Damp + Heat = Mould. Here’s how to fight and eliminate exterior mould

Mould mayhem, what actually works to eliminate it: our DIY tests, results & recommendations

When Cyclone Tam swept through the North Island over Easter, it didn’t just blow a few branches around — it left behind a hot, sticky mess. Literally.

The combination of heavy rain and follow-up humidity created perfect conditions for mould, mildew, and algae to thrive. And thrive it did —on decks, fences, outdoor furniture, concrete… anywhere it could get a foothold.

We started seeing it everywhere — and not just on client properties, but on our own. So, we decided to run a few tests at home (so you don’t have to), to find out what actually works to get rid of mould and spores.

This isn’t a sales pitch — just real-world feedback, a few hard-won lessons, and one solution that surprised us enough to share it.

Let’s start with what doesn’t work on getting rid of mould

1. Bleach

Yes, it’s everyone’s go-to. Yes, it looks like it works.

No, it doesn’t actually kill mould spores.

Bleach can clean surface mould — but it doesn’t penetrate into porous materials like timber or concrete. What’s worse, some mould species can metabolise bleach and use it as food, which makes the problem worse long term.

That means the deeper mould roots — called hyphae — survive untouched.

Worse still, when bleach hits the surface, it creates a sudden chemical shock. The mould senses this threat and often retreats further into the material to protect itself.

That’s not all — some mould species can actually metabolise bleach, using it as a food source to fuel future growth. 😬

So, while you might think you’ve killed the mould after a good scrub with bleach, you’ve really cleaned the visible part and possibly triggered a defensive growth spurt beneath the surface.

If you're scrubbing away and thinking you've won the battle, you're likely just pushing the spores further in and prepping them for acome back tour.

2. Vinegar

This one’s a bit more promising. But it has it’s limits.

White vinegar is a mild acid — and according to both AI and Google — it can kill around 80% of common mould species. Its power comes from the acetic acid, which gives it a low pH of about 2.5. This acidity disrupts the structure of mould, stopping it from growing or spreading. (Yup, AI and Google teamed up for that gem.)

Unlike bleach, vinegar has both antifungal and antibacterial properties, and it can soak into slightly porous surfaces like timber or outdoor fabric, making it more effective than bleach at reaching mould that’s sunk in beneath the surface. So it’s not just wiping the top — it’s helping prevent it from coming back.

It’s also non-toxic and kid- and pet-friendly, which makes it a great option if you’re cleaning in spaces where safety matters.

Here’s what happened when I put it to the test:

🪑 Fabric furniture: It took a bit of dabbing (okay, a lot of dabbing), and I still need to give it another go — I paused so I could get this written and out to hopefully help you and others.

✔️Outdoor composite furniture: The black mould on our wooden-look furniture pretty much vanished. (See pic below)

🪵 Rough-sawn timber fence: The vinegar helped loosen the black mould, but it wasn’t magic. I had to scrub, and I can still see traces of it. It would be VERY time-consuming to do the whole thing.

✖️Concrete & green algae: Total flop. It didn’t touch the green stuff. Lots of scrubbing, no satisfying results.

So, in short, vinegar is a great starting point, especially for smaller jobs and smoother surfaces.

But if you’re tackling hard landscaping — like decking, fencing, or concrete— or green algae in particular, you’ll need something a bit tougher. And something that does not involve you scrubbing for ages.

‍

3. A mould spray from the hardware store

The label promised big things — it claimed to kill, eliminate, clean mould and mildew and boasted that it works on wood, plasterboard, plastic, tiles, fabric, and more.

We tested it on both the composite wooden furniture and the fabric of our outdoor seating. We followed the instructions to the letter. When it didn’t work the first time, we gave it a second go — just to be sure.

The verdict? It did absolutely nothing. Not even a little improvement. Maybe there are others that do a better job? We did not want to keep spending money to find out.

We’re not naming the brand because this isn’t about calling anyone out — it’s about offering real, helpful solutions.

Also, just because this did not work does not mean that others won't. We did not want to test a multitude of store-bought products. Maybe someday we will try some more, and we can share which ones worked.

Which leads us nicely to the one we do recommend — and use ourselves:

4. The concentrated treatment we use professionally

This product kills mould, eliminates spores, and makes clean-up so much easier — especially on porous or textured surfaces like concrete, rough timber, or decking. And no scrubbing marathons required. 

We use it regularly in our soft washing services, and we trust it so much that we use it at home, too.

We don’t sell it.

We don’t get a kickback for sharing it.

We’re just passing it on because we know how frustrating mould problems can be— and this product actually works.

Want the name of the product?

If you’re not in Northland or Auckland (where we service),we’re more than happy to tell you exactly what we use so you can hunt it down locally.

Just send us a email and we’ll flick you the name of the product.

We’re not sharing it openly online — not because it’s a secret, but because we’re trying to avoid becoming the unofficial national distributor 😉

A few extra tips if you go the DIY route

- This product isn’t bleach — but we still recommend a spot test on any fabric, just to be safe.

When we’re using it on client jobs, we only apply it to shade sails and umbrellas — not cushions or other outdoor fabrics — simply because different materials and dyes can react in unpredictable ways.

If you’re thinking of trying it on your outdoor furniture fabric, just proceed with caution and always test a small hidden patch first.

- The smallest container is 5L (it’s concentrated), so it’s perfect for sharing with neighbours or friends.

Bonus reminder: check your gutters

One of the sneakiest ways mould can enter your home is through water damage from overflowing gutters. If your gutters are blocked, all that rainwater from the next storm won’t drain properly — it could sneak into wall cavities instead. We have seen it happen countless times.
That’s where the real damage starts: rot, internal mould, and long-term repairs that cost way more than a simple gutter clean.

If you’re in our region (northland and Auckland and want us to take a look, just reach out. If not — make sure you get someone reliable to clear them before the next big downpour.

Final thoughts

After the cyclone, we felt that sharing this was one of those “right things to do” moments.

Mould is frustrating, damaging, and stressful. The weather has made everything harder. The last thing anyone needs is to waste money on ineffective products or scrub the same fence for two hours only to see the mould grow back.

So — if this helps even one person get a better result, we’re happy.

And if you're in Northland or Auckland and want a hand? We’d love to help.

Stay safe, stay dry, and give that mould the boot.

‍