.

How does an interlock actually hold a roof together?

An interlock holds a roof together by turning each panel edge into a hook. During installation the crew starts at the eaves with a starter strip, sets the first course, and fastens it along the top flange. The next course hooks over that flange, covering the fasteners, and its side edges engage the panels next to it. Because each piece is restrained on every side, a gust pulling at one panel is resisted by all the panels it is locked into. The joints are formed channels, so water moving across or up the roof is turned back downhill. The panels can still slide slightly within the locks as the metal warms and cools, which keeps temperature movement from loosening fasteners over many seasons.

What parts make up an interlocking roof system?

An interlocking roof system includes much more than the panels you see. Under them is the roof deck, which must be solid, and an underlayment that acts as a secondary water barrier. At the eaves is a starter piece that the first course locks into. Along the rakes, ridges and hips are trim pieces made for that profile. Then there is flashing: the metal that turns water at chimneys, sidewalls, dormers, skylights, valleys and vent pipes. Flashing is where most roof leaks start on any kind of roof, so on a New England house with a big brick chimney, two or three dormers and an ell added a century after the main block, it is the most important part of the job. A system roof is designed so all these parts fit together.

Is standing seam an interlocking roof?

Standing seam is related but usually described differently. It uses long vertical panels that run from eave to ridge, joined at raised seams that are crimped or snapped together, with concealed clips holding the panels to the deck. The panels are joined along their long edges rather than locked on all four sides like shingles. Interlock® offers Standing Seam as one of its profiles, alongside the interlocking Slate, Cedar Shingle, Shake and Mediterranean Tile shingles. The choice is mostly about the look of the house. Standing seam has a clean, linear appearance that suits modern homes, barns and some farmhouses. The shingle profiles suit traditional Colonials, Capes and Victorians, where a seamed metal roof can look out of place.

Why are interlocking roofs used on New England homes?

Interlocking roofs are used on New England homes because they address the region's main roof threats at once. Locked edges resist the wind that comes with nor'easters and summer thunderstorms. Covered fasteners and formed joints help keep wind-driven rain and snowmelt out. Aluminum interlocking panels don't rust in salt air from the Cape to Down East Maine. Weight is the other reason. Many older houses have rafters sized for wood shingles, and Interlock® panels weigh roughly 0.4 to 0.7 lb per square foot, which leaves the framing's capacity for snow. Our Job Book shows 5,036 completed Interlock® roofs in the six states since 1999, with Brockton, Lowell, Worcester and Plymouth among the towns with the most.

Are there limits to what an interlocking roof can do?

Yes. An interlocking roof needs enough pitch to drain, and Interlock® shingles require at least 3/12. Flatter porch roofs and rear additions need a low-slope system. It also cannot fix problems inside the house. Ice dams form when attic heat melts snow that refreezes at the cold eaves, and the cure is air sealing, insulation and ventilation. Ice damming is excluded from the Guardian Lifetime Limited Material Warranty, which otherwise covers the material and finish for the original purchaser while they own the home. And an interlocking roof is only as good as its installation; the locks and flashing have to be done the way the system was designed, which is why Interlock® is installed only by certified crews.

Common questions