Washington's Climate and What It Means for Sheet Metal Trim
Washington state has two dramatically different climates divided by the Cascades. Western Washington — home to Seattle, Tacoma, Olympia, and Bellingham — is one of the wettest metropolitan regions in the continental US. Seattle receives about 38 inches of annual rainfall (mostly as persistent drizzle from October through April), while the Olympic Peninsula can exceed 140 inches per year. Moisture management and corrosion resistance are the dominant design considerations for sheet metal trim west of the Cascades.
Eastern Washington — Spokane, the Tri-Cities, Yakima, and the wheat country — has a semi-arid continental climate with cold winters (Spokane averages 25 inches of snowfall), hot dry summers, and significant freeze-thaw cycling. The contrast with Western Washington means that trim specifications appropriate for Seattle are not always optimal for Spokane, and vice versa.
Washington's coast — from the Olympic Peninsula south to the Columbia River — is exposed to Pacific storm systems and has some of the highest design wind speeds in the state, combined with persistent salt air in marine-facing exposures.
Common Roofing Systems and Trim Applications
Western Washington commercial construction is predominantly low-slope single-ply — TPO and EPDM dominate the Seattle commercial market, with modified bitumen on older building stock. Steep-slope construction is a major market segment in the Seattle suburbs, the Eastside communities, and throughout Puget Sound:
- Aluminum coping caps on flat-roof commercial buildings throughout the Seattle metro
- Drip edge and eave trim for the significant steep-slope construction roofing market — cedar shake replacement is a major driver, with metal roofing growing rapidly as a premium re-roofing option
- Aluminum fascia cap on new commercial construction throughout western Washington
- Counter flashings and sidewall flashings on the large stock of 1960s–1990s commercial and institutional buildings
- Valley flashing — wider profiles preferred in high-rainfall environments to manage increased runoff volume
Sealed hem specification note: In Western Washington's wet climate, open hem details trap moisture and accelerate corrosion and biological growth at the hem. Always specify closed, sealed hems on any trim installed in shaded or perpetually damp exposures. Galvalume and aluminum are strongly preferred over galvanized steel.
Washington Building Code Context
- Washington State Building Code: Adopts IBC with Washington state amendments. The Seattle Building Code has additional amendments for the city of Seattle.
- Washington State Energy Code: Among the more stringent energy codes in the US — particularly affects continuous insulation requirements on commercial walls and roofs, which in turn affects trim profile back-leg dimensions to span insulation thickness.
- Coastal wind zones: The Washington coast and the San Juan Islands are subject to elevated design wind speeds from Pacific storm systems. Verify wind design requirements for coastal projects.
- Seismic design: Western Washington is in a high-seismic zone due to the Cascadia Subduction Zone. Parapet bracing requirements under seismic design categories D and E affect coping anchor design similarly to California.
Common Trim Profiles for Washington Jobs
How Trimgy Ships to Washington
- Seattle / Tacoma metro: 3–5 business days
- Olympia / South Puget Sound: 3–5 business days
- Bellingham / Skagit Valley: 4–6 business days
- Spokane / Eastern Washington: 4–6 business days
- Tri-Cities / Yakima: 4–6 business days
Frequently Asked Questions — Washington
What sheet metal specifications are recommended for Washington's wet climate?
Washington's Pacific maritime climate delivers 35–60 inches of annual rainfall in Seattle and up to 140 inches on the Olympic Peninsula. Sealed hem details are critical — open hems trap moisture and promote corrosion. Galvalume and aluminum are strongly preferred over galvanized steel. Kynar-coated painted finishes maintain adhesion better in consistently wet conditions.
How does moss and algae affect sheet metal trim in Western Washington?
Western Washington's high humidity and moderate temperatures create ideal conditions for moss, lichen, and algae growth. Smooth Kynar-coated aluminum or Galvalume surfaces are less hospitable to biological growth than textured surfaces. Trim profiles with tight sealed hems minimize moisture trapping that feeds biological growth.
Does the Seattle building code have specific requirements for roofing trim?
Seattle follows the Washington State Building Code (IBC-based) with Seattle amendments. The Seattle Energy Code has requirements affecting roofing assemblies for commercial applications. Coastal areas of the Olympic Peninsula have higher design wind speeds due to Pacific storm exposure.
Does Trimgy ship to Washington state job sites?
Yes. Trimgy ships via LTL freight to Seattle, Tacoma, Spokane, Olympia, and all points across Washington state. Enter your job-site zip for a real-time freight quote. Typical transit to the Seattle metro is 3–5 business days.
What roofing systems are most common in Western Washington?
Western Washington commercial construction is heavily toward low-slope single-ply — TPO and EPDM dominate the Seattle market. Steep-slope construction in the Seattle suburbs uses cedar shake, composition shingle, and increasingly standing seam metal. Custom eave trim, rake trim, and drip edge are common on the growing premium commercial metal roofing segment.