Does Moss Damage Your Roof? Yes — Here's the Mechanism

In most of the country, moss on a roof is a curiosity. West of the Cascades it is the default condition of any shaded slope that nobody maintains. It is also one of the most preventable causes of roof damage we deal with.

Two ways moss shortens a roof's life

First, moisture. Moss acts like a sponge sitting directly on the roof surface, keeping shingles and the substrate below damp long after the rain stops. Sustained moisture promotes rot.

Second, lifting. As growth thickens it wedges into the gaps between shingle courses and works the edges upward. A lifted shingle is no longer a shingle doing its job — it is an opening pointed uphill, exactly where wind-driven rain will find it.

There is a third, less obvious cost: moss and the debris it traps wash into your gutters, clog downspouts, and back water up under the lower courses of roofing.

Why the pressure washer is a mistake

It is tempting, and it is the single most common way homeowners create a leak. Pressure stripping asphalt shingles blasts off the protective granule layer that shields the asphalt from UV, and it can drive water up under shingle courses. You trade moss for accelerated aging.

Moss should be brushed off, the debris cleared, and the surface treated with a roof-friendly solution to inhibit regrowth. That is the process we use.

Keeping it from coming back

• Keep the roof and gutters clear of needles and leaves — debris is where moss establishes first
• Trim branches back so shaded slopes get light and air and dry faster after rain
• Have growth removed while it is still thin, before it lifts shingle edges
• Plan on a periodic maintenance visit; in this climate no roof stays moss-free on its own

If your roof already has green on it, the question is not whether to deal with it but whether you deal with it before or after it lifts a shingle.