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Black Mold and Deck Stains

Moldy Deck

Moldy Deck

Black Mold and why it Grows in Certain Deck Stains

Exterior deck stains are subjected to the elements making them prone to natural contaminants. A deck stain is there to shield the wood from harsh weather and the damage it can cause such as rot and decay. In providing protection sometimes the deck stain itself can be compromised.

Black mold is one of the natural occurrences that can affect a wood stains effectiveness and longevity. There are several reasons why black mold can grow on or in a deck stain but here are the most common reasons.

Over Applying Deck Stain

When a deck stain is over applied to the wood it can affect the curing or drying time of the stain. When this happens the deck stain remains tacky for several days or longer allowing dirt and tree pollen to become embedded. Once these contaminants have intruded the stain they can attract and begin to grow black mold even after the stain finally cures.

Tips: To avoid curing issues by over applying a wood stain, work in small areas or one board at a time. Let the stain soak into the wood for several minutes then use a brush or a stain rag to wipe any excess drips or puddles.

Inexpensive or Cheap Deck Stain

Going with a cheap deck stain may save you money initially but may cost you more in the long run. Cheaper deck stains consist of a cheap grade linseed oil that lacks the mildewcides that help fight black mold.

Tips: Do some research and buy a quality deck stain that is not prone to mold and mildew.

Humid Climates

In some areas black mold is almost impossible to avoid. Hot and humid climates or areas next to water are always more prone to all sorts of mold, mildew, and algae. In this situation black mold can grow on top of a deck stain and jeopardize the overall appearance.

Tips: In this case, use a deck stain that fights against these contaminants and increase the frequency of deck maintenance to keep black mold from becoming a problem.

Oil Based Stains

Oil based stains contain both natural any synthetic oils. These oils can feed to growth of mold “in” the stain itself. All stain manufactures add mildewcides and algaecides to help prevent this. Some like TWP have a EPA registered wood preservative ingredient.

Tips: Use a oil based stain that has a proven track record of not attracting mold and does not turn black in color.

Water Based Stains

Water based stains so not feed the growth of mold like an oil but still can attract mold once the stain starts to fail.

Tips: Choosing a water based decking stain with zinc oxides will minimize mold.

Have a question or comment about black mold and your deck? Please ask below.

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91 responses to “Black Mold and Deck Stains”

  1. Jen says:

    We had our deck stained/sealed last year. Right after, our house was power-washed and the guy also lightly power washed the newly stained/sealed deck. We didn't know, but probably should have re-sealed it? About 3 weeks later, noticed black mold growing on the deck. Trying to take care of it this year. My question is…I don't want to remove the stain as it's still in very good condition…so should I just use a cleaner (not a brightener) and then a sealer on top of it?

  2. vicki says:

    live in sw Pa, deck is 3 yrs old, low to the ground, 6 inches one side to 18 inches opposite. used TWP stain. had to power wash black mold off 3 times last year, this year, 2 times and restained. it is back. will Behr premium help or hurt? or should we just let it go and start over with pavers/concrete?

    • Deck Stain Help says:

      Vicki, nothing can stop mold from growing on top of any surface and we strongly suggest avoiding Behr products.

  3. Jo-Ann says:

    Hi, we have a 2 year deck that is attached to our above ground pool. We sealed it with Thompsons water seal (honey gold not sure if that matters)… .
    Well, now we have what looks like black mold growing all over it, so before looking or reading anywhere I used some water + bleach and was able to get it off in some spots but most spots the water just beaded up. So once that appeared not to work I looked online and got even more confused on what to do. Could you please help me, also I'm really confused as to how the wood can grow anything with the chlorine from the pool that gets on it.
    As my husband says I should get to the point lol
    Ok so what to use to remove the water seal? Then what to remove the mold? Then what to use to prevent and protect the deck? And finally what kind of seal / stain would be better for chlorine traffic?
    And we live in Georgia where humidity is almost a constant in the summer and the deck is in direct sunlight the majority of the day.
    Thank you Jo-Ann

    • Jo-Ann, you will need to strip and possibly sand of the Thompsons deck stain. This should remove the mold as well but if not a good cleaning with a deck cleaner like Restore A Deck should help. Brighten the wood when done. Try Defy Extreme for this deck. Nothing will repel the chlorine but it will help prevent the mold.

  4. Adam says:

    I just stained my mahogany deck with armstrong Clark mahogany after stripping the old failing stain, but it looks like black mold is already appearing in some areas. I was careful not to over apply, but after two days the stain is still not completely dry (but not shiny/tacky), and overall the stain penetration looks very uneven. After reading all the positive recommendations from pros about Armstrong Clark I am sure this is due to something I did wrong and not a product issue. Possibly it wasn't mixed well enough beforehand. I was thinking about lightly washing with a oxidized cleaner and then lightly restaining with a second coat, but not sure if this would make the problem worse by using too much stain too soon.

  5. Courtney says:

    We built our house one year ago. We had our second-story deck pressure washed and stained with Deckscapes at the end of May. The weather here has been unusually rainy for the past four months. The underside of the deck was not stained (we were told it was not recommended).

    The top looks fine. However, the underside now looks terrible (there are large black spots and streaks (where the rain runs between boards) and smaller yellow spots, which I assume is mold and mildew. The person who stained it said he didn't know what to say except that it's been unusually rainy and that it would not hurt anything. However, it does not seem like it should look that bad, and the edges of some of the boards even look "jagged", I'm concerned they will rot.

    We would like to know what we should do next. We are considering having the underside stained, but is there something specific we should use? Do we need to treat the bottom with something first?

  6. Mark Walsten says:

    Have applied Penofin red label stain onto new cedar deck 1 month later black mold on all surfaces what should
    II do now?

  7. Sarah, is the mold on top of the stain or under the stain? Test by seeing it it wipes off with a wet rag.

  8. Big Al Smith says:

    I have a pressure treated pine deck about 10 years old, never cleaned or sealed. It was a bit moldy and very gray so I applied a stripper (diluted sodium hydroxide with some surfracants) and then power washed. The power washer was a 4000 psi 3.5 gpm using the 40 degree (white) tip. I was a bit worried about getting too close to the wood so I washed from a foot or so away. It seemed to come clean but after drying the results were a bit uneven and there was quite a bit of very fine furring which appeared to maybe be the the remaining gray layer. So I power washed again this time closer to the wood. This time the wood looked very clean and new when finished AND the furring seems to be gone. My question is about the furring. My first assumption was that I had cleaned to close or with too much chemical but now I think maybe I just didn't get close enough the first time to actually remove the deteriorated top layer of wood, is this correct?

  9. Steve Young says:

    Wow!! What a great site – I have gleaned more information and learned more in this one site than the rest of the web combined regarding the care of my deck. Thanks to the site developers as well as the people who have contributed questions, answers and comments. This is a DIY'ers dream come true.
    Thank you, thank you and finally thank you!!

  10. Susan C. says:

    Our wood deck is on the north side of house, no roof, gets lots of rain. At last cleaning, 4+- years ago, did not restain just applied sealer. So color and protection are uneven. We are not prepared to start over by stripping, sanding, etc.

    I want to know what kind of cleaner will get rid of the dirt, mold and algae that have accumulated without compromising what protection still remains on the deck. Prefer something I can spray on and scrub with a push broom. (I have already used clorox and water, which helped with algae but not with dirt and mold.)

    • Susan C, since it is has been four years it is not possible to not compromise the old coating with a cleaner. Best to prep with a cleaner and brightener like the Restore A Deck System but you will need to apply a new sealer or stain after.

  11. Giggi says:

    Have a deck similar to Big Al. Seven years old on the Southside of the house, lots of trees, stains, never sealed or cleaned. Plan to sell in two years. Immediate plan is to replace split boards, tighten it up, clean with some type of "stripper" and then power wash. Do the same thing again in two years, trying to replace any bad boards six months prior to the cleaning / power washing and then staining. Any problem with this plan? Somewhat concerned about the letting it set for two years without staining before repeating the process. Thanks.

    • Giggi, you should prep with a deck cleaner and brightener like the Restore A Deck Kits. No need for a stripper since it does not have an old coating. In addition it would be a good idea to have a coating applied now and again in 2 years.

  12. Sandra Piker says:

    We stained our 5 year old deck for the first time last fall. It was thoroughly cleaned per instructions and allowed to dry completely prior to applying the stain. A few months ago I noticed what appeared to be black mold showing up in several areas on the deck. Those areas are beginning to rot! How can this happen so quickly and what can I do to salvage the remaining boards? It seems to be spreading.

    • Sandra, can you tell if the mold is on top of the stain or growing \”in\” the stain? Nothing can stop mold from growing on top of a surface. This usually can be cleaned off easily. If the mold in growing in the stain then most likely you sued a oil based stain that is \”feeding\” the growth the of the mold.

      • tat says:

        what should someone do/use…exactly, in this situation ?
        I have the SAME problem and we did use an oil base as highly suggested by a reputable paint store .

        • Strip off and start over if the mold is imbedded in the stain. If on top try a light cleaning to remove.

          • tat says:

            lovely…not a job I enjoy to start off with and this shouldn't have happened, IMHO.
            Thanks for your help though, much appreciate it .
            Should we go with a water base stain this next time after cleaning it?
            Plan to clean it with just Clorox/water mix since the budget is very very tight here, is that allright ?
            There are other smaller identical black spots popping up all over, looks like soot and the whole deck really needs to be started over and it's a big deck. :<(

          • Bleach will make the spots go away if mold. Use about 1 part beach to 5 parts water. It may bleach or lighten the stain though. The other reason for black spots are rust. Did you use anything metal on the wood for prep?

  13. Acamp14 says:

    I have a 14 year old cedar siding house I just bought that I would like to restain. I am planning on using TWP 100 for the stain, but I need some advice as to how to prep my siding. there is old stain on there, but I am not sure of the product that was used or how long ago it was stained. There is black mold and mildew present on the upper level and the boards close to the ground are very dull and dirty. I am not sure whether i should just use a cleaner and brightener or a stain remover and brightener?

  14. clint says:

    I have a 3 year old cedar deck in Iowa. I used a clear oil based wood sealer 2 years ago after a light power wash and 3 day wait, nothing else. Since, it has become very dull and has the black mold or mildew on quite a bit of the deck surface and that smooth whiteish substance is visible when the deck is wet. I'm inclined to powerwash all this off, let it dry, then stain. I have read lots of conflicting items on whether power washing the mold off is good or bad…? After cleaning what is the best type of stain to use? Tint or clear. I like the wet look but did not get this effect with the last stuff I used.

  15. Samantha says:

    Hi! We recently moved into a home with a 10+ year deck. The wood seems to be in great condition (no cracks, holes etc), except it was covered in black mold. We used a power washer to remove the mold and now we are left with wood frays that are likely to cause splinters. Are there any products we could use to seal and stain the wood without having to sand it first?

  16. Cathy says:

    I just spent the last two weeks stripping two year old Flood deck sealer off my redwood deck in Northern California. I then cleaned and brightened it. It was grueling work but now looks great. I bought Penofin Blue Label to put on it but am afraid of having to strip it again in two years. Penofin says I can just wash it in two or three years and reapply the stain. Is this true? I was so disappointed with the Flood product. it deteriorated quickly and had mold growing in many areas of my deck.

  17. Serena says:

    I just bought a house that has a 10 year old deck that doesn't see a lot of sun and is in a damp area. It doesn't look like it's ever been cleaned or stained. It has mold/mildew engrained in the wood. I've cleaned it with bleach and TSP five times, power washed twice, and sanded once. Nothing works. The last time I cleaned it, the run off ran clear – no more dirt and mold is being removed. Not sure what to do to get the mold out of the wood. There are green patches everywhere. Is it even possible? If it's not possible, can I go ahead and seal it anyway or will that cause even more problems? What kind of sealant do you recommend for this?

  18. briankbax says:

    First time on site. Looks very informative. A very subjective question. I have 7 year old cedar deck that we are replacing with composite or PVC. Don't know which way to go. Have been looking at Gorilla Deck, Fiberon, Ever Grain, or Lumber Rock. Any advice would be greatly appreciated. I don't want WPC (Trex) as it tends to get swelling on the ends and can mold, I think. Also, we are going to use the existing joists but they have black mold growing on them. they were once stained with a water-based semi-transparent stain but now has mold on the surface. I was just going to mix bleach and water and spray it and leave it. What do you think?

  19. Emily says:

    We have a fairly new cedar deck and we used a bleach based cleaner which ended up turning it white and tissue paper like. We ended up renting a power washer to clean it off which only did it partially..and we had to end up sanding the entire deck down by hand! We now have black spots on parts of the wood…what do we do about it now? Re clean it with Restore A deck or wet and forget etc… Please help!,

  20. stephen says:

    We replaced all of our wooden deck boards and railing boards with wooden boards on a 15' x 30' deck. We waited 9 months and in the spring aggressively powerwashed the floor boards :( ,and lightly powerwashed the rails. We then washed with soap and water and rinsed with a hose. It rained before staining. We waited two days for it to dry then started rolling the railing with CWF which took 3 days. Several days later I started rolling the floor boards which also took several days to complete. I believe there was sunshine with no humidity during this entire time period. The wood looked unevenly stained so I sealed it again. I didn't do this after the recommended 20 minutes, instead I resealed over a period of a few days beginning about 24 hours after initial application. The weather was the same. The deck still looked unevenly stained so I then spot stained those areas. I know the deck was completely sealed because water beaded up to the point where it was necessary sometimes to squeegie the deck to remove the water.

  21. Mick says:

    I just finished building a western red cedar deck and, not surprisingly, as I now learn, it has started to get grey/black stains which arrived after the first rain for several weeks. After a chat with Armstrong Clark I had planned to wait until spring then brighten the wood with oxalic acid solution and then put on a Armstrong Clark semi-transparent stain. Now that the black marks have begun to show up, should I change this plan. Presumably by April/May (in BC) the marks will be much more widespread and i am concerned that they may not be removable by then. Any suggestions would be most welcome.

    • If you stain now then you will probably need to redo in the Spring. Best to wait. You can remove mold.

    • Tim says:

      Western Red Cedar reacts with iron and steel when water is added to make black/grey stains. Did you use zinc screws and galvi. nails? Did you cut any steel or do any grinding where the black parts are? A new cedar deck should start to mold for a year at least. ( I live in BC)

  22. Lyndsy says:

    Does a dry below system under the deck cause mold to grow on top deck

  23. Tanja Meier-Smith says:

    Is there any product that will kill the mold on my oiled deck without stripping or damaging the colour?

  24. Douglas says:


    I just received delivery of new redwood (all clear heartwood) and was surprised that
    nearly every board has marks on it. It could be mold, but is not dotted throughout the board but instead has marks, some larger but some smaller, sometimes lines that go clear across the 5 1/2 width, perhaps 3-10 different places on each each board. The same type of black marks on at least one board appear to be where the straps were, but they are not all strap marks Before seeing this, I was planning on waiting 2-3 months, then cleaning, then staining. Should I alter my plan and clean now to make sure it cleans up before installing, and, what is the best way to clean in when I do proceed. Thank you, I really appreciate your site! Doug

    • Not sure if these marks can me cleaned off or if they will need to be sanded off. Either way it should be done now then allow the wood to weather for the 2-3 months, clean, then stain.

  25. Dorrie says:

    Comments please on this! I just got done watching this old house video where after they got done washing the deck they sprayed something called borate on it to prevent mold and rot. It was 5 parts water to 1 part borate. Sprayed it on and let it dry. Is that a good idea ?? I've never heard of borate and wouldn't know where to even get it.

    Next on this video they took 1 gallon of oil based stain and added a quart of varnish to it. Wouldn't that change the formula of the stain if you varnish was added?

    • That is a bad idea to change the formula of a stain and even worse to add varnish to a horizontal surface. Borates are more for preventing insects. It does not help with mold and rot. We have never bothered to use a borate.

      • Dorrie says:

        Thank you. It all sounded weird to me as I was watching it. Now if I could just get my deck done here .. handyman guys went on a 2 weeks vacation and are not done yet replacing boards. Then with all the rain we have here in Ohio looks like I will be staining in the fall.

  26. Stephanie says:

    We just had our deck stained. The person who stained it did not prep the deck before hand. Therefore the stain is over top of dirty and mold wood. We tried to power wash, but it seems to be tearing up the wood. Is there a deck paint like killz or something we could just paint over top of it? What are your suggestions?

  27. Mike Becker says:

    I have a question on a mold issue.
    I stained a pergola last yr. Prep was as follows sanded mill glaze off with 80 grit and pressure washed with tsp, bleach and water. Stained 4 days later with everything dry for those days. Sikkens SRD was what I used for stain. The pergola is near a pool, and a lake. 1 year later it has some black mold in places. Do I need to clean and sand and restain if so what product should I use?

  28. sue marcum says:

    we have dark spots on our new deck made with pressure treated wood. We have not used a water sealer yet such as "Thompsons". How do we clean this off?

  29. Cheryl Suehl says:

    Clear coated a new cedar deck. Looks like there is mildew forming under the clear coat. We let it dry for several days after cleaning it before coating it. What can be done?

  30. Michele says:

    I allowed my new redwood cedar to weather for 9 weeks. I am from Minnesota. Today, I hand sanded it and cleaned with ready to use Cabots wood cleaner for new wood. I used a deck scrub brush on the walking surfaces where a bit of plain old dirt existed. On the kick boards designed from tongue and groove cedar there are 3 small areas that are discolored. Mold or mildew I assume that didn't wash off. Should I allow to dry, sand, and repeat the cleaning process in these areas??

    • It depends on what the discoloration is. It may come off with cleaning or if tannin stains then a wood brightener should fix it. Make sure to brighten all the wood and not just the affected areas.

  31. Don says:

    I've been reading the posts and it seems that I have mold both on top and I betted in the stain. Being that it's going to be October. Is it best just for me to wait till spring?

  32. Gloria says:

    I just cleaned off the black mold and mildew with Dawn and a deck brush. It works great! We had 10 days of rain!

  33. rhoda Paul says:

    what stains have a track record of not attracting mold or turning black on 2 redwood decks that where one get full sun and the other no sun?

  34. Kevin G says:

    we just built new cedar deck in May, live in Chgo suburbs, used oil based B. Moore arborcoat natural sealant, then 1-2 mos. later the deck was full of large dark stains? Used product recommended by JC Light Pint Store, Lo max and bleach to attack the black stains, mold probably. Applied this mixture again just last week, next day, cedar looked clean, looked great dry, but when it rained next day, dark stains then appeared? How to fix in spring after winter passes in Chgo?

  35. C white says:

    Resurfacing deck with new super select KDAT. Pressure washed framing to try to rid of mold. Not all was removed as it had penetrated into 2x10s. Deck next to stream and minimal sun throughout day. Should we replace framing, if so what should be done to precent from reoccurring. Or is it unavoidable in certain areas l(south) and synthetic decking would be better

  36. jimi9746 says:

    Installed new KDAT on existing joists, now trying to decide between the TWP100 vs Defy Extreme. What are the criteria that should make the final decision between the two products. Do not want to wait till Spring and subject the KDAT to the wet/cold cycle before giving it protection.

    In Atlanta where we've had a month of rain, and have had the deck covered with heavy tarp and decking has had zero water since applying. I know that Defy Extreme and TWP 100 are the top rated products, but ….

  37. john says:

    i used %100 linseed on a deck after reading a "this old house" article. in a year it was blackened badly by mold and some wood was indented from mold eating – not a simple matter of keeping clean. (i didnt clean with mildewcide and unsure if any product lasts or is safe to continaully apply)

    i later found in the (same?) "this old house" article: "use a quality linseed product containing mold inhibitors". (i dont believe i've found one yet, no products are forthcoming about it)

  38. Kathryn says:

    So I used a deck cleaner, cleaned the deck well looked great. Let the deck dry for couple of days, deck gets a lot of sun. Put a water based deck stain down and it looked good. By fall I noticed black areas showing up again. Do not understand?

  39. Julie says:

    I live in the Pacific NW, so have been very careful to use Restore-a-deck parts 1 and 2, with pressure washing in between. Then I let the deck and rails dry for several days , and sanded them all smooth to help incorporate the 4 year old untreated PT deck into the 1 year new untreated cedar addition. It all looked fabulous, and I then stained in sections. The old deck looks quite good, and the new deck and rails also looked spectacular, for about 5 days. Then they developed black small dots all over them ! They were that areas that had dried really well before staining, and 5 days of sunshine after ! I tried scrubbing with alcohol, then soap- nothing helped ! HELP !!! What went wrong, and how can I fix this ????? I could understand easier if the old deck went spotty, but it did not.

  40. Jeannine Cary says:

    We put up all new wood on our deck. The floor is pressure treated and the rails are cedar and powder coated aluminum. We stained the cedar Sikkens. I have noticed this black powder (like soot) only on the top rail (arm rest)of deck. If you took your hand or wet towel and wipe the top rail you would see black on your hand/rail. I tested the deck floor (that is pressure treated wood) and nothing. I check the stained cedar that is vertically on the deck and nothing but everything that is horizontal in cedar has this black powder. It had just rained heavily 3 days prior and my deck is facing southern exposure in a 3+ acre lot that is open. I live in upstate NY Washington County. What could it be? I would say some type of mold but our deck is in direct sunlight from about 8am-5pm

  41. Ann says:

    I have a 9yr old Tigerwood deck. Workers sanded it dien and applied Ipe oil based stain. now i have what looks like mold. I kive at 8k ft in CO so very low humidity. What stain would you suggest for Tiger wood? Shoukd I sand it dien, then clean, brightner, restain in spring or now before winter? thank you. Also i dont see how to post a pic, inly youtube video url or :(

  42. Teresa says:

    My deck was stained 2 years ago. The deck was power washed then allowed to thoroughly dry during the hot summer. Black Mike has now appeared. I bleached it with a brush and it looked good for 2 months. Now the mold is reappearing. What do I do?

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*This is first and foremost a help site from our experience as wood restoration contractors. All stain and prepping manufacturer directions were followed with our reviews and ratings. We offer no guarantee of similar results. Take inconsideration that wood and deck stain results may differ due to prepping procedures, different wood types, exposure to UV radiation, natural weathering, etc.