Mould vs Mildew vs Lichen: Telling Them Apart on Your Roof
Mould vs Mildew vs Lichen: Telling Them Apart on Your Roof
8 min read · 1,550 words
That dark streaky stuff on your roof — is it mould? Mildew? Lichen? Most homeowners aren't sure, and honestly, it's easy to mix them up. But it matters, because each one behaves differently, damages your roof in different ways, and needs a different approach to treat properly.
Up here in the Northern Rivers, we deal with all three regularly. The humidity rolling in off the coast, the warm wet summers, the tree canopy shade — it's basically a five-star resort for anything that grows on a roof. Across the 58 exterior cleans we've done for households from Tweed Heads to Evans Head, roof growth is one of the most common things we're called out for. So here's how to actually tell what you're looking at.
What Is Roof Mould?
Mould on a roof typically shows up as dark patches — black, dark green, or brownish — often spreading in irregular shapes. It tends to look soft and slightly fuzzy if you get close enough. Mould is a fungus, and it loves moisture. On roofs, it usually takes hold in areas that stay damp longest: under overhanging trees, on the southern-facing side (less direct sun), or near valleys where water pools.
In the Northern Rivers climate, mould can establish surprisingly fast — a wet La Niña summer can kick off a full outbreak within a few months on an unprotected roof.
Why it matters beyond looks:
- Mould holds moisture against your roofing material, accelerating deterioration
- On terracotta or concrete tiles, it can work into the surface and cause pitting over time
- Left long enough, it can get under ridge capping and into the roof cavity
Roof mould treatment typically involves a low-pressure biocide wash rather than blasting it with a pressure washer — high pressure on tiles can strip the surface coating and make regrowth worse.
What Is Mildew — And Is It Different?
Mildew is actually a specific type of mould, but it behaves differently enough that it's worth separating out. Where mould tends to be three-dimensional and fuzzy, mildew is flat and powdery. It often shows as white, grey, or light yellow patches. It spreads in a more uniform, sheet-like pattern rather than the irregular blotching of other moulds.
On roofs, mildew is less common than full mould growth, but it does appear — particularly on painted metal roofing (Colorbond-style) and on older fibro or asbestos-cement roofs (yes, there are still a few around, especially in older areas like Lismore and Mullumbimby).
Mildew is generally a bit easier to treat than deep-set mould, but don't ignore it. It's a sign your roof surface isn't drying out properly between rain events, which points to either a drainage issue, too much shade, or both.
If you're not sure whether you're dealing with mould or mildew, the texture is your best clue — mildew wipes off relatively easily with a damp cloth (not that you should be climbing up there to test it). Mould resists wiping and tends to stain the surface beneath it.
What Is Lichen — And Why Is It the Worst of the Three?
Lichen is in a different category altogether. It's not a fungus on its own — it's a symbiotic organism made up of fungi and algae living together. That combination makes it incredibly tough and slow-growing, but once it's established, it's genuinely difficult to remove.
On roofs, lichen looks like crusty, irregular patches — often grey-green, orange, or pale yellow. It has a rough, almost flaky texture and it's firmly attached to the surface. Unlike mould or mildew, you can't wash lichen off with a standard clean. It bonds to roofing material at a microscopic level.
Why lichen is the most damaging of the three:
- It physically etches into tile surfaces as it grows, causing surface degradation that doesn't reverse
- The root-like structures (called rhizines) penetrate into concrete and terracotta tiles, breaking down the material from within
- When lichen dies (either naturally or after treatment), it leaves behind a rough, pitted surface that's more vulnerable to future growth
- It's a slow process — lichen can take years to cause visible structural damage — but by the time you notice it's bad, there's already real deterioration underneath
Lichen is also the thing most likely to void a tile manufacturer's warranty if it's left untreated, something worth checking if your roof is less than 20 years old.
If you've got lichen patches and want to understand the broader maintenance picture, our pre-winter exterior audit checklist covers roof assessment alongside the rest of your property's exterior.
How to Tell Them Apart at a Glance
Standing on the ground with a decent pair of eyes (or binoculars), here's a rough guide:
- Dark, irregular, slightly fuzzy patches: probably mould
- Flat, powdery, lighter-coloured patches: probably mildew
- Crusty, rough, firmly attached patches in grey-green or orange: almost certainly lichen
- Dark streaking running down from ridgeline: often algae or early-stage mould spreading with rainwater
In practice, you'll often see all three on the same roof in different spots — shaded sections getting mould and lichen, sunnier sections getting mildew or algae streaking.
What Actually Works for Roof Mould Treatment (and the Others)
The right treatment depends on what you're dealing with:
- Mould and mildew: Low-pressure soft wash using a biocide solution. This kills the organism at the root rather than just shifting it surface-level. High pressure can damage tiles and actually spread spores.
- Lichen: Requires a specific treatment — a biocide applied and left to work over time (often weeks), followed by a careful low-pressure rinse once the lichen has died. Trying to scrub or blast live lichen usually makes it worse.
- Ongoing prevention: A post-treatment sealant or inhibitor slows regrowth significantly, especially important in the Northern Rivers where conditions for regrowth are almost always present.
What doesn't work: domestic bleach mixed in a garden sprayer. It'll kill surface growth temporarily but doesn't penetrate, and it can damage mortar between tiles and strip colour from Colorbond roofing. We see a fair bit of DIY bleach damage on roofs — sometimes worse than the original growth problem.
Worth reading if you're thinking about timing: why getting your roof sorted before winter matters, especially given how quickly growth takes hold after a wet season.
Ready to get a proper look at what's on your roof? Book a roof assessment with EcoClean and we'll tell you exactly what you're dealing with and the best way to treat it.
When Should You Actually Do Something About It?
The honest answer: earlier than you think. Most people wait until growth is visually obvious from the street, but by that point it's been establishing for months — sometimes years in the case of lichen.
A good rule of thumb for the Northern Rivers: inspect your roof (from the ground, safely) twice a year — once before the wet season (October-ish) and once after (around April-May). Catching growth early means a straightforward soft wash. Leaving it means more aggressive treatment, longer treatment time, and sometimes surface damage that can't be undone.
If your roof is under heavy tree canopy — common in areas like Brunswick Heads, Ocean Shores, and the Tweed hinterland — bump that up to quarterly checks. Shade and leaf debris create the perfect environment for all three types of growth to accelerate.
Got questions or want to talk through what you're seeing on your roof? Schedule a time with EcoClean here, or if it's easier to just ring it through: 0489 271 982 — happy to chat through what your place needs.
— Kolt @ EcoClean
FAQs
What is the difference between mould and lichen on a roof?
Mould is a fungus that appears as dark, fuzzy or patchy growth and sits on the roof surface. Lichen is a combined fungus-algae organism that physically bonds to and etches into roofing material, making it much harder to remove and more damaging over time. Mould responds to a standard biocide soft wash; lichen requires a specific treatment process and longer dwell time.
Can I treat roof mould myself with bleach?
DIY bleach treatments kill surface growth temporarily but don't penetrate deeply enough to eliminate the root of the problem. They can also damage tile mortar, strip colour from metal roofing, and spread spores if applied with high pressure. A professional low-pressure biocide wash is more effective and avoids surface damage.
How do I know if I have lichen or just dirt on my roof?
Lichen has a distinctly crusty, rough texture and is firmly attached to the surface — it won't wipe or rinse off easily. It often appears in irregular grey-green, orange, or pale yellow patches. Dirt and general grime tend to be more uniform and wash off readily. If patches have a rough, almost flaky appearance and have been there for a while, lichen is the likely culprit.
How often should I have my roof treated for mould and lichen in the Northern Rivers?
For most Northern Rivers homes, a professional roof clean every 2-3 years is a reasonable baseline. Homes under heavy tree canopy, on south-facing slopes, or in high-humidity areas closer to the coast may need attention more frequently — annual checks are sensible. Catching growth early means simpler, less costly treatment each time.
