Moss and algae on patios, walkways and roofs: what removes it and what brings it back

Moss and algae on patios and roofs grow for the same reason, but they do not do the same damage, and the fix that is right for one surface ruins another. A pressure washer is a fair tool on a concrete walkway and the one thing the asphalt roofing industry tells you never to point at a shingle. A moss killer that works on the patio may not be legal there in Ontario. And the black streaks on the roof, the most visible of the three, are according to the shingle makers the least harmful. This guide sorts out which growth you have, what removes it, and what has to change so it does not come back by next spring.

Is it moss, algae or lichen, and does the difference matter?

It matters mostly on the roof. The Asphalt Roofing Manufacturers Association (ARMA) states that there is no scientific evidence that algae damages asphalt shingles, while moss can lift or curl the leading edges of shingles, raising the risk of blow-off in wind and, in severe cases, moving water sideways into the roof deck. On paving, the Royal Horticultural Society's advice is that algae, lichens, liverworts and mosses do not damage the surface they grow on; the problem there is that they make it slippery.

Why does it grow on the north side and under the trees?

Because those are the surfaces that stay wet longest. ARMA explains that moss takes in water through its leaves rather than roots, so it needs a moist surface to survive, and that in North America it tends to grow on north-facing roof planes, which get less direct sun and stay damp longer than south-facing ones. Overhanging branches add shade and drop debris that holds moisture and feeds the moss. The RHS makes the same point for paving: algae grow faster in shady areas with poor air circulation, and mosses favour damp, shady conditions.

Treat green growth near the house as a drainage clue as well as a cleaning job. The City of Mississauga lists green moss growing on your concrete foundation among its drainage warning signs, alongside patios sloped toward the house and water lingering more than a day next to the foundation. If the moss is on the foundation wall or on the slab right beside it, find out where that water is coming from before you clean anything — the downspouts are the first place to look, and our post on how often eavestroughs need cleaning under mature trees covers why they overflow.

How do you get moss off a patio or walkway without chemicals?

With a stiff broom and a narrow tool for the joints, which is the method the RHS puts first. It also notes that as non-chemical methods are effective, there is no need for moss killers or chemical patio cleaners, and that whatever you use, the growth returns if the conditions stay the same. It is a job a homeowner can do without buying anything.

1. 1Brush the whole surface with a stiff broomA stiff-bristle broom removes established film and cushions on concrete, pavers and flagstone. The RHS suggests a wire brush for stubborn growth; test it on a hidden corner first on anything with a sealed, coloured or textured finish.
2. 2Pick the joints cleanA pointed weeding tool or an old knife lifts moss out of the gaps between slabs. Sweep the debris up rather than into the lawn or the flower bed next to it.
3. 3Decide whether you need water at allThe RHS says a pressure washer removes growth quickly but uses a lot of water for a temporary result, can make damp problems worse where drainage is already poor, and calls for goggles.
4. 4If you do wash it, control where the water goesWash water carrying dirt, organic matter or any cleaning product must not run into a catch basin or the street. Our guide to pressure washing a driveway or patio sets out what Halton, Peel and Toronto allow.
5. 5Keep brushing where people walkThe RHS notes that brushing on a regular basis stops growth from taking hold. Front steps and the path to the door are the surfaces worth doing every season; a shaded corner of the patio nobody uses can be left alone.

Which moss killers are legal on an Ontario patio or driveway?

Only products whose active ingredients are all on Ontario's Allowable List. Ontario's cosmetic pesticide ban covers lawns, vegetable and ornamental gardens, patios and driveways, and the Ministry of the Environment, Conservation and Parks states that you may only use pesticides that solely contain active ingredients found on the List of Active Ingredients Authorized for Cosmetic Uses. A product can be federally registered, sold in Canada and still not be usable on your patio in Ontario.

The list is published as open data by the province. Ingredients on it include:

• Acetic acid (the active ingredient in vinegar-based herbicides).
• Ammonium soaps of fatty acids. Health Canada describes ammonium soap of fatty acids as a domestic herbicide for the control of weeds, algae and moss in areas including vegetable and flower gardens, driveways, patios and gravel.
• Soap (potassium salts of fatty acid) and the other fatty-acid soaps listed.
• Iron (ferrous or ferric) sulfate and the other iron compounds on the list.
• Several copper compounds.

To check a product in the store, turn it over and find two things. First, the P.C.P. registration number, which Health Canada lists among the things a registered pesticide's label must carry. Second, the active ingredient line, which must match the Allowable List in full: one extra ingredient that is not on it and the product is not allowed on a patio in Ontario. Then check that the label names the use you have in mind, because Health Canada's rule is to use a pesticide only for the purpose stated on the label.

Health Canada offers one non-product option for the joints: pouring boiling water on weeds growing between patio stones. It also cautions that homemade pesticide recipes are not scientifically evaluated and carry no label directions, which is a reason to stay with the broom or a registered product rather than a kitchen mixture.

How is moss removed from a shingle roof without damaging it?

Without a pressure washer and without a brush. ARMA's guidance is blunt: never use a pressure washer on an asphalt shingle roof, because it causes granule loss and very likely premature failure of the roof, and do not use any type of brush or broom either, since the friction can loosen granules. The method it describes instead is chemical and slow.

1. ARMA recommends a 50:50 mix of laundry-strength liquid chlorine bleach and water, applied with a sprayer.
2. It is left on the roof for 15 to 20 minutes and not allowed to dry completely, because a dried solution may not rinse off fully.
3. The roof is then rinsed thoroughly with low-pressure water.
4. Algae disappear and wash away with later rain. Moss loosens over time and can then be removed with a leaf blower; heavy moss may need more than one treatment.
5. Landscaping and surrounding areas are protected from the bleach, and the person applying it wears eye protection and rubber gloves.

ARMA also advises working from a ladder or walkboards where possible, to avoid standing on the shingles at all. That is the honest line between a homeowner job and a hired one: a single-storey garage roof reachable from a properly set ladder is one thing, a two-storey slope is another. Our post on ladder work on a two-storey house covers where that line sits. Two further cautions from Health Canada apply to anyone using bleach: never mix it with other cleaning products, including anything containing ammonia or acids, even vinegar, because toxic gases such as chloramine can result.

Remember where the rinse goes. Everything that comes off the roof runs through the eavestroughs and out of the downspouts, so the same storm-sewer rules apply to a roof wash as to a driveway wash. Check where each downspout discharges before you start: that water should not reach the driveway or a catch basin, and ARMA's caution about protecting landscaping from the bleach applies at the downspout outlets too.

What stops it from coming back?

Changing the shade, debris and standing water that let it grow. Both ARMA and the RHS say the growth returns if conditions stay the same, so cleaning on its own is a temporary result.

• Trim branches back from the roof so more sun reaches it and less debris lands on it (ARMA). Before cutting, check whose tree it is: a street tree belongs to the municipality, and our post on municipal and private tree rules covers permit thresholds. Oaks have their own timing, explained in our oak wilt pruning window post.
• Clear roof debris regularly with a leaf blower or another non-abrasive method, blowing down the slope so debris is not driven under the shingle edges (ARMA).
• Keep the eavestroughs clean, and do not let an upper roof's downspout drain straight onto a lower roof; extend it into the lower gutter instead (ARMA).
• Make paving shed water. The RHS recommends a slight slope so water does not stand, shallow gravel-filled channels along the edges of paths and patios to take run-off, and keeping drains clear of leaves.
• Prune overhanging shrubs beside walkways to let sun and wind dry the surface (RHS).
• Rake gravel paths regularly, which the RHS notes stops algae and moss establishing on loose surfaces.

Late September is a sensible time to do the debris part, before the leaves come down. ARMA's point is that debris on a roof holds moisture and feeds moss, so a roof that goes into winter under a layer of wet leaves is the setup it describes. The RHS adds that algae build-ups often occur after wet winters.

When is it fine to leave it alone?

On any surface nobody walks on, and, for algae streaks on the roof, whenever the look does not bother you. The RHS encourages leaving these growths where they create no hazard, such as on stone features and walls, and notes they provide habitat and food for small creatures and nesting material for birds. The cases worth acting on are narrower than the whole yard:

• Act: steps, the front walk, the path to the side door, a deck used through the winter — anywhere someone walks.
• Act: moss cushions on shingles, and moss or debris sitting along the roof's lower edge and gutters.
• Optional: black algae streaks on the roof. ARMA treats them as an appearance issue, though it notes they reduce the performance of highly reflective "cool" roofs.
• Leave: lichen on a garden wall, moss on a boulder or planter, green on a patio corner nobody steps on.

If the roof is the part you want handled, the gutter cleaning and repairs service includes a roof wash and moss treatment option, done as a soft wash or low-pressure rinse where suitable and aimed at keeping the gutters cleaner for longer. Tell us which slopes are affected and whether branches overhang them when you ask for a quote.