Washington law prohibits burning of waste, includes large fine.
Washington State law prohibits burning of solid waste, including garbage or rubbish, Chapter 173-425 Washington Administrative Code (WAC), Outdoor Burning.
Prohibited items to burn:

Solid waste: Burning solid waste, such as garbage, rubbish, paper, cardboard, or junk mail, is illegal.
Construction debris: Burning construction debris, lumber, or treated wood is illegal.
Other materials: Burning dead animals, asphalt, petroleum products, paints, rubber products, plastics, metal, or anything that emits toxins or bad odors is illegal.
Burn barrels: Using a burn barrel is illegal.
The fine can be up to $10,000 per day for illegal burning.
Where burning is allowed, only natural vegetation or firewood is legal to burn.
Learn more and/or report illegal burning.