Washington · Roof lifespan guide
How long does a metal roof last in Washington?
It depends on the metal, the coating, the fasteners and the installation. Steel lasts about as long as its coating keeps it from rusting. Aluminum doesn't rust, and aluminum has been known to last over 100 years. That is a property of the metal, not a promise; what we and the manufacturer put in writing is in the warranties, set out below.
The metal
Aluminum has been known to last over 100 years
Aluminum doesn't rust. A property of the metal, not a promise.
The material
Guardian Lifetime Limited Material Warranty
Material and finish, for the original purchaser's ownership. Transferable once.
The finish
30 Year Alunar® Coating Warranty
On the paint finish that carries the color.
What decides how long a metal roof lasts
"Metal roof" covers very different products. Four things decide how long one lasts on your house.
The metal
Steel rusts wherever its protective coating is scratched, cut or worn through, so a steel roof lasts about as long as its coating. Aluminum doesn't rust. It forms a thin oxide layer on its own surface, which is why aluminum has been known to last over 100 years.
The coating
The paint finish decides how the color holds up to sun and weather. Interlock® roofs carry a 30 Year Alunar® Coating Warranty on the finish, separate from the material warranty.
The fasteners
Exposed screws through the weather surface rely on rubber gaskets that age in sun and cold. Interlock® panels lock to each other on all four sides with the fasteners concealed, so there is no exposed screw head for water or wind to work on.
The installation
Flashing at walls, chimneys, valleys and skylights, the underlayment, the ventilation of the attic below, and the slope (every Interlock profile needs a minimum 3/12) decide how a roof performs for decades. Wind resistance is warranted to 120 mph when installed to manufacturer specification.
What Washington weather does to a roof
A roof here works through months of rain, moss and needles under the trees, salt air on the water, snow in the foothills and dry summer heat every year. Each of those finds a different weak point.
Months of rain
West of the Cascades a roof stays wet for much of the year, and winter storms drive rain sideways under loose shingle tabs. Wood soaks it up and rots; asphalt loses granules. Aluminum absorbs no water and can't rot, and interlocked panels leave no tab edge for wind-driven rain to get under.
Moss and shade
Under tall firs and cedars, moss and needles collect on any roof. Oregon State University Extension says moss doesn't damage roofing directly but holds moisture and debris against it, lifts asphalt shingles and speeds the decay of wood roofs. It can attach to metal too, so keeping the roof and gutters clear still matters; on aluminum there is no wood to rot and no shingle tab to lift.
Salt air on the water
Along Puget Sound, Hood Canal, the San Juan Islands and the coast, salt spray rusts steel wherever its coating is cut or scratched. Aluminum has no steel to rust. Sun is the other test: the Alunar® finish carries the color, and its 30-year coating warranty is what to read.
Foothill snow and ice
In the Cascade foothills and on higher ground, snow and freeze-thaw are part of every winter. Metal sheds snow; where a slide would land on an entry, a deck or a walkway, snow guards go in, planned at the estimate. Ice dams come from heat escaping into the attic and from ventilation, not from the roof covering; no roof prevents them on its own, and ice damming is a stated exclusion of the material warranty.
Metal, asphalt and cedar compared
Published life expectancies are averages. Local weather, the building and its design, material quality and maintenance all move them, as the study below says itself.
| Roof | Life expectancy | Basis | What to know |
|---|---|---|---|
| Asphalt shingles | About 20 years | NAHB / Bank of America study (2007) | A common roof here. When it is done, it comes off or, if suitable, is roofed over once. |
| Wood shakes | About 30 years | NAHB / Bank of America study (2007) | Needs cleaning and re-treating, and in a wet climate grows moss, splits and rots as it ages. Always torn off before a new roof. |
| Interlock® aluminum | Aluminum has been known to last over 100 years | A property of the metal, not a promise | What is promised is in the warranties: the Guardian Lifetime Limited Material Warranty and the 30 Year Alunar® Coating Warranty. |
Asphalt and wood shake figures: National Association of Home Builders and Bank of America Home Equity, Study of Life Expectancy of Home Components, February 2007 (a US study). Full reference at the end of this page.
The warranties, stated separately
How long aluminum can last and what is promised in writing are two different things. Here is what is in writing on an Interlock® roof installed in Washington.
- Guardian Lifetime Limited Material Warranty, from the manufacturer, Interlock Roofing: material and finish, for as long as the original purchaser owns the home, transferable once. The second owner is covered to 50 years from substantial completion. Workmanship is excluded, and ice damming is a stated exclusion. Full warranty (PDF)
- 30 Year Alunar® Coating Warranty on the finish.
- 10 Year Limited Labor Warranty from Interlock Metal Roofing (PNW) Inc.: our 10 Year Limited Labor Warranty covers the installation, for ten years from the Substantial Completion Date. Labor warranty
- Wind resistance warranted to 120 mph when installed to manufacturer specification.
Replacing a roof you'll otherwise replace again
Most of the roofs we replace in Washington are asphalt. Cedar shakes, concrete tile and natural slate always come off first; a single layer of asphalt can be roofed over only if our inspection finds it suitable. The Interlock® profiles (Slate, Cedar Shingle, Shake, Mediterranean Tile and Standing Seam) weigh roughly 0.4 to 0.7 lb per square foot and are made in North America by Interlock Roofing at its plant in Delta, BC, then installed in Washington by our own company, as we have done since 1997.
Book a roof consultation at your house, or over Zoom to start, and you get a written price after your roof consultation.
Metal roof lifespan: questions, answered
How long does a metal roof last?
It depends on the metal, the coating, the fasteners and the installation. Steel lasts about as long as its protective coating, because it rusts where the coating fails. Aluminum doesn't rust, and aluminum has been known to last over 100 years. That is a property of the metal, not a promise; what is promised is in the written warranty.
How long do metal roofs last in Washington?
Long wet seasons, moss, salt air on the water and snow in the Cascade foothills are hard on roofing that absorbs water or relies on exposed screws. An aluminum roof with concealed fasteners, installed to manufacturer specification with good attic ventilation, avoids the usual failure points. Its material and finish are covered by the Guardian Lifetime Limited Material Warranty and the 30 Year Alunar® Coating Warranty.
Does a metal roof last longer than asphalt shingles?
In published figures, yes. A 2007 study by the US National Association of Home Builders and Bank of America put the life expectancy of asphalt shingles at about 20 years and wood shakes at about 30, noting that local weather, building and design, material quality and maintenance all affect it. Aluminum has been known to last over 100 years.
How long does a steel roof last?
A steel roof lasts about as long as its metallic coating and paint keep water off the steel; where the coating is scratched or cut, steel rusts. We install aluminum, which doesn't rust.
What does the Guardian Lifetime Limited Material Warranty cover?
The Guardian Lifetime Limited Material Warranty covers material and finish for as long as the original purchaser owns the home, and it transfers once. Workmanship is excluded from it; our 10 Year Limited Labor Warranty covers the installation. The finish has a 30 Year Alunar® Coating Warranty. Read the full terms before you sign.
Will the color fade?
All exterior finishes change with years of sun. The Alunar® finish on Interlock® roofs carries a 30 Year Alunar® Coating Warranty; the terms set out what is covered.
Will a metal roof stop ice dams?
No roof stops ice dams on its own. They come from heat loss into the attic and from ventilation, and ice damming is a stated exclusion of the material warranty. We look at ventilation and insulation with you at the estimate.
What maintenance does a metal roof need?
Keep gutters and valleys clear of leaves, needles and moss, trim back branches that shade the roof, look the roof over after a major storm, and keep attic vents open. With concealed fasteners there are no exposed screws to re-tighten or re-seal.
Does moss hurt a metal roof?
Moss can attach to any roof that stays shaded and damp, metal included, according to Oregon State University Extension, which says moss holds moisture and debris rather than damaging roofing directly. On aluminum there is no wood to rot and no shingle tab for it to lift; keep the roof and gutters clear, and trim the trees that shade it.
Sources
- National Association of Home Builders / Bank of America Home Equity: Study of Life Expectancy of Home Components (February 2007)
- Guardian Lifetime Limited Material Warranty (full document, PDF)
- 10 Year Limited Labor Warranty (Form 5335)
- ICC-ES Evaluation Report ESR-1790, Interlock Roofing, reissued September 2025 (PDF)
- Oregon State University Extension Service: How to control roof moss and prevent long-term damage
- Interlock Roofing: the aluminum roofing system
Page reviewed October 2026.
Related
- Roof replacement
- Cedar shake roof replacement
- Condo & townhouse roofing
- Fire-resistant roofing
- Metal roof cost
Communities we serve in Washington
- Seattle, WA
- Tacoma, WA
- Vancouver, WA
- Bremerton, WA
- Everett, WA
- Kent, WA
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Get an aluminum roof priced.
A written price after your roof consultation. Interlock Metal Roofing (PNW) Inc. · 5608 17th Ave NW, Suite 1574, Seattle, WA 98107-5232 · 2219 Rimland Dr, Suite 301, Bellingham, WA 98226-8660 · 26910 92nd Ave NW, Suite 425, Stanwood, WA 98292
