Keeping moss off with zinc strips in Vancouver, WA

Zinc strips are thin bands of zinc or zinc-coated metal installed near the ridge of a roof. When rain runs over them, it carries a small amount of zinc down the slope, which makes the roof surface less friendly to moss and algae. They do not remove existing moss, and they are not a cure on their own, but on a clean roof they slow regrowth noticeably, especially on the slopes nearest the strip.

Moss prevention is more than metal, though. Shade, debris and dampness are what let moss take hold. Our crews install zinc strips along the right lines, and we also look at trimming back overhanging limbs, keeping valleys and gutters clear, and applying periodic treatments. Combined, those steps can stretch the time between full moss cleanings on a shaded Clark County roof.

How zinc strips work on a roof

Rainwater is slightly acidic, and as it runs over a zinc strip it dissolves a tiny amount of the metal. That zinc-laden water flows down the roof below the strip, coating the surface in a way moss and algae do not like. The effect is strongest just below the strip and weakens with distance. On a short slope, one strip near the ridge may protect most of the roof. On long slopes, a second row lower down helps cover the bottom courses.

Zinc does not kill thick moss that is already growing. Old moss shields the roof surface and keeps the treated water from reaching it. That is why we install zinc on clean roofs, ideally right after a moss removal. Zinc also wears slowly over the years as it dissolves, so strips eventually thin out. We check them during later visits and replace them when they have lost much of their metal.

What our moss prevention visit includes

We plan zinc placement slope by slope. North and east slopes under trees get priority, along with any slopes below valleys and dormers where debris gathers. Strips are fastened under the shingle course near the ridge with sealed nails, so only a narrow band of metal shows. On some roofs we use a zinc-coated ridge cap instead, which hides the strip in the ridge line and suits homes where looks matter most.

• Zinc or zinc-coated strips near each ridge
• Second row on long or heavily shaded slopes
• Sealed fasteners hidden under shingle laps
• Limb trimming advice for roof shade and debris
• Gutter and valley clearing to keep the roof drier
• Follow-up treatment on the worst slopes

Other ways to slow moss growth

The single biggest factor in roof moss is how long the roof stays wet. Overhanging limbs from firs and maples keep the roof shaded and drop needles that hold moisture. Trimming them back so sun and air can reach the roof often makes a bigger difference than any product. We point out which limbs are causing the most shade and debris, and you can decide whether to have an arborist trim them.

Clean valleys and gutters matter too. Debris dams slow water and give moss a place to start. A light moss treatment applied in early fall, before the long rains, also helps on the worst slopes. Some newer shingles include algae-resistant granules with copper, which slow black streaks. When a roof is due for replacement anyway, choosing those shingles is another way to reduce growth over time, along with lighter colors.

What zinc strips cannot do

It is worth being clear about the limits. Zinc strips will not clean an already mossy roof, and they will not stop growth completely on a roof buried in deep shade with no sun. Their effect fades toward the bottom of long slopes, and runoff from the strip can leave faint lighter marks on some shingles. Zinc runoff into ponds or gardens directly below can be a concern, so we discuss placement if you have either.