How to Get Rid of Black Spots on a Concrete Patio

Backyard not looking as good as it should? Here’s how to get rid of black spots on a concrete patio so you can make your yard look amazing all summer long. Try these easy DIYs yourself before your next backyard party!

For all its strength and endurance, concrete sure gets dirty easily, doesn’t it—especially if you live in an area with lots of moisture in the air. Algae growth is one thing, but black spots are a whole different animal (or, more accurately, organism).

Getting rid of them is hard—power washers won’t do much, for example—but it is not impossible. By the end of the present article, you will know what those black spots are, their causes, and, quite importantly, how to get rid of them so your patio will look as good as you and yours deserve.

Black spots on concrete can have varying causes. Let’s go over them.

Poorly Done Concrete Job

It’s rare, but it happens; if the concrete is of low quality, or the mix was not appropriate, this can give origin to black spots on the surface. When this is the case, it will usually be spotted not long after the job is done.

Water Damage

This can happen if there is a malfunction in your draining or irrigation systems, which will bring water to your paved area. A good power washing is usually good enough to take care of these blemishes.

Ink, Oil, and Other Chemicals

Spills happen, especially in areas such as workshops or garages. When this occurs, dirt will cake on top, darkening the stain. Clearing these blemishes is not too difficult; here’s what you’ll need.

  • Cleaning gloves.
  • Soft-bristled brush, or a sponge if you prefer.
  •  An ammonia based cleaner.

How to Remove Oil Spots From Concrete

Once you’re all geared up, proceed with the following steps:

  1. Begin by thoroughly sweeping the area, to remove superficial dirt.
  2. Put your gloves on, and prepare an ammonia solution: 1 cup of ammonia and ½ gallon of water. Stir with your brush or sponge.
  3. Apply the solution on top of the stain, and let it sit for about 5 minutes. Scrub vigorously. Repeat this step (apply ammonia solution, wait 5 minutes, scrub), until the spot disappears completely. When it’s gone, wipe the area with a tower damp with warm water. Allow to air dry.

Alternatively, you can (after sweeping the area) cover the stain with mineral spirits (wearing gloves remains a good idea for this), then apply cat litter or cornmeal on top. Let the patch work overnight, then sweep the whole thing away. By now, the stain should be gone; wipe with a towel damp with warm water, and let it air dry.

Editorial image for How to Get Rid of Black Spots on a Concrete Patio of clean concrete backyard patio

Algae Growth

This is one of several growths (the other two are discussed separately further down) that can appear on areas of your concrete floor where moisture and shade combine.

Beyond making your area unsightly, algae does not cause any special harm, although it can make the affected area slippery.

How to Remove Algae From Concrete

It is, however, easy enough to remove; all it takes is the right cleaner. This one, for example, requires no scrubbing, although it does take a while to yield results.

Mold Growth

This one is not too hard to remove either, at least not when caught early on—which is fortunate, as it can be harmful when inhaled, and should therefore be tackled with all due haste; if allowed to proliferate, it might become so big a problem you’ll have to call a professional.

DIY Patio Mold Removal

For those cases in which mold is still in a small area, you can opt for one (or more) of the following remedies:

  • Load a pressure washer with hot water, and blast the affected area. If your floor is slabs, remember to avoid the junctures, or you might wash the grout away and compromise the whole thing. After the pressure washing, scrub any stubborn patches with a deck brush, and rinse with a hose. The water carrying the mold off should be channeled away appropriately, into the nearest drain.
  • Give it a pass with a steam cleaner; scrub with your deck brush as necessary, then hose down thoroughly. Here, you might also want to ensure the resulting wastewater is appropriately drained away.
  • Pour vinegar (cleaning vinegar might be even better) into a spray bottle and apply all over the area. Let the vinegar sit there for 15 minutes, then scrub and hose down to rinse.
  • If you want to save yourself the scrubbing, you can opt for the commercial cleaner we discussed for algae (which will admittedly take a while to act), or a product like this one, which doesn’t require scrubbing and provides results quite soon after applying.
  • Be warned, though—it is pretty potent. Don your protective gear, and remember to replace the cap on the bottle, or it might leech off the spray nozzle and cause havoc where it is stored, as it has happened to some customers. Given its strength, it is advised to try it out in a more inconspicuous corner; if there is any unwanted change, it might be best not to use the product.
  • Don’t let it near your plants, keep the area off limits while you work, and mind your ventilation.

Spots Caused by Lichen

Many of us are familiar with algae and mold; removing either of them is easy enough. Anybody who’s dealt with lichen, however, can confirm that clearing it out can be a monumental task. The question is, why?

It begins with the specific type of organism we’re dealing with. It is, at its core, a formidable life form: extremely long lived (centuries, or even millennia), and widely spread—it can even thrive in the Arctic)—, it is carried across great distances by the wind, which is how it makes it all the way to your concrete patio or driveway, where it begins to take over if left unchecked.

We said ‘organism’, but it is actually ‘organisms’, plural. Do you remember the term ‘symbiosis’ from your biology class? Lichen is a prime example of it—a fusion between a bacteria or algae, and a fungus (or several).

The latter acts as an anchor (which is what makes lichen so hard to get rid of—it is quite literally rooted into your slabs) and traps nutrients and moisture; the former processes said nutrients and moisture, providing sustenance for the entire lichen structure.

Pretty cool, all in all—until it starts to turn your concrete floor into a smattering of black spots. It’s unsightly, on that we all agree.

But is it harmful?

Fortunately, it is not; the fungi acting as an anchor relies on the surface’s pores to take root, but it does not break the concrete down in the slightest. It does not release spores either, as mold does, so there is no worrying about harm due to inhalation.

Now that we know ‘what’ it is we’re dealing with, let’s get on with the ‘how’.

Use the Right Products to Remove Lichen

As we have mentioned before (and as you might have experienced in the past), lichen cannot be removed through the means used for other types of growths. Weed killers, for example, can often cause the opposite effect: as certain parts of the formula break down, the lichen can absorb them and supercharge its development.

Does Patio Magic Get Rid of Black Spots?

Not all countermeasures will have such an undesirable effect, but most will still be ineffective. Patio Magic, a well-reputed cleaner known to be quite powerful against algae, has been observed to struggle against black spots, likely because there is a missing ingredient in its formula—which we will discuss forthwith.

How to Remove Lichen From Concrete

In a word: bleach. This is what has been used most successfully in lichen situations. For this, a popular ratio would be 3:1—3 parts bleach in 1 part water.

Apply it all over, let it sit for a full night, then rinse by hose and let it dry. Likely, you will notice a difference afterwards, but you might need more than one application to fully get rid of the lichen; this is not unusual.

A remedy that enjoys wide popularity is a particularly strong version of chlorine—sodium hypochlorite at concentrations of 14 to 15%.

This is more easily found in the UK; in the US, one can find it in concentrations of 12.5%, intended for using on pools. Bear in mind that this one is a fair bit stronger than even the mold remover we discussed a few paragraphs back (which contains 4-6% of sodium hypochlorite); handle with appropriate caution (protective gear, plenty of ventilation, cordon off the area, and keep away from plants).

The process is the same: apply all over, rinse the following morning, let it dry, and reapply as necessary. Potent though they are, both bleach and its stronger version (known in some places as dairy bleach) are generally considered safe for hard surfaces such as concrete, though it doesn’t hurt to perform a test on some area that is not so visible.

Prevention Matters

When it comes to algae, mold and lichen, a key part is moisture; if you tackle that, then these three will at least take longer to appear. To this end, it is often recommended to seal the concrete, so as to prevent it from absorbing moisture. For best results, apply only after on clean, dry concrete.

How Do You Get Black Marks Off Indian Sandstone?

Temples, homes, works of art, tools—these are but a few of the documented uses of sandstone. In recent times, it has been steadily gaining popularity for paving, due to its looks, low maintenance and relative affordability when compared to other materials. The variety imported  from India—hence the name—, specifically, is widely praised in the UK for its unique looks, and also because it tends to be cheaper than Yorkstone, so named because it is quarried in the Yorkshire area.

Lichen growth can also occur on Indian sandstone, but it shouldn’t be treated like concrete. An often recommended alternative, specifically formulated for this type of paving and growth, is SmartSeal Patio Cleaner Xtreme, which has been tested by some professionals and found to be quite effective where other similar products have been known to fail.

How do I Get Rid of Black Spots on Concrete FAQ

Our Final Thoughts

Black spots on concrete, depending on their cause, are usually not impossible to remove. That said, they should be taken care of as soon as possible, especially if it happens to be algae—which may cause someone to slip on it—, or mold, which can be harmful if its spores are inhaled. And remember: it’s okay if it seems to take more than one application of your chosen remedy.