Chimney Waterproofing for Puget Sound Rain

Seattle's rain is less about volume than about how long it lasts. NOAA's 1991 to 2020 normals put Sea-Tac at about 39 inches a year spread over roughly 156 days with measurable rain, and Everett's Paine Field logs rain on about 178 days, close to every other day. Brick on a chimney rarely gets a chance to dry out between October and May, and wet masonry is where mortar loss, flaking faces and inside stains begin.

A water repellent slows that down. It is a clear, penetrating product made for masonry, not paint and not a shiny sealer. It lines the pores so liquid water beads and runs off, while moisture already inside the chimney can still leave as vapor. We only apply it to a stack that is sound, because no coating bridges an open joint, a cracked crown or a gap in the flashing.

Signs you need this

• Brick that stays dark for days after the rain stops
• White powder (efflorescence) streaking down the stack
• Brick faces flaking or popping off
• A damp patch on the chimney wall inside the house
• Moss growing on the brick or along the flashing

How we do it

1. Look at the whole stack firstAt the free inspection we check the crown, cap, flashing, joints and brick faces and photograph every place water can get in from above.
2. Repair before we coatOpen joints, a cracked crown or loose flashing are fixed first and priced separately, so the repellent never hides an active leak.
3. Wait for dry brickWe brush off moss and loose material and book the work for a dry spell. Repellent soaks into dry masonry and only skins over wet masonry.
4. Saturate every faceWorking from the top down, we flood each side until it stops taking product, with roofing, siding and windows masked off.

Why not paint the chimney or use a glossy sealer?

Film coatings stop water going in, but they also stop it coming out. Moisture reaches brick from inside too, through warm flue gases and water that wicks through the masonry. The National Park Service's guidance on historic masonry makes the point that masonry must be able to let moisture migrate out, or the units get damaged. Trapped water pushes the brick face off behind the film. A breathable repellent keeps rain out and lets vapor leave.

Does moss on the chimney matter?

Oregon State University Extension describes moss as holding moisture against the surface it grows on, and on shingle roofs it can lift the edges and let water reach the layers beneath. On a chimney, that means wetter brick and wetter flashing. We brush it off by hand before treating. We do not pressure wash, which can damage shingles, and we keep zinc moss products off copper flashing because zinc sulfate corrodes copper.

How long does a repellent last in this climate?

It wears with sun, rain and time, so plan on having the stack looked at again in a few years. A simple check is to splash water on the brick. If it beads, the treatment is still working. If it darkens and soaks in, it is time for another coat. We note the date of the treatment on your inspection report so you know when to ask.