Washington gives you a narrow window each autumn when the weather is still workable and the problems have not started. Using it properly prevents most of what goes wrong between November and March.
Gutters and drainage
The highest-value item west of the Cascades. Gutters clogged with needles and leaves overflow, and overflowing gutters put water against your foundation exactly when the ground is least able to absorb it.
Clear them, check downspouts run freely, and confirm water is discharging well away from the house rather than pooling at the base. Under heavy conifer canopy — much of King, Pierce and Snohomish counties — this may need doing twice through autumn rather than once.
Heating, before you need it
Book service now rather than in October, when every contractor in your area is booked and the first cold morning has already arrived.
Furnace searches in Washington multiply enormously between summer and autumn, which tells you exactly when availability collapses. A system serviced in early autumn gets an unhurried appointment; the same call in November competes with everyone whose heat has already failed.
Change filters, and if the system has not been looked at in a couple of years, have it properly serviced.
Outdoor plumbing
Disconnect and drain hose bibs. Shut off and drain any outdoor supply that can be isolated. Blow out irrigation systems before the first hard freeze rather than after.
East of the Cascades this is not optional maintenance — it is the difference between a normal winter and a burst line. West of the Cascades it matters less most years, which is precisely why people skip it and then get caught by the occasional hard snap.
Roof and moss
Autumn is your last comfortable chance to look at the roof before the wet season. Check for lifted or missing shingles, debris in valleys, and moss growth.
Treat moss rather than removing it aggressively, and never pressure wash an asphalt roof — it strips the protective granules and takes years off the roof’s life. If growth is heavy or the roof is steep, this is worth paying someone with fall protection to do.
Crawl space
Worth a look before the wet season, particularly in Western Washington. Check for standing water, damp ground, condensation on ducts and pipes, and any smell. Confirm the ground vapour barrier is intact.
Problems found now are manageable. The same problems found in February have had months to develop.
Seals and draughts
Check weatherstripping on doors and windows, and caulking around openings. Small gaps that are irrelevant in August cost real money and comfort through a Washington winter, and they are cheap to address.
East of the Cascades specifically
Attic insulation matters more here than almost anything else, because inadequate insulation is what causes ice dams. Heat escaping into the attic melts the snow pack from below, and the meltwater refreezes at the cold eave.
Check insulation depth and confirm that soffit vents are not blocked. Getting this right in October prevents a problem that presents in January as a roofing failure.
Trees
Washington windstorms bring down limbs. Anything overhanging the roof, resting on the house, or obviously dead is worth addressing before the storms rather than after.