Moss on a Tacoma roof: remove it, treat it or replace the roof

Why moss grows so well on Tacoma roofs

Moss needs water and shade, and Tacoma supplies both. The city's own climate table shows roughly 40.81 inches of precipitation spread across about 155 days a year, so roofs stay damp for months at a time. A shaded north slope may barely dry out between November and April.

Trees add the shade. The City of Tacoma measured tree canopy at about 20% of its land area in 2017, but coverage ranged from 3% to 64% across census blocks. A house in a leafy block can carry a roof that never sees full sun, while one a few streets away stays clean for years.

Oregon State University Extension notes that roof moss follows shade, which is why it collects on north faces, under overhanging limbs and in the strip below a taller roof plane.

What moss actually does to shingles

Moss is not just a cosmetic problem. OSU Extension says it holds moisture against the roof and can lift shingles. A thick mat acts like a sponge, keeping the shingle surface wet long after rain stops, and its roots work into the gaps between tabs.

Once a shingle edge lifts, wind can get under it and rain can run backward under the course above. Fir needles and debris caught in the moss make the mat thicker and dam water in valleys. Over a few seasons the granules loosen and wash into the gutters, and the bare asphalt underneath ages faster.

Why we do not pressure wash a mossy roof

A pressure washer clears moss in minutes, which is why it is tempting. A Seattle Public Utilities moss fact sheet warns that pressure washing can force water under shingles or damage them. The spray strips granules, breaks the seal strips that hold tabs down, and drives water up under the courses where it soaks the deck.

We see the results later as leaks and bald patches that were never there while the moss was. On cedar shakes the damage is worse, because high pressure tears the soft wood fibers apart.

Remove: brushing it off by hand

The gentler method works better. We brush moss off from the top of the roof downward with a stiff brush or a plastic scraper, so we move with the shingle laps instead of against them. Thick clumps get loosened by hand. The roof should be dry enough to walk safely, and steep roofs need harness anchors.

Timing matters. The SPU fact sheet notes moss revives with the rains and is most visible from early winter to late spring. That is when you notice it, but removal on a dry day in late spring or early fall, followed by treatment, gives the roof the longest clean stretch.

After brushing we clear the gutters and downspouts, because the debris has to go somewhere and it usually goes there.

Treat: zinc strips, sprays and copper gutters

Treatment slows regrowth; it does not replace removal. Zinc or copper strips installed near the ridge release metal ions with each rain, which discourages moss on the slope below. They work best on the upper part of the roof and lose effect further down, and they do little where moss is already thick.

Check your gutters before choosing a treatment. OSU Extension warns that zinc treatments corrode copper gutters, so a house with copper gutters or downspouts needs a different product. Zinc runoff also ends up in the soil below the downspouts, which is worth thinking about near vegetable beds.

The cost box shows a published national range for cleaning and prevention treatment. Roof size, pitch and how thick the moss is set where your roof falls in it.

When moss means the roof needs replacing

Sometimes the moss has done its damage already. After cleaning, look at the shingles from a ladder or ask us to. These signs point toward replacement rather than another round of cleaning:

• Shingle tabs curled, cupped or lifted across large areas
• Bare asphalt where granules have washed away
• Soft or spongy decking underfoot in shaded sections
• Leaks or attic staining under the mossiest slope
• A roof already near the end of its expected life

Trees and shade: the long-term fix

OSU Extension calls trimming trees the long-term fix, because removing shade removes the conditions moss needs. Thinning limbs that hang over the roof lets sun and wind dry the surface after rain. An arborist can do that without harming the tree.

Trees are not going anywhere in Tacoma. The city's urban forest plan targets 30% canopy by 2030, and residential property holds 44% of the current canopy. So for most owners the realistic plan is a mix: thin the limbs over the roof, clean on a schedule, and treat between cleanings. Our roof maintenance visits fold that into one routine.