Trees, Moss and Algae: What Shade Does to a Roof
Shade and retained moisture are what biological growth needs. Understanding the difference between algae, moss and lichen tells you what is happening on your roof and how serious it is.
A mature treed lot is one of the nicest things about an older Ontario neighbourhood and one of the harder environments to keep a roof in. The trees themselves are only part of it. What they really do is create shade, and shade means a surface stays damp for hours or days longer after every rain, dew or snowmelt than the sunlit slope on the other side of the ridge. Prolonged dampness is what biological growth needs, and prolonged dampness is what most roofing materials handle worst.
Three Different Organisms, Three Different Problems
People use moss as a catch-all for anything green on a roof, but the three common growths behave differently and matter to different degrees.
- ✓Algae. The black streaks running downslope on asphalt roofs are a cyanobacteria colony. It needs moisture and very little light, and on many shingles it can make use of the mineral filler in the product. It is largely a cosmetic problem in itself, though the dark colour raises the surface temperature slightly and it indicates a surface that stays damp.
- ✓Moss. A true plant, with root-like rhizoids that anchor into any texture the surface offers. Moss holds water like a sponge and keeps it against the roof indefinitely, and as a clump thickens it physically lifts the edges of shingles and works its way into keyways and laps. Moss is the one that causes real damage.
- ✓Lichen. A partnership between a fungus and an alga, growing as flat crusty patches. It bonds tenaciously and can etch into the surface it grows on. Removing it often removes some of the surface with it, which is why lichen on an asphalt roof is usually a sign that the roof is late in its life.
Why the North Slope and the Shaded Corner
Growth follows moisture, and moisture follows shade. A north-facing slope in Southern Ontario receives little direct sun through the winter and only glancing sun in summer, so it dries slowly after every wetting event. Add an overhanging tree, a neighbouring building, a tall hedge or a dormer that casts a shadow, and you get a specific area that is essentially never dry. That is where the growth appears, and it is why the pattern on a roof is usually so sharply defined: heavy on one plane, nonexistent on the plane facing the other way, and a distinct patch under the one big maple.
What Moss Does to a Shingle Roof
Asphalt shingles are a layered, water-shedding system that depends on each course overlapping the one below and on water running off quickly. Moss defeats both. Growing in and under the laps, it lifts the shingle edges and creates a path for water to travel sideways and upward by capillary action rather than straight down and off. It holds a reservoir of water against the surface, extending the wet time enormously and driving freeze-thaw damage on the shingle edges through the winter. And when it is removed, it takes granules with it, because the rhizoids grip the granule layer. A shingle roof carrying established moss is being damaged continuously, and the longer it stays the less can be salvaged.
Why Metal Is Much Less Hospitable
A metal panel is a smooth, hard, non-porous surface with a cured coating on it. There is nothing for rhizoids to grip, no granule layer, no mineral filler to feed on, and no keyway or lap for a plant to establish in. It also dries far faster than a textured surface, because water sheets off rather than being held in a rough matrix. In deep, permanent shade under heavy tree cover you can still get algae film or growth taking hold in accumulated debris — the growth is living in the dirt on the roof rather than in the roof — but it is a fraction of the problem it is on a shingle roof, and it does not damage the panel the way moss damages shingles.
The Real Issue Under Trees Is Debris
For a metal roof under mature trees, the maintenance question is not growth. It is organic material collecting where the roof cannot shed it.
- ✓Valleys. This is where debris from two planes converges and where a wet mat of leaves and needles can build up, hold moisture indefinitely, and back water up under panel edges during heavy rain.
- ✓Behind snow guards and any roof-mounted equipment, which act as small dams that collect exactly what they are designed to collect.
- ✓Eavestroughs and downspout outlets, which under a mature canopy will block every year without fail, and often twice a year.
- ✓Low-slope sections, dormer roofs and any transition where water slows down and drops its load.
- ✓Behind chimneys and on the upslope side of any penetration, where the debris pile is out of sight from the ground.
- ✓Conifer needles specifically, which are small enough to pack densely, slow to break down, mildly acidic as they decay, and very good at holding a layer of wet grit against a finish.
Abrasion, Limbs and Other Physical Damage
Branches that touch the roof are a slow, mechanical problem. Every windy day they sweep across the surface, and over years that abrades a coating. On a shingle roof the same motion strips granules. Beyond abrasion, overhanging limbs deliver everything the tree produces directly onto the roof: sap, seeds, berries, and the bird and squirrel traffic that comes with a highway into the attic area. Dead limbs above a roof are a genuine hazard in a wind event, and a limb heavy with wet snow can come down on a roof it would clear in summer. Trimming so that nothing overhangs and nothing touches solves several problems at once and is usually the single most effective thing a homeowner on a treed lot can do.
There is a reason growth is often absent in a clean stripe below a metal ridge cap, a chimney flashing or a vent. Rainwater running over zinc or copper picks up trace metal ions that suppress biological growth downslope. It is a real effect, and it is why zinc strips are sold for shingle roofs — though on a badly shaded roof they only treat the area directly below them.
Cleaning Without Wrecking the Roof
More roofs are damaged by enthusiastic cleaning than by the growth being cleaned. A few rules hold across every surface.
- ✓Never pressure wash an asphalt roof. It removes granules wholesale and drives water up under the courses. Soft washing with an appropriate solution and low pressure is the accepted method.
- ✓Do not use wire brushes, abrasive pads or scrapers on a coated metal panel. Once the coating is scratched to substrate you have converted a cosmetic issue into a corrosion issue.
- ✓Check any cleaning product against the coating manufacturer's guidance before it goes on a metal roof. Strong alkalis and some degreasers will dull or damage a finish.
- ✓Work downslope, and rinse thoroughly, so that nothing is driven up under laps and nothing is left to concentrate as it dries.
- ✓Protect plantings below. Runoff from roof cleaning is hard on gardens and lawns, and worth pre-wetting and rinsing for.
- ✓Do not walk on a wet, mossy or snow-covered roof, and be very careful on metal in any condition. Where possible, work from a ladder or hire it out.
- ✓Clear the eavestroughs at the same time, since everything you remove from the roof ends up in them.
Frequently Asked Questions
Do metal roofs grow moss?
Very rarely in any meaningful way. There is no porous surface or granule layer for moss to anchor into and the panel dries quickly. Where you do see growth on a metal roof it is almost always living in accumulated debris on the surface rather than in the roof itself, which means the fix is cleaning the debris off rather than treating the roof.
How much clearance should I keep between trees and the roof?
Enough that no branch can touch the roof in wind, that the canopy is not directly over it, and that dead wood is removed from anything within falling distance. Beyond the debris and abrasion, that clearance also lets the roof see sun and wind, which is what keeps it dry.
Are the black streaks on my shingles damaging the roof?
Algae staining itself is mostly cosmetic. What it tells you is more useful than what it does: that slope stays damp, and dampness is what shortens the life of an asphalt roof. If moss is present as well, that is a different matter and worth acting on.
Will a zinc strip fix my moss problem?
It will help the area directly downslope of it and it will not do much elsewhere, so a single strip at the ridge of a badly shaded roof is a partial measure at best. It also does not remove what is already growing. Reducing shade and clearing debris address the cause; the strip addresses a symptom in one band.
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