Moss, Streaks and Staining on a Metal Roof

Metal roofs stay cleaner than shingles, which is one of the reasons people choose them. They do not stay clean forever, particularly on shaded slopes and under trees.

Most of what appears is cosmetic. Some of it is worth acting on. The important part is that the wrong cleaning method causes more damage than anything it removes.

What you are actually looking at

• Organic growth. Moss, algae and lichen, on north-facing slopes and anywhere overhung by trees. Moss holds moisture against the surface, which is the reason it is worth removing rather than a purely visual concern.
• Streaking. Vertical marks running down the slope, usually airborne dirt washed into patterns by rain. Cosmetic.
• Debris in the valleys and behind penetrations. Needles and leaves collecting where water is meant to run. This one matters, because it dams water and holds it against the roof.
• Rust spots in isolated places. Different from the rest of the list. Usually a cut edge, a scratch, or a fastener rather than the coating failing generally.
• Chalking or fading. Gradual, even change across a whole slope. This is the coating aging, and it is what a coating warranty concerns itself with.

What actually matters

Rank them like this. Debris sitting in valleys and behind chimneys matters most, because it holds water where the roof needs to shed it. Moss matters next, for the same reason on a smaller scale. Rust spots are worth investigating, because they tell you something has broken the surface. Streaking and even fading are appearance.

Nothing on that list is an emergency. All of it is easier dealt with early.

How roofs get wrecked by cleaning

Here is the part worth reading twice.

Do not pressure wash a metal roof. High pressure can drive water up under laps and into places the roof was designed to shed it from, and it can damage the coating that is the entire basis of the roof's long life and its warranty. A coating warranted for decades can be undone in an afternoon.

Do not use harsh or unknown chemicals. Some cleaners are incompatible with coated steel finishes. Some will strip the finish. Others will run off onto your planting.

Do not scrape it. Wire brushes and metal scrapers create the scratches that later become the rust spots you were worried about.

Do not walk on it casually. Metal roofs are slippery, particularly when damp or mossy, and there are right and wrong places to put weight on a profiled panel. This is the most common way people get hurt on their own roof.

What good practice looks like

Gentle. Low pressure, appropriate cleaning agents for a coated steel finish, working down the slope so water goes where the roof expects it, rinsing thoroughly, and protecting whatever is planted below.

Before any of that, check what your coating warranty says about cleaning and about which products are acceptable. It is worth two minutes and it is exactly the situation warranties are written to exclude.

Prevention is mostly trees

Overhanging branches cause most of it: shade keeps the surface damp, and what falls off them collects. Cutting back what overhangs the roof does more than any cleaning schedule. Keeping the eavestroughs clear does the rest, because a blocked gutter keeps the eaves wet.

Highland cleans metal roofs, including roofs we did not install, and will tell you when a roof does not need it. If you are unsure what the marks on yours are, send us a photograph when you get in touch.