How to Remove Black Stains from Your Wood Deck

Keeping your wooden decking in good condition is crucial if you want it to last a long time and look amazing. Unfortunately, wood is prone to staining and it’s very common for people to get black stains on their wooden deck, usually caused by mildew.

To remove the black stains from your wooden deck, you need to clean the deck effectively. It’s best to use a deck stripper to remove all of the existing paint or stain. Then pressure wash the surface free from any stripper residue, this also helps remove any black stains. Lastly, specific chemicals should be applied to kill mildew before the deck is stained again.

Black stains on wooden decking are very common and they are often down to poor stain application or lack of maintenance. But if you remove the stains, there are some simple steps you can take to stop them from coming back. Read on to learn more about dealing with black stains on your deck.

What Are the Black Spots on my Deck?

The black spots on your deck are usually caused by mildew. Mildew is a type of fungus that can live in damp environments, such as on the underside of the decking. Mildew causes these black spots and it thrives in warm, moist environments, which means it’s more prominent during the summer months.

What is Causing My Deck to Turn Black?

There are a number of reasons why you have a problem with mildew. It may be because you did not clean the decking properly before applying wood stain. If the mildew is already present, it is locked in by the wood stain and it thrives over the coming months, leaving black stains on the wood. This is a significant problem because if mildew can grow underneath the stain, that means there is moisture under there too. If left unchecked, this leads to more serious rot and decay, which is not so easily undone.

Using poor-quality deck stain products is another common cause of black stains. Good-quality products will contain mildewcides to kill mildew and inhibitors to stop it from growing in the first place. However, cheap products will not contain these ingredients, meaning that you are more likely to get black stains.

Sometimes, black spots are not caused by mildew, they are caused by tannins. Tannins are found in plants and soil, but they can also be present in wood. If the wood comes into contact with water, it will absorb some of the tannins and this causes brown stains on your decking. You can often tell if it’s tannins that are causing black spots because these are more widespread randomly across the deck rather than being clustered together in one area.

How to Remove Black Stains from Your Wood Deck

Although mildew stains can ruin the look of your deck and cause damage, they can be dealt with by following these steps:

1. Strip The Existing Wood Stain

The first step to take is to remove the existing wood stain with a deck stripper. You can buy deck stripping solutions from your local DIY store or hardware store, you can also buy an earth-friendly option called SmartStrip on Amazon that many people have had success using on wooden decks. Be sure to read the instructions carefully. Some wood stain strippers require you to apply them with a brush and some are ready to spray directly onto the wood.


After applying the solution, leave it on for the recommended amount of time before scrubbing over it with a stiff brush. Once scrubbed down, use clean water to rinse off all of the product until no more product is running off. If there is any residue left after washing, re-apply until everything has come up. You may need to use this product multiple times if the black staining is particularly bad or you used a dark wood stain on the decking.

2. Clean the Wood

The next step is to thoroughly clean your decking using a heavy-duty cleaner. Make sure that all of the stripper residues have been removed and scrub away any dirt or mildew stains. If possible, use a pressure washer to blast any cracks between boards and completely remove mildew stains from every inch of your decking.

Sometimes stripping and pressure washing wood causes loose fibers and an unpleasant looking deck. if you have this issue you can use a random orbital sander to sand the deck smooth. Throw on an 80 grit sanding disc and begin sanding until things are smooth.

3. Kill and Prevent the Mildew

Once cleaned, you should use a topical fungicide product to kill mildew underneath. A recommended product used at this time is Nisus Boracare Wood Treatment. This will stop it from spreading into causing black stains again in the future. Just because you can no longer see any black stains, that does not mean that mildew is not present, so make sure that you don’t skip this step.


Once the mildewcide has finished working, clean the decking down thoroughly again and let it dry. It is vital that you give the decking several days to fully dry. If you move on to the next step while the decking is still slightly damp, you will experience a lot of problems with moisture in the wood once the stain is applied.

4. Stain The Deck (or Paint)

When the decking is completely dry, you are ready to apply a new stain. You can paint again too if youd like. Make sure that when you apply it, it comes into full contact with all parts of the wood. This will ensure that no mildew can grow underneath in the future. Use your preferred type of stain and follow the instructions on the pack to get good results.


I like using Thompson WaterSeal Stain because it is affordable and provides great water and moisture resistance. This helps prevent any new black stains and mildew from forming in the future.

When choosing a wood stain, make sure you use products with mildewcides and inhibitors in them. Even though you have used a fungicide already, this extra layer of protection is still welcome and it will stop more black stains in the future.

Alternative Ways to Clean Black Stains from Wood Decking

You can use some alternative methods for cleaning the black stains from your wooden decking.


Bleach is particularly effective for cleaning stains caused by mildew. Simply dilute a small amount in water and apply it to the stained wood using a stiff brush. Leave the bleach to soak into the stains for about 20 minutes before rinsing off with fresh water.


Detergent is another common household item that people use to clean their decking. It is particularly effective for black spots caused by tannins, but can also be used to deal with mold. However, if there is deep-set mildew and moisture, it will only clean the surface without dealing with the underlying issue.

White Vinegar

White vinegar is another effective way to kill mildew and remove black spots. It is also more cost-effective and less toxic than a lot of other deck cleaning options.

Mix 3 parts water with 1 part white vinegar and apply it to the stained wood using a stiff brush. Allow the solution to soak into the stains for about 20 minutes before rinsing off with fresh water.

Trisodium Phosphate

TSP is an effective cleaner that can be used for a wide range of stains, not just black marks. It breaks down the bonds in the wood and lifts off oil, grease, and grime. This means it can effectively clean old mildew stains without causing damage to your decking boards. It is also preferable to other strong cleaning chemicals like Oxalic acid because it will not lighten the wood.

Pressure Washing

Pressure washing your decking is a quick, easy way to get rid of black spots without any fuss. It blasts off dirt and mildew stains from the wood and brings back the natural color of the boards. Pressure washing is a good option because you don’t need any other products. However, you have to be careful with the pressure because you risk damaging the wood. Soft wood like cedar or pine should never be washed at more than 500-600 psi. Harder woods may withstand slightly higher pressure, but you should always use a head with a wide spread.

Should You Avoid Chemicals on Your Wood Deck?

Many people are concerned about using chemicals on their deck and whether it will cause damage. The short answer is, it depends. Some chemicals are designed for use on wooden decking and have been formulated in the right way, so if you follow the instructions, they are safe to use.

However, certain chemicals can cause long-term damage to wood and should be avoided. Chlorine and other pool chemicals, undiluted bleach, and cleaning products containing lye and acid. As a general rule, you should look at the labeling on chemicals and if they have warnings to keep away from children and pets, this is a sign that they are probably too strong for your decking.

Now, How Can You Prevent These Stains from Returning?

Maintenance is key if you want to prevent stains from returning. It is best to check your deck regularly for signs of mold and mildew, so you can take action as soon as you spot it.

Following the steps outlined above to clean your decking, and using a good-quality wood stain, should stop the stains from coming back. You should also give your decking a deep clean and restain it every few years, at the very least. This regular maintenance will prevent mildew growth and stop black stains.

Black stains on decking are very common and it almost always comes down to two things; lack of maintenance or poorly applied wood stain. If you can solve these two problems, you can tackle black stains for good.

Leave a Comment