Black streaks under siding joints are one of the more common calls we get from homeowners across Whatcom County, from the neighborhoods of Bellingham out to Ferndale, Lynden, Blaine, Birch Bay, Sudden Valley, and the islands. The pattern is easy to recognize once you know what to look for, but it is often mistaken for ordinary dirt until it keeps coming back after cleaning.
The telltale sign is a dark streak, usually charcoal gray to almost black, running vertically downward from a horizontal lap joint, a butt seam where two pieces of siding meet end to end, or the bottom edge of trim and J-channel. Sometimes the streak has a faint greenish cast, which usually points to algae mixed in with the grime. The staining tends to widen as it travels down the wall, fanning out below the joint the way a stain fans out below a leaky gutter seam.
You will usually find the heaviest streaking on north- and west-facing walls, on elevations shaded by mature conifers, and on any wall that catches wind-driven rain off Bellingham Bay or the Strait. Walls under a roofline without a drip edge, or under a gutter that has been overflowing, show streaks that line up almost perfectly with the drip line. Rub a gloved finger across a dry streak: if a chalky, dark residue comes off easily, you are most likely looking at surface algae or airborne grime. If the mark is embedded, slightly raised, or feels slightly soft when you press near it, that is a different problem worth a closer look, which we cover below.
It helps to distinguish this from a few look-alikes. Rust streaks from corroding fasteners tend to be a narrower, orange-brown line radiating from a single point, not a joint-wide band. Efflorescence, the chalky white mineral deposit sometimes seen near foundations, is the opposite color and texture entirely. True black streaking at a joint is almost always a moisture-and-organic-growth story, sometimes with a rot component underneath.
Three things combine to produce this pattern in our part of the state, and it is rare to find just one acting alone.
The first is simply how much rain Whatcom County gets and how slowly things dry between storms. Long stretches of low-intensity, persistent rain from fall through spring keep siding joints damp for days at a time, and the low winter sun angle on north-facing walls means those surfaces barely get a chance to dry before the next system rolls through. Water that would evaporate quickly in a drier climate instead sits at the joint, feeding whatever organic material is already there.
The second is wind-driven rain off Bellingham Bay and the Strait. Homes closer to the water, and homes on more exposed lots in Blaine, Birch Bay, and parts of Bellingham's waterfront-facing neighborhoods, take rain sideways during winter storms. That horizontal push forces water up and under lap joints and behind trim edges that were only ever designed to shed water moving straight down. Once moisture gets behind a joint, it stays there far longer than it would on a sheltered wall, and it carries dust, pollen, and organic debris with it that becomes food for algae and mold.
The third factor is biological growth. Whatcom County's damp, mild climate and heavy tree cover, especially around Sudden Valley, wooded lots outside Lynden, and shaded lots throughout Bellingham, are close to ideal conditions for algae and lichen. These organisms feed on the organic content in dust, pollen, and sometimes the wood fibers or cellulose in older siding and trim, and they concentrate wherever moisture lingers longest, which is exactly at the joints.
There is also a rot component that shows up less often but matters more. Older cedar lap siding, or any siding with a butt joint that was not properly flashed with house wrap or flashing tape behind it, can develop wood rot in the trim board or sheathing right at that seam. As the wood breaks down, tannins and rot byproducts leach out and streak down the face of the siding below, often looking very similar to an algae stain from a distance. In the foothill areas toward Mount Baker and out past Deming, occasional freeze-thaw cycling adds another layer: water that gets into a hairline gap in caulking freezes, expands the gap slightly, and then lets in more water on the next thaw, gradually working the joint looser year over year.
Most of the time, no. A light black streak on a shaded wall that has been there for a year or two without spreading is very often just surface algae, which is a cosmetic issue rather than a structural one. It is worth cleaning for curb appeal and to slow further growth, but it is not an emergency.
The picture changes when the streak is paired with other symptoms. If the siding near the streak feels soft or spongy when you press on it, if the joint itself looks like it has separated or bowed slightly, if the streak reappears heavily within weeks of a cleaning, or if you can see daylight or feel a draft at that seam from inside, those are signs that water has gotten past the siding and into the wall assembly rather than just sitting on the surface. That is worth an inspection sooner rather than later, because wood rot in sheathing or framing spreads quietly behind the finish material and is much more expensive to fix once it has traveled beyond a single joint.
If a repair does end up requiring the wall to be opened up, whatever goes back together needs to meet the current Washington State Energy Code for the building envelope, including the continuous insulation and air-sealing requirements that apply to exterior walls. That is not something to worry about for a simple cosmetic streak, but it is a real consideration once you are past cleaning and into rebuilding a section of wall.
For streaking that turns out to be cosmetic, the fix is usually a soft wash cleaning followed by resealing the joint with an appropriate exterior sealant, sometimes paired with a small trim repair if the J-channel or drip cap has deteriorated. That kind of work is generally a modest expense, often in the low hundreds of dollars per affected area, and it can extend the life of otherwise sound siding significantly.
When we open up a joint and find isolated rot in the trim board or a small section of sheathing, spot repair is often still reasonable: cut out the damaged material, replace it, correct the flashing detail that let water in, and reinstall matching siding. That work climbs into the low thousands depending on how much material needs replacing and how accessible the wall is.
Replacement becomes the more honest recommendation when streaking shows up at multiple joints across a wall or across the house, when the siding is older cedar lap nearing the end of its practical service life, or when opening even one joint reveals rot that has clearly traveled along the sheathing rather than staying contained. In that situation, patching one joint just means the next one fails a year or two later, and a full re-side, while a considerably larger investment, ends up costing less over time than a string of repeated repairs. We will tell you honestly which situation you are in rather than defaulting to the more expensive answer, and we will also tell you plainly that pressure washing alone, without addressing why water is sitting at that joint in the first place, is a temporary fix at best; the streaks will return.
The most effective prevention is getting the water management details right at every joint: proper lap orientation so upper courses shed over lower ones, correctly installed drip caps above windows and trim, and house wrap or flashing tape that directs any water that does get behind the siding back out rather than trapping it. On homes exposed to wind-driven rain off the bay or the Strait, this detailing matters even more than it does on a sheltered lot.
Keeping gutters clear and properly pitched prevents the overflow streaking pattern we see so often below rooflines, and trimming back conifers and shrubs from north- and west-facing walls lets those surfaces get more airflow and occasional sun, which slows algae growth considerably. A gentle soft wash once every year or two, rather than aggressive high-pressure washing that can force water behind panels and make the problem worse, keeps organic growth from establishing a foothold. Caulking at joints does not last forever in this climate; plan on inspecting and refreshing sealant every few years rather than assuming it is permanent.
If you are re-siding or doing a larger repair, insist that the new wall assembly meets current Washington State Energy Code requirements for insulation and air sealing. Getting that right the first time avoids both the energy performance shortfall and the moisture problems that come from an improperly detailed wall.
It will usually remove the visible staining, but if the underlying cause is moisture collecting at a joint due to wind-driven rain, poor flashing, or shade from trees, the streaks tend to come back within a season or two. A soft wash paired with correcting the water path at that joint gives a much more lasting result than washing alone.
The dark growth on exterior siding is almost always algae or lichen rather than the indoor mold species people are usually concerned about, and it is generally not a health hazard sitting on the outside of your home. That said, if streaking is paired with signs that water has gotten into the wall cavity, it is worth having that checked, since sustained moisture inside a wall can eventually support mold growth on the interior side of the sheathing.
Surface algae rubs off as a dry, chalky residue and the siding underneath feels solid. If the area near the streak feels soft, spongy, or slightly bowed when you press on it, or if the streak has a brownish rather than black or greenish tint, that points toward wood rot in the trim or sheathing rather than a purely cosmetic stain, and it is worth a closer look before it spreads.
Yes. North- and west-facing walls that stay shaded longer and walls that catch wind-driven rain off Bellingham Bay or the Strait tend to show the heaviest streaking, while south- and east-facing walls that get more sun and less direct wind exposure usually stay drier and cleaner.
If you are seeing black streaks at your siding joints anywhere from Bellingham to Ferndale, Lynden, Blaine, Birch Bay, or Sudden Valley, Alpine Exteriors can take a look and tell you honestly whether you are dealing with a simple cleaning job or something that needs a repair. Alpine Exteriors is licensed in Washington under contractor license ALPINI751OW, and we would rather give you a straight answer about what your siding needs than talk you into more work than the situation calls for.