Every spring in Simcoe County, homeowners step outside after a long Ontario winter, look up at their rooftop, and may notice something that was not there in the fall. A green cushion on the ridge. Dark streaks down the shingles. A flat, crusty patch that looks painted on. Or a blotchy, dark mass near the soffit that does not look like anything familiar.
The phone comes out. Six browser tabs open. None agrees on whether it is moss, mould, algae, or lichen. Most of the results are written for homeowners in Florida or the Pacific Northwest, and none of them speak to Ontario weather specifically.
This guide fixes that. After years of assessing roofs across Barrie, Innisfil, Orillia, and the surrounding County, JN Roofing has seen every version of roof growth that local weather produces. Below is a plain-language identification guide for all four common types and what each one means for your home.
Why Do Simcoe County Roofs Get So Much Roof Growth?
Local humidity and shaded roof conditions are the main reasons moss, algae, and lichen show up on roofs in this area.
Simcoe County sits between Lake Simcoe and Georgian Bay. In our experience, roofs in this area tend to stay wet for longer periods than roofs further inland, especially in the shoulder seasons when temperatures swing above and below freezing on the same day. That extended dampness is exactly what organic growth needs to take hold of asphalt shingles.
Many Simcoe County homeowners notice a clear surge in visible growth every April and May. Spores that sat dormant through winter activate the moment air temperature and ground moisture rise together. A patch that was barely visible in March can look fully established by mid-May.
Certain conditions accelerate this considerably: overhanging tree branches that prevent the roof from drying out, low-slope roof planes that drain slowly, poor attic ventilation (which keeps the underside of the roof deck damp), and existing shingle damage that holds moisture more readily. When JN Roofing assesses a roof showing growth, we check for all four of these conditions. Treating the growth without addressing what is causing it means the problem returns the following spring.
What Does Moss Look Like on a Roof?
Moss is the easiest of the four growth types to identify. It forms a raised, cushion-like layer that is visibly three-dimensional from ground level.
The colour ranges from bright green in wet conditions to grey-green when it dries out. It almost always shows up first along the roof ridge and on north-facing slopes where sunlight is limited, then spreads downward toward the eaves.
Moss grows by embedding fine root-like structures into the shingle granule layer. These hold moisture against the shingle surface continuously, and over time, that retained moisture lifts shingle edges and creates the conditions for freeze-thaw damage during Ontario winters.
On the north slope of Simcoe County homes, moss is one of the most consistent findings on our spring assessments. It often surprises homeowners how quickly it builds up from the small patch they noticed the previous autumn.
What Do Algae Streaks on a Roof Look Like?
Algae appears as dark grey or black vertical streaks running down the roof plane from the ridge toward the eaves.
This is the most commonly misidentified roof growth. The streaks look serious and are often mistaken for mould or for staining caused by a leak. In reality, algae (specifically a cyanobacterium called Gloeocapsa magma) feed on the limestone filler in asphalt shingles. Rainwater carries the colony progressively downward, which is why the pattern is almost always vertical streaks rather than patches.
Algae is not penetrating the shingle, and it is not evidence of a moisture problem inside the roof. It is primarily cosmetic in its early stages, though over several years, it does accelerate shingle wear by retaining heat and contributing to granule loss. It is also self-seeding; left alone, a streak on one section of the roof can spread across the entire slope over two or three seasons.
What Is Lichen on a Roof?
Lichen appears as flat, circular, grey or greenish-grey crusty patches that look almost fused to the shingle surface.
Unlike moss, which sits on top of the shingle, lichen embeds root-like structures directly into the shingle granule layer and, over time, into the asphalt itself. Those embedded structures physically pull granules loose when the lichen is disturbed.
Lichen is the most tenacious and structurally damaging of the four common roof growths. It grows slowly but persistently, and it is the growth type where professional assessment is most strongly recommended before any treatment is attempted. Do not attempt to pressure-wash lichen off your roof; the force required to dislodge it will strip granules at the same time, accelerating the damage you are trying to prevent.
What Does Mould on a Roof Look Like?
Mould on a roof typically appears as dark greenish-black blotchy patches without the clean streak pattern of algae. It concentrates near areas of persistent moisture, roof valleys, flashings, and soffits.
This is the growth type where professional assessment matters most. Unlike moss, algae, and lichen, which primarily affect the shingle surface, mould is often a symptom of moisture that has already worked its way below the surface into the roof deck or attic. If it has, the problem is no longer cosmetic; it can degrade the decking material and affect indoor air quality if spores enter the living area through ventilation gaps.
If you suspect mould rather than moss or algae, the most important thing is to get a professional set of eyes on both the roof and the attic before anything is cleaned or treated. JN Roofing assessments cover both.
Moss vs. Algae vs. Lichen vs. Mould: Quick Comparison
| Factor | Moss | Algae | Lichen | Mould |
| Appearance | Raised green or grey-green cushion | Dark grey or black vertical streaks | Flat, circular, crusty patches | Dark blotchy patches, no clean edge |
| Texture | Soft and raised | Flat and smooth | Hard and embedded | Soft and irregular |
| Where it appears | Ridge and north-facing slopes | Running down the roof plane | Any surface; worst in shade | Near valleys, flashings, soffits |
| Urgency | Moderate – treat within the season | Low to moderate – cosmetic at first | High – causes ongoing shingle damage | High – may indicate deeper moisture |
| Safe to leave untreated? | No, spreads and lifts shingles | Short-term yes, long-term no | No, damage to shingles over time | No, needs investigation |
| Recommended next step | Book a professional assessment | Book a professional assessment | Book a professional assessment | Book a professional assessment |
Should I Remove Roof Growth Myself, or Call a Professional?
For most Simcoe County homeowners, a professional assessment is the safer first step, even when the growth looks like a simple moss or algae case.
There are two reasons. First, misidentification is common – a patch that looks like early-stage algae can be mould that has penetrated the deck, and treating the surface without addressing what is underneath makes the underlying problem worse. Second, most of the conditions that cause roof growth (tree shade, attic ventilation issues, low-slope drainage) are not things a DIY surface clean addresses. Treating the growth without treating the cause means it comes back the following spring.
Professional roof cleaning can be reasonable in a narrow set of conditions: early-stage moss or algae only, a single-storey accessible roof with no visible shingle damage, and no sign that the growth is concentrated near valleys, flashings, or soffits. In those cases, a specialist roof cleaning company will use a purpose-made cleaning solution formulated for asphalt shingles, applied with low-pressure equipment and worked from the ridge downward. Pressure washers are not recommended on asphalt shingles at any setting, as the force strips granules, shortens the life of the roof, and can void the manufacturer’s warranty. Roof cleaning is a specialized service and not something JN Roofing offers directly. If your assessment shows your roof is a good candidate for cleaning rather than repair, we’ll tell you honestly and can point you toward reputable cleaning specialists in the area.
If the growth is lichen, if it looks like mould, if it has been present for more than one season, or if your roof is two storeys or steep-pitched, the answer is a professional assessment. Book one before any cleaning begins.
What to Ask Before You Hire Someone to Inspect Roof Growth
A few questions will quickly tell you whether a contractor knows what they are doing:
- Do you assess the attic as part of a roof growth inspection, or only the exterior shingles?
- Will you identify the specific growth type and explain what is causing it?
- Do you provide a written summary of findings?
A contractor who cannot answer these clearly is not the right fit. At JN Roofing, our assessments cover the exterior shingles and the attic space, identify the growth type, and give you a plain-language explanation of what we found and what we recommend.
Not Sure What Is Growing on Your Roof?
No article can tell you with certainty what is on your specific roof. The identification guide above gives you a starting point, but a professional set of eyes on the shingles and in the attic is the only way to know for sure.
JN Roofing has been assessing and repairing roofs across Barrie, Innisfil, Orillia, and Simcoe County for years. When you book a free assessment with us, we check the exterior surface, inspect the attic, identify the growth type, and give you a straight recommendation for next steps. No obligation, no pressure.
Spring is the best time to catch these problems. A growth treated in May is a much simpler job than the same growth in October.
Book a Free Roof Assessment: jnroofing.ca/contact/
Frequently Asked Questions
What is the difference between moss and mould on a roof?
Moss is a raised green or grey-green cushion that builds up on the shingle surface, usually starting at the ridge. Mould is dark greenish-black blotchy patches that concentrate near valleys, flashings, or soffits and often indicates moisture has worked below the surface. Moss is a surface issue you can catch early. Mould is a warning sign that warrants a professional assessment.
Is moss on my roof dangerous?
Moss is not an immediate roofing emergency, but it does cause real long-term damage. It retains moisture against the shingle surface, lifts shingle edges, and accelerates granule loss. In Simcoe County, freeze-thaw cycles drive that moisture into shingle cracks every winter, which shortens the life of an asphalt roof faster than most homeowners expect.
How do I know if what is on my roof is moss or algae?
Moss is raised and three-dimensional with a cushion-like texture you can see from the driveway, and it is usually green or grey-green. Algae is flat dark streaks running vertically down the roof with no raised texture. If you can see visible thickness, it is moss. If it looks like dark painted streaks, it is algae.
Can mould on a roof affect indoor air quality?
Yes, if the mould has reached the roof deck or attic space. Spores can enter the living area through ventilation gaps or unsealed penetrations in the ceiling. Surface-only mould on the exterior shingles is less likely to directly affect indoor air, but dark blotchy patches near soffits often mean that the boundary has already been crossed. A professional assessment of both the roof and attic is the only reliable way to know.
Can I remove moss from my roof myself?
Roof work carries real safety risk and asphalt shingles are easily damaged by the wrong cleaning method, so most homeowners are better off leaving moss removal to a specialist. For early-stage moss on a small, accessible roof, a professional cleaning company can apply a purpose-made low-pressure treatment safely and without voiding the shingle warranty. For anything else, lichen, suspected mould, multi-season growth, or a two-storey roof, a professional assessment is the right first step to confirm what you’re actually dealing with. Pressure washers are not recommended on asphalt shingles at any setting.