Why chimneys are the wettest part of the house
A chimney is the one element exposed on all four sides, standing above the roofline, with no overhang protecting it. Masonry is porous by design. Water gets absorbed, and in a freeze-thaw climate it expands as it freezes and breaks the material apart from the inside.
Crestline sits in the worst band for this — roughly 4,600 to 5,200 feet, with enough winter to cycle through freezing repeatedly but not enough sustained cold to simply stay frozen. That repeated cycling is what splits crowns and pops brick faces. At Green Valley Lake's 7,200 feet you get snow sitting against the stack for weeks, melting into the masonry and refreezing, which is harder on a crown than rainfall ever is.
The four entry points, in the order we check them
- Flashing at the roof junction. The most common single source. Step and counter flashing should be let into the mortar joint, not caulked over as a temporary fix that then becomes permanent.
- The crown. The sloped slab at the top that sheds water off the structure. Thin unreinforced mortar-wash crowns crack early, and once cracked they let water into both the brick and the flue.
- A missing or failed cap. Without one, rain falls straight down the flue onto the smoke shelf and damper, and animals nest in the passage.
- The masonry itself. Saturated brick and open mortar joints absorb water directly through the face.
It is frequently more than one at once, which is why tracing matters more than guessing. Fixing the flashing on a chimney whose crown is split just moves the complaint.
The mistake that makes things worse
Here is the part that costs people real money. A masonry chimney has to be able to dry outward. Moisture that gets in — and some always does — needs a path back out through the face.
Many consumer masonry sealers are film-forming acrylics. Applied to a chimney they do stop liquid water entering, and they also stop the wall drying. Trapped moisture then freezes inside the brick and spalls the face off. The product marketed to prevent the damage causes it.
The correct category is a vapour-permeable siloxane repellent. It stops liquid water penetrating while letting water vapour escape. If you are buying something off a shelf, the word to look for is breathable or vapour-permeable, and if the tin does not say it, assume it is not.
Order of operations
Waterproofing is the last step, not the first:
- Trace where water is actually entering — crown, cap, flashing, or the masonry face.
- Correct that source. Repellent applied over an active leak seals moisture in.
- Repoint any joint that will take water.
- Let saturated masonry dry before treating it.
- Then apply a breathable siloxane repellent, saturating the surface rather than skimming it.
Done in that order on sound masonry, a siloxane treatment is one of the better-value preventive measures available and holds for several years. Done first, it is worse than doing nothing.
Want a professional to take a look?
Baker Chimneys Incorporated serves 12 communities across the San Bernardino Mountains and the valley below. Free, no-obligation estimates.
Frequently Asked Questions
The usual signs are damp patches on ceilings or walls beside the chimney chase, white efflorescence staining on the exterior brick, a rusty damper or standing water in the firebox after rain, and a musty smell from the fireplace in wet weather.
You can, if the crown, cap, flashing and joints are already sound and you use a vapour-permeable siloxane rather than a film-forming acrylic. If any of those four are not sound, sealing locks the water in and you will spall brick faces off within a few winters.
A properly applied siloxane treatment on sound masonry generally holds for several years, depending on exposure. We record what was applied and when, so reapplication is scheduled on evidence rather than on a sales cycle.