This post was updated on April 4, 2025
Preventing Mildew on a Deck
Mold growth is a common problem that can arise during the deck staining process. Today we talk about what causes your deck to turn black from mold and how to prevent it. If you have encountered mold during your deck staining project, we want to hear from you. Feel free to leave a comment below with pictures of your deck stain projects.
How to Prevent Mold on a Deck from Returning.
One of the biggest enemies of a deck is mildew and mold. With moisture being ever so present outdoors, the mold is bound to find its way onto your deck. Typically, mold will begin to appear on the more shaded sides of a deck. Anywhere there is less sun means there is more moisture present making conditions ideal for mold growth.
Once mold infiltrates a surface it only continues to spread if conditions are favorable. Mold can get under wood stains and sealers and cause them to fail prematurely. Mold will also rot and decay wood if it is neglected and jeopardize the integrity of the deck and decrease its longevity.
Mildew On Decks
To remove mold from a deck, clean the surface with a deck cleaner containing sodium percarbonate or a mix of bleach and water. We are not fans of bleach on wood but sometimes this is best to help get rid of the mold. If you use bleach, mix at a ratio of 1 part bleach to 5 parts water. Use a stiff brush or pressure washer to clean away the mold, dirt, grime, and graying. After cleaning a wood deck, the surface should be recoated with a protective deck stain. Composite decks can also be protected with the proper sealer.
Best Cleaner to Prevent Mildew on Deck Stains
Pro Tip: The RAD Guard will remove the black mildew and green algae stains from your wood and deck stains and will prevent them from returning for 1-2 years: RAD Guard Mildew and Algae Cleaner and Preventer
To prevent mold from reoccurring you can use a product called RAD Guard Mildew and Algae Cleaner and Preventer. This product can prevent mold, mildew, lichen, and algae from appearing. For maintenance even if mold is not present, spray RAD Guard onto the deck’s surface and let it do the work for you. No scrubbing, rinsing, or pressure washing involved. This is a non-caustic product that contains no bleach or acidic material. It can be used on almost any type of wood or composite decking.
Apply RAD gaurd to your deck surfaces every 12-24 months as a mold and mildew preventative. Even if no mold is present, RAD Guard will kill and prevent unseen mold spores from growing. It is safe to spray on top of most wood stains and sealers. Wind and rain will continue working with the RAD Guard product overtime to keep mold away. Use as needed to prevent mold on a deck and to keep all your exterior wood surfaces clean and mold free.
Please Ask Any Questions Below
Hi. Thank you for answering my question. My husband and I had a new deck built; six months later, he started putting a sealant on the deck. Shortly thereafter, we started seeing mold underneath the sealant. We have mold everywhere. What do we do now? It’s very disappointing.
Remove the sealer first and then use a proper deck stain: https://www.deckstainhelp.com/the-5-best-deck-stain-reviews-and-ratings/
Try Restore A Deck Stains or Defy Extreme Stains as they do the best for mildew prevention.
Used RAD cleaner and brightener then stained with RAD light walnut in spring of 2021. Now I have green mold/mildew. What is the best course of action?
That is algae on top of the stain. Try a product called “wet and forget” and see if that helps.
Deck stained in 2020, looked great for a few months…then dark brown and dirty! Now we are removing the grime/mold and were advised to strip off the oil based stain and use a water based stain. Not sure if this is good advice. The deck is in the sun all day long-south facing
What stain brand did you use? Some like Penofin or Super Deck are more prone to issues like this. You will need to strip to fix.
We used PPG Prolux-used to be called Sikkens. We are cleaning it then stripping it. But how to prevent it from happening again? Some tell us to use a water based
stain
Make sure to brighten after stripping. Try TWP stains as they are not prone to darkening or attracting mildew if you want an oil-based stain. Other options would be water-based like the Restore a Deck Stains:
https://www.deckstainhelp.com/restore-a-deck-wood-stain-review/
Thank you!
My mold is a rounded ball of green stuff mostly walnut size but round with a flat bottom on the deck. Last one was a big one bigger than a lemon on glass table.
I have to reapply TWP 100 to my deck every year, and this is getting old. I’m hoping that you will have a suggestion that will allow two years between applications.
KDAT PT pine was put down over existing framing in 2015. Per your suggestion I started with a light coat of TWP 100.
I prep with Restore-A-Deck, making two passes over the RAD Cleaner with a scrub brush and then a pressure washer. In some areas a fairly thick layer of “mung” comes off with the pressure washer. (More on this later.) I make sure to flood the deck thoroughly to remove the RAD Brightener. I have tried different applicators, but I’m currently using a brush for the TWP 100.
Our house is in a very damp location, so per your suggestion I am applying Wet & Forget to the deck every 4 months or so.
After a year, areas underneath furniture and a deck storage box still look great and still bead water. Shaded areas look fair and no longer bead. Sunny areas have turned dark and no longer bead. It is these sunny areas where the thickest layer of “mung” is found.
This is what is weird. Mold should be thickest in the shaded and covered locations, not the sunny locations. I’m not sure what I am dealing with.
I finished this year’s Restore-A-Deck prep yesterday, and plan to stain in the next few days. Any suggestions that you might have to extend the life of the stain coating, including a possible change of product, would be appreciated.
Thanks.
Post some pictures.
Sorry. Too late. Deck has already been cleaned and brightened with RAD and pressure washer. I’ll put a tickler in my calendar to take pictures next spring. Thanks. (Cross-post with RAD)
Don’t know if these will provide any information. Under furniture and along shady wall still looks pretty good. Open, sunlit areas (now cleaned) were dark with a layer of something that came off with RAD Cleaner and pressure washer.
I’m getting mold on my deck and don’t think it will be favorable for staining before winter. Is there anything I can do to prevent things from getting worse until next year?
No, and it will be fine until you prep and stain next year.
I have lichen growing between the boards of the cedar deck. I just had the deck cleaned, sanded and stained. Should I just live with it? Or should I try to clean between the planks with 30 second cleaner? Or should I try to file down the lichen and stain it?
Please advise! Thank you…
The 30-second cleaner will not prep a deck. Try scrapping it off if you cannot pressure wash it off.
I found there are mold under my deck, most on the wood part and only a little on the AZEK matirals (attached the picture), would like to ask your kind advice how should I deal with this issue, thanks.
Try a bleach mixture of 1 part bleach to 4 parts water and spray it on those areas to kill the mildew and mold. Make sure to wear protective clothing.
Can I use the Wet and Forget as well? Thanks
I doubt it will work for this.
I have what looks like black mildew on my 2 year old decks and after 90 days I had someone stain them. Had him stain them with a clear stain because I love the wood look. But I have noticed black mildew forming them and I don’t know what to do. Can’t get in touch with this guy because he has moved away. And my decks aren’t built close to the ground. What do you suggest. Thank you so much
Strip it off and redo. Try Defy Extreme Clear.
Ask a Question or Post a Review…thank you 🙂
I have this deck that barley gets any sun at all. I finished it with something like a tough coat. As you can see it is now peeling and has mold and mildew. I want to either clean it or strip it and clean it. In the end I would like to go with a stain that won’t peel and will help with the mold and mildew. Any suggestions on how I can make this visually pleasing, please advise.
Thank you,
Joe
You will have to power sand this off as strippers will not remove it. Once removed, use one of these top brands: https://www.deckstainhelp.com/the-5-best-deck-stain-reviews-and-ratings/
Can I use Wet and Forget on cedar deck before standing.
I ment staining. Had mold in or under new stain.Striped and sanded.still small amount left.
As long as it dries after applying you should be able to.
Very informative website! I have a cedar deck in great shape but time for it’s restaining this summer here in suburb of Chicago. I use a brand called Rymar Extreme Westher semi transparent wood sealer stain from Wisconsin.
Question:
Can I use the Wet n Forget this August to let it work over the fall/winter season and then power wash lightly/re stain next year 2020?
No point in doing that as you would just wash it off in the Spring.
Can we use sealer over wet and forget on a deck?
Probably not.
Should I put a layer of Poly on the deck after I put on the Stain
No: https://www.deckstainhelp.com/can-you-apply-a-clear-sealer-or-polyurethane-over-a-deck-stain/
I built a new redwood deck last year and sealed it with transparent cedar from Armstrong Clark. Over the winter the entire deck was over run with mold (north facing deck with very little sun in the winter). I just pressure washed the entire deck with water only and it removed all of the visible mold. What should I do now before repeating the staining process? Is there any stain that prevents mold over the wet winter months?
No stain can stop mold from growing on top of the stain. If clean, just lightly recoat.
Use wet and forget
We have a five month old deck that is showing signs of mold. We used the treated pine and were told to wait a year before treating. I’m concerned as we have a lot of money invested. Just not sure what to do we have had so many different opinions.
Clean off the mildew with this: https://www.deckstainhelp.com/restore-a-deck-cleaner-system-review/
We have a covered front porch that we started refinishing a year ago, but we only got half way finished, so we covered it up to prevent it from getting wet (while well covered rain does blow in when it’s windy). The wood is maple, and just like a wooden floor inside the house. We just lifted the cover off, ready to complete the job, and discover some signs of black mold in between the floor boards in a few places, as our covering job was not completely successful. The wood, though, is still structurally sound. We thought we would give it a sanding with our hand-held belt sander, then seal and varnish it as we did the other side. That way we will address the cause of the mold. Do you think that will be enough? We are hesitant to apply anything else, as we already have to blend it in with the finished side. I don’t want anything that would bleach it, because it gets sun and it would continue to bleach, and then we would have spots where it is lighter than the rest of the floor. (we already have a problem, because we used a product on part of the floor that we couldn’t get all out. It was a treatment that we layed down then covered with paper. By DuPont. We used the remover and sanded it down several times with professional sander, but could not remove it, so if you have any ideas about that, that would be an extra bonus)
You cannot use a varnish on outside wood. It will peel, blister and attract mildew. You are doing this incorrectly, Sand all to bare wood and use a penetrating semi-transparent stain like the TWP that is designed for outdoor applications.
After the winter snow disappeared our cedar deck which we have left natural has a greyish film on it. It reminds me of a wet tissue that has dried in the sun. Is this Mold or mildew? We have wet it and swept it but it remains in difficult areas. Can you explain what happened and is there something we need to do in the future?
Post a picture.
Here are a couple of photos.
Sheila
Looks like wood pulp. Not sure why it is there. Pressure wash to remove.
Thanks for the reply. It turns black when wet and can then be brushed off. But it is hard work. We live in Ontario. Could it have something to do with all the snow we get. We don’t shovel it in the winter. Also we were told pressure washing harms the cedar. There is no other finish on it.
You can use a pressure washer as a rinsing tool.
Pressure wash it with a bleach free deck leaner then use a clear wood sealer on it. Ask at a ‘good’ diy store that knows about it.
We made the mistake of hiring an inexperienced person who pressure washed our 6 month old deck and started staining the next day. This was 3 years ago. We have had constant black mold issues on our deck. It does face the east and some woods, so only gets the am sun. We have had it professionally pressure washed twice and restrained twice. We started using wet and forget and it has helped, but lots of mold in places. My husband is ready to tear it down and pay for composit. I cannot believe there is not a product out there that is capable of sealing wood? Any suggestions are welcome!
Could be related to the stain itself or your environment. Please post a picture if possible.
Also what brand is the stain that was used before?
Sherwin Williams deckscape oil based stain
Strip it all off and try a stain that is not prone to the issues you have had before. Try TWP 100 Series or Restore A Deck Wood Stain.
That sounds like something we should consider.. do you think a more solid stain repels the mood from wood better? We used semi transparent, but notice most people going w solids?
Most people do not go with solids due to issues of peeling. Semi-transparent penetrating stains are best for future maintenance. Mold can grow just as easily on a solid color stain as a semi-transparent.
Our deck is on the North side, so in the shade almost all day plus our dryer vent is directly under it. Thus we have lots of green mold growing on it. Will the Wet and Forget work on this?
Not 100% sure but it will not hurt to try.
Wet and forget..
Do not use bleach on any cellulose material with mold infestation. It causes a reaction with the mold, releasing mycotoxins into the air. This can make a person VERY ill, especially if they are part of the 25% of the population that has no natural protection against mold toxicity.
Instead, use a combination of washing soda (soda ash) and real TSP (trisodium phosphate) in water. Spray with a garden sprayer, let it set for a few minutes, and hose off. Scrub with a brush if the mold is deep. In the case of mold remediation in a basement or home, you would leave it on (no rinse) and let it dry. This kills the mold and it will not return.
What is the best protective deck sealer. We get Moldova every summer.
Try Defy Extreme Clear.
Does it prevent mold permanently?
And does that mean once you use wet and forget you never have to re-Stain your wood deck again?
No nothing stops mold forever and no you still have to stain every 2-3 years.
Our deck is 1 foot above a carport with a metal roof. What do you recommend I use without damaging metal roof?
Not sure if any stain brand r prepping products will damage metal.
serching for composite deck sealer,need help
Defy makes one.