Is an Engineer Required for My Deck Project in Washington?

Washington cities — Seattle, Spokane, Tacoma, Bellevue — generally require an engineer-sealed plan set for any deck that leaves the prescriptive path, and the PE seal is the standard requirement for the structural sheets. The seal must show the engineer's name, Washington PE number, and date, with the engineer in responsible charge. Simple ground-level decks can often be permitted from the prescriptive span tables, but ledger-attached decks, decks over 30 inches above grade, and decks carrying a hot tub or roof load are treated as engineered work in Washington. Owner-builders don't escape the requirement: the building department asks for the seal regardless of who drew the plans, and the engineer must design the deck for Cascadia seismic bracing and wet rot.

The engineer designs the footings for deep footings in saturated soils — Puget Sound lots sit wet most of the year, so the engineer details post bases that keep framing out of standing water, and details the ledger connection with ledger flashing that survives nine months of rain — the engineer details through-bolts and flashing sequences that keep the band joist dry behind siding. Guards, stairs, and lateral bracing go on the sealed plan set — guard height and infill spacing, stair stringer sizing, and the bracing that keeps the deck from racking under Cascadia seismic bracing and wet rot. Bring a plot plan with setbacks, the deck's dimensions and height above grade, and photos of the house wall where the ledger will attach. Cascadia seismic bracing shapes Washington decks — lateral bracing and positive house ties are engineered for earthquake loads standard prescriptive tables skip.

Use this checklist before engaging an engineer for your deck: • Engineer of record holds an active Washington PE license, verifiable on the state roster • Footings designed for Cascadia seismic bracing and wet rot on the lot • Ledger connection detailed to the house band joist, never the veneer • Lateral bracing and hold-down designed for Cascadia seismic bracing and wet rot • Guards and stairs drawn to code (guard height, infill spacing, stringers) • Hot tub or roof loads called out and framed for where the deck carries them • Post bases and hardware specified for the local exposure • Engineer confirms they'll respond to plan-check corrections on sealed sheets