Synthetic Slate vs. Metal Slate Roofing

What is the difference between synthetic slate and metal slate?

The difference is the material. Synthetic slate tiles are molded from plastics, recycled rubber, or polymer and mineral blends, then colored to look like stone. Metal slate is formed from sheet metal, most often aluminum or steel, and coated with a factory finish. Synthetic slate is usually installed as individual pieces, nailed in overlapping courses like real slate. Interlock's metal slate comes as panels that each show several slate faces and lock to the panels around them. Both exist to give homeowners the slate look without the weight, cost and fragility of quarried stone. Because the category labels cover many different products, two synthetic slates or two metal slates can perform quite differently. The useful comparison is product to product: ask each manufacturer for weight, ratings, warranty terms and installation requirements.

How do they compare on weight?

Both are far lighter than natural slate, and aluminum slate is among the lightest options available. Interlock aluminum panels weigh roughly 0.4 to 0.7 lb per square foot. Synthetic slate weights vary by brand and thickness, so check the manufacturer's published data for any product you are considering. In general, molded tiles need enough thickness to be stiff and impact resistant, which adds material. On most homes either option can be installed without reinforcing the framing. Weight becomes a deciding factor on older New England houses with modest rafters, on long-span roofs, and in snow country in Vermont, northern New Hampshire and inland Maine, where the structure needs as much capacity as possible left for snow.

Which handles New England weather better?

Aluminum has a clear advantage in two of New England's toughest conditions: salt air and freeze–thaw. It does not absorb water, so freezing cannot split it, and it does not rust, so coastal exposure on Cape Cod, the Islands, the Rhode Island shoreline or the Maine coast is not a corrosion threat to the panels. Synthetic products are also designed to resist moisture, and many perform well. Their long-term behavior depends on the polymer formulation and UV stabilizers, since years of sun can change the color or surface of any plastic. Ask how long the specific product has been on roofs in a cold climate. Neither type prevents ice dams. Those are caused by heat escaping into the attic and refreezing at the eaves, and the fix is insulation and ventilation.

How do fire and impact ratings compare?

Ratings depend on the specific product and how it is installed, so ask any supplier for the listing behind the claim. Fire ratings in particular usually apply to a tested assembly, including underlayment and deck, not to the shingle by itself. For Interlock, the correct statement is Class A Fire Rated Available, as a complete assembly. The underlayment and deck are part of that rating, so the assembly details matter. For impact, Interlock aluminum shingles are listed to UL 2218 Class 4 under the Intertek listing for Delta Building Products. Many synthetic slates also carry impact ratings. Hail is less frequent in New England than in the central states, but falling ice and branches during winter storms are a real source of impact damage, so a documented rating is worth having either way.

What about color, appearance and fading?

Both can look convincing from the street, and appearance comes down to the specific product. Synthetic slate often has a thicker edge, which can mimic the profile of real stone. Metal slate gets its depth from the formed shape and the interlocking courses. Color stability is where materials differ. Interlock finishes are baked-on coatings backed by a 30 Year Alunar® coating warranty. Synthetic tiles are usually colored through the material or on the surface, and fading behavior varies with the formulation. Interlock colors that suit a slate look include Deep Charcoal, Black and Hunter Green. Whichever product you consider, look at an installation that has been through several New England winters and summers, not just a new sample. That tells you more about long-term color than any brochure.

Which should you choose?

Choose based on documented performance for your conditions, the warranty terms, and the installer. If you live near salt water, want the lightest roof, or are in heavy snow country, aluminum's resistance to rust and water absorption is a strong argument. If a particular synthetic product has a long record in cold climates and a warranty you are comfortable with, it can also be a reasonable choice. The installer matters as much as the material in both cases. Interlock aluminum roofs carry the Guardian Lifetime Limited Material Warranty, covering material and finish for the original purchaser while they own the home, transferable once with the second owner covered to 50 years from substantial completion; workmanship is excluded. If you are in a historic district, check with your local historic district commission before choosing either product.

Common questions