Winter in the San Juan Islands is not defined by snow. It is defined by water and wind. Roughly 40 inches of rain fall here each year, and the heaviest of it arrives in November, driven sideways by gusts that run about 20 percent stronger on the west side than they do over in Anacortes. Add salt air off every shoreline and the moss that thrives on shaded north slopes under Douglas fir, and you have a climate that finds every weak point in a roof. The good news is that a few hours of preparation each fall can spare you a leak, a claim, and a very inconvenient winter repair.
This guide walks through what to check, in what order, and why it matters.
Start With a Ground-Level Look
Before anyone climbs a ladder, walk the perimeter of your house on a dry day and look up. You are hunting for obvious trouble: lifted or missing shingles, metal panels with loose seams, sagging gutter lines, and dark streaks of moss on the north slopes. Bring binoculars. Half of what you need to know is visible from the yard, and it is far safer than a wet roof in a rising wind.
The Moss Problem
Moss is not just cosmetic. It holds moisture against the roof surface, lifts shingle edges as it grows, and creates the damp, shaded conditions that rot decking over time. If you see thick green mats, plan a treatment before the wet season, not during it. A professional moss treatment typically runs an estimated $400 to $1,600 depending on roof size and severity. Never pressure-wash asphalt to remove moss; you will strip the protective granules and shorten the roof's life dramatically.
The Fall Storm-Prep Checklist
Work through these tasks in order. The early items are the cheapest and prevent the most expensive failures.
Task Why It Matters Do It Yourself or Call a Pro 1 Clear gutters and downspouts Backed-up water finds its way under eaves and behind fascia DIY if single-story and comfortable 2 Cut back overhanging branches Wind-whipped limbs abrade panels and drop debris Pro for large limbs 3 Treat and monitor moss Prevents moisture retention and edge lifting Pro for full-roof treatment 4 Inspect and reseal flashing Chimneys, skylights, and valleys are the top leak points Pro 5 Check fasteners and seams Salt air loosens and corrodes cheap hardware Pro 6 Confirm attic ventilation is clear Blocked vents trap moisture and invite rot DIY inspection 7 Verify ice-and-water membrane at valleys and eaves These channels take the brunt of wind-driven rain Pro to confirmIf you are searching online for https://sanjuanroofingco.com/ in the weeks before the storms hit, understand that the busiest window for island roofers is exactly when the first big front rolls through. Booking your inspection in September or early October means you are not waiting in line behind everyone whose roof just started leaking.
Focus on the Water's Real Path
Island rain rarely falls straight down. It is pushed under shingle courses and up beneath flashing by wind that does not let up for hours. That is why the details matter more than the field of the roof.
Flashing and Penetrations
Every place something pokes through your roof is a potential leak: the chimney, the plumbing vents, the skylights, the wall intersections. Flashing seals those transitions, and it is where most winter leaks begin. Look for cracked sealant, lifted metal, and rust. On a coastal roof, flashing should be a corrosion-resistant metal, and near saltwater that means marine-grade coatings and stainless or equivalent fasteners rather than the cheap galvanized hardware that pits within a few seasons.
Valleys and Eaves
Valleys funnel enormous volumes of water, and eaves are where wind-driven rain can back up under the first course. Both should be protected by an ice-and-water membrane beneath the roofing. If your roof was installed on a budget years ago, this protection may be missing exactly where you need it most. A fall inspection is the time to find out.
Know Your Roof's Age and Limits
Different materials tolerate island winters differently. A well-installed standing-seam metal roof with 24-gauge, marine-grade Kynar 500 coating can shrug off five decades of salt and storms. Architectural asphalt does well for 20 to 30 years but degrades faster on shaded, moss-prone slopes. Cedar is beautiful but demanding in this wet climate and needs vigilant upkeep. Flat and low-slope sections, common on modern island homes, rely on TPO, PVC, EPDM, or torch-down membranes, and their seams deserve close attention before winter.
If your roof is near the end of its expected life, a fall storm-prep visit is also the moment to get a realistic sense of timeline and cost. A targeted repair might run an estimated $450 to $3,500, while a full replacement lands somewhere in the $11k to $40k-plus range depending on material and complexity. Ferry logistics for crew, materials, and off-island disposal typically add roughly 10 to 25 percent over a comparable mainland job, so budget accordingly. These are estimates, not quotes.
After the Storm
Once the first big system passes, do another ground-level walk. Look for displaced material, new debris, and any interior signs of water: stains on ceilings, damp attic insulation, or a musty smell. Catching a small intrusion in December is a repair. Ignoring it until spring is often a decking replacement. Document anything you find with photos and dates, which will help enormously if you ever need to file an insurance claim.
Preparation is not glamorous, but on these islands it is the difference between a roof that quietly does its job all winter and one that ruins a holiday weekend. A few hours in the fall buys you months of peace.
About the Author
Dana Whitcomb is a Salish Sea home-maintenance writer and former general contractor who spent a decade weatherproofing houses against Pacific Northwest winters. She is happiest explaining the unglamorous seasonal chores that quietly save homeowners thousands.