Metal strips at the ridge are one of the few genuinely passive roof maintenance measures — install once, and they keep working for years with no further attention. They are also frequently oversold and occasionally installed in ways that do very little. Here is what they actually do and what to expect.
Zinc and copper are toxic to bryophytes and algae at low concentrations. When rain falls on an exposed strip of either metal, a small quantity of metal ions dissolves into the runoff. That runoff then flows down the slope below, and every surface it crosses receives a dilute dose sufficient to suppress new colonisation.
You can verify the principle on almost any Seattle roof without buying anything: look for the clean vertical stripes running down the slope beneath any galvanised vent flashing, metal chimney cap, or satellite mount. Those stripes are not coincidence. They are the same chemistry operating accidentally, and they are the most persuasive argument for the deliberate version.
Zinc is the common choice. Less expensive, widely available in strip form, and effective. It does deplete over time as it dissolves — that is how it works — so the effect diminishes across many years rather than lasting indefinitely.
Copper is more effective at lower concentrations and lasts considerably longer. It costs more. It also weathers to a patina that some homeowners like and others do not, and it can stain lighter-coloured surfaces below it.
For most Seattle roofs zinc is the sensible economic choice. On a heavily shaded north slope under permanent canopy — the hardest case — copper's greater potency is worth the difference.
This is the practical detail that determines whether strips work on your roof.
Protection comes from runoff carrying dissolved ions downhill, and that solution becomes progressively more dilute as it travels and as it is joined by rain falling directly on lower sections. The concentration at the eave of a long slope is a fraction of what it is a few feet below the strip.
Practically, a single ridge strip gives strong protection for the upper portion of the slope, decent protection through the middle, and progressively less toward the bottom. On a short slope that may cover the whole roof. On a tall roof, or a long slope, the lower third can remain essentially untreated — which is exactly where moss is often worst, because that is where runoff lingers and debris collects at the gutter line.
The fix is straightforward: additional strips partway down the slope. It is less tidy, and it is what actually works on a large roof.
Strips should be installed near the ridge with a portion of the metal exposed to rainfall. If the strip is entirely tucked under the ridge cap with nothing exposed, rain never contacts it and no ions are released. This sounds obvious and is a real and common installation error.
Fastening should be minimal and, wherever possible, under the shingle course above rather than face-nailed through the field, since every face fastener is a penetration. Fastener metallurgy should be compatible with the strip to avoid galvanic corrosion at the fixing points — the same principle that applies to flashing near saltwater.
By far the best moment to install is during a re-roof. Someone is already up there, the ridge is already open, and the incremental cost is minimal. Retrofitting later means either lifting ridge caps or accepting a face-fastened installation.
Will: suppress new moss and algae establishment on the slope below, for years, with no maintenance.
Will not: kill an established mature moss mat. Existing growth has to be treated and removed first; strips then keep it from coming back. Homeowners who install strips over heavy existing moss and wait for results are disappointed, reasonably.
Will not: compensate for the underlying conditions. A roof under dense permanent canopy, receiving continuous organic debris and never drying, will eventually grow moss regardless. Strips slow the process substantially; they do not repeal it.
Against the alternative — treating chemically every one to two years indefinitely, with the associated cost, roof access, and runoff each time — a properly installed strip system is inexpensive, passive, and environmentally lighter because it delivers a very small continuous dose rather than periodic concentrated applications.
Combined with algae-resistant shingles specified at re-roof time, which carry copper-infused granules distributed through the surface, it is close to the strongest passive defence available in this climate. Neither is expensive. Both are decisions made once, at the moment the roof goes on, and then forgotten about for twenty years.
This post is part of our in-depth coverage of moss removal & gutter care in Seattle.
They suppress new establishment on the slope below by releasing trace metal ions with each rainfall, and they work for years passively. Two limits: protection weakens further down a long slope, so additional strips help, and they will not kill an established mature moss mat.
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