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Why Are My Patio Slabs Turning Green?

Why Are My Patio Slabs Turning Green?

You’re here because it’s time to clean the green stuff on your patio slabs again, right? Well, you’re not alone.

Many homeowners are in a constant battle with them, and a common question that they always ask is why are their patio slabs turning green.

Patio slabs turn green because of algae or mold that grows abundantly in damp places that don’t get a lot of sunlight. Weather, location, and surface are factors that affect their growth. Unfortunately, if you already have them on your patio slabs, it’ll be a constant battle to get rid of them.

In this article, we’ll talk about the reasons why your concrete slabs are turning green and how you can slow down their growth.

But since it’ll be a recurring issue, we’ll also provide you with some tips that you can use to get rid of them effectively. 

What’s the Green Stuff on My Patio Slabs?

Many people ask this question, and let me tell you that millions of homeowners worldwide also deal with this issue.

It’s really not that uncommon to have the green stuff on your patio slabs. When left unchecked, it will keep spreading and will be even more unsightly.

It can get very slippery too, which can cause accidents for unsuspecting guests.

The best way to explain why they are on your patio slabs is to figure out what they are in the first place. So, what’s that green stuff? Well, it can either be mold or algae (birdbaths get them too).

Yes, these are living organisms hanging out on your slabs and will continue to multiply if you leave them unchecked. Aside from being unsightly, they can even cause permanent discoloration on your slabs.

Mold or algae would be a concern for a wooden patio because they can cause structural damage. Algae on concrete are relatively harmless, aside from making the surface too slippery.

On the other hand, mold can cause several health concerns, including throat pain, headaches, and even lung illnesses.

So, regardless of what you have on your patio slabs, it’s important to get rid of them (we’ll talk about that later).

Why Are They Growing on Patio Slabs?

Three factors contribute to the growth of mold or algae on your patio slabs: weather, location, and surface. Now that you’re wondering why there are green deposits on your patio slabs, it’s safe to assume that you’re in an area that has the perfect condition for them to grow. There will always be a constant battle against them, so it’s important to equip yourself with the knowledge to do just that.


Weather is the most significant factor that promotes the growth of mold and algae. Simply put, homeowners in places with a humid environment will always find their patio slabs with either of them.

Both mold and algae reproduce from spores that are carried by air currents. Wherever these spores land, they’ll grow and multiply as long as it has the right environment.

The season also affects their growth, and you’ll notice that they thrive well during fall and winter. Now, algae and mold prefer warm and humid environments, but they also need enough shade to thrive.

A few days of warm weather, coupled with the lack of sun that fall or winter brings, create the perfect environment for these organisms to grow.

Unfortunately, there’s nothing that you can do about it. Algae and mold will keep growing, not because you can’t get rid of them, but because you’re living in an area that allows them to thrive.

But there’s another factor that you can use to minimize or at least slow down their growth—location, and structure.

Location and Structure

The location and structure of your patio slabs can also affect the growth of mold and algae. UV rays are harmful to these organisms, and you’ll notice that there’s less of them in areas that are getting constant direct sunlight.

So, one of the best ways to slow down their growth is to ensure that all areas of your patio are exposed to direct sunlight.

Of course, you can’t just relocate your patio, but you can reduce its shade to minimize mold and algae growth. It may require you to remodel or reconstruct, but it’s still not enough to prevent them from growing.

That’s why many homeowners just let them grow and deal with them regularly.


Another factor that affects the growth of these organisms is the surface. Soft surfaces (wood) and irregular-shaped surfaces (slabs) create the ideal place for them to thrive.

Of all the three factors, creating a hard, straight surface is the most plausible option for you to minimize or slow down their growth.

However, before you start remodeling your patio slabs to make them less ideal for mold and algae to grow, you need to remember that it won’t stop them from growing.

Even if the surface is not ideal for them, the weather and your location may still work in their favor.

So, instead of slowing down their growth, it would be best to find out how you can effectively get rid of them. It may sound like a lot of work, considering that they’ll keep growing, but it’s not, and it won’t cost you a lot either.

How To Remove the Green Stuff From Patio Slabs

Before we proceed, here’s a disclaimer: there are commercial products that you can use to help you remove mold and algae from your patio slabs. These tips aren’t the only options that you have either.

But what I’m about to share with you are the ones that work best for me and don’t have the recurring expense that commercial products have.

So, here are the 5 things that you can do to get rid of them:

  • Pour boiling water on the surface. Mold and algae will be hard to remove if you brush them without boiling water. I find this option to be the easiest because it helps remove them easier, and I can use a regular hose to get rid of them completely.
  • Use a heater or fan to dry the surface. Removing mold and algae from concrete slabs will be much easier if you can dry the surface before using a scrub or deck broom.
  • Use a pressure washer after pouring boiling water or drying out the surface. You may want to use a pressure washer directly, but I find it easier to pour boiling water on the green deposit before I blast it out. I don’t usually recommend it at first because you’ll need equipment for it, but if a pressure washer is readily available, you can use it. It’ll even help you get rid of the green discoloration on the surface.
  • Use bleach or vinegar if your patio slabs are away from other plants. It’s a practical option for anyone because bleach or vinegar can instantly kill mold or algae, making it easier for you to remove them. However, you have to be careful when using them because they can also damage nearby plants.


If your patio slabs are turning green, you can expect that they’ll keep coming back, regardless of what you do.

Sure, they’re unsightly and can cause accidents, but unfortunately, your place creates the perfect environment for them to keep growing.

So, instead of spending money to slow them down, the best way for you to deal with them is to figure out how you can effectively get rid of mold and algae.

Just make sure that you don’t brush them without preparation because these green deposits can also cause permanent discoloration on your patio.


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