This post was updated on February 27, 2024
Black Mold Mildew on Deck Stains
We appreciate it when people write in and ask for advice when something goes wrong during their deck staining projects. Today we talk about what makes your deck prone to black mold growth after application and how you can prevent it. We appreciate your input, so feel free to leave a comment below with pictures of your deck stain projects.
Black Mildew and why it Grows in Certain Deck Stains
Moldy Deck
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 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 while increasing the frequency of deck maintenance to keep black mold from becoming a problem.
Oil-Based Stains
Oil-based stains contain both natural and synthetic oils. These oils can feed the growth of mold “in” the stain itself. All stain manufacturers add mildewcides and algaecides to help prevent this. Some stains like TWP have an EPA registered wood preservative ingredient.
Tips: Use an 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 do 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.
Best Cleaner to Remove Mold/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
More information Black Mold & Mildew Decks
- How to Clean a Deck Stain From Mold and Mildew
- Mold and Mildew on Wood Decks
- Black Mold and Deck Stains
- How to Remove Mold and Algae From Decks
- Restore-A-Deck Wood Stain Review
Mildew on Decks
Have a question or comment about black mold and your deck? Please ask below.
We have a large black greasy/sticky area on our deck. It is near a hummingbird feeder. Could there be a connection?
Yes. Feeds mildew growth.
Thank you for your quick response and advice.
We apparently live in a swamp. After prep with R-A-D products and applying TWP 100 stain our PT pine deck is always covered in mold within 6 months. (We did $20K of drainage improvements around the house. The deck is 2-4 feet above grade.) Is there a more mildew-resistant product than TWP 100 and/or a mildewcide that could be added to the (TWP?) stain to increase the resistance?
TWP already contains an EPA registered mildewcide in the stain. A deck stain like the TW will help prevent mildew from growing in the stain but no deck stain will or can stop mildew from forming on top of a deck stain. It is most likely just an issue with your environment that will have to be dealt with annually.
Thanks. Is there a surface spray mildewcide you recommend that could be applied periodically without degrading the TWP 100?
Not that we have tried. Maybe the “Wet and Forget” would help?
We have green mildew on deck from lack of sun. We tried Behr for mold. It didnt work
, it pealed off like paint. Looking for suggestions
Picture please.
See picture below.
Only way to fix this is to power sand it all off.
once the black is apparent on top of new deck board and in newly applied Behr stain, what do I do. Do i have to strip and startover, will this compromise my new deck. No black was apparent before we stained the new deck after waiting several months. Can I stain over with a solid stain, if so will the black cause my deck to rot?
Strip and or sand it all off and start over is the best option.
Brand new 4″ Tigerwood. Not even finished putting it all down yet (with Stainless steel Camo screws), and not yet oiled/sealed. Tiny black spots have started appearing over large areas. In So Cal, the deck is in the sun a good part of the day, not really been wet yet, although squirted the hose underneath a few days ago to rinse off concrete a couple feet below the deck . But the area where it seems to be popping up most isn’t even over that concrete and is raised about 4 feet off the ground. Doesn’t seem to wash off with dish soap and brush or sanding (which I only tried a very little bit on one small spot by hand). I was planning on Screen sanding the deck with lowish grit like 100 or 120 before Ipe Oil. Now what ?????
Post a picture.
Nothing happens when I click on the Camera ICON to try to attach pic
I tried a different browser. Tried to get a pic showing the black specs. They are appearing out of nowhere over entire boards and sections of the deck.
Try a wood brightener and see if that removes the spots.
Any idea why so many would appear so suddenly?
No idea.
Any Idea what would cause these to appear suddenly out of the blue over large sections?
Those spots look more like stains from metal filings. Any chance you were drilling anything metal near the deck and got some filings on it? Tiny black spots would show up overnight if any moisture is present. Sanding with 60/80 grit will take it out.
I hand scrubbed my deck,looked great. But rained for 2 wks. Black spots (small dots) appeared afterwards so I sprayed it down and scrubbed some of the areas. Some places came up easily and when dried are gone some did not. If these spots are mold and I stain over it,what then? I realise this sounds quite stupid but figured the worst is stain will not last in those areas.
Did you try to kill the mold with bleach? Also try a wood brightener in case the spots are not mold but rust spots.
I painfully hand brushed (2) coats of oil based Arborcoat Stain on to a very large deck of dried out, clean, treated lumber. Black mold slowly appears throughout, every 2-3 months. Looks great when power washed, but is there any products that will slow down the infestation?
Not really. Maybe a product called Wet and Forget?
I live on a lake in a very humid southern city in South Carolina. I like the idea of a penetrating oil stain but am concerned about mildew and mold. Specifically,
what is the best stain for me. I last used an Olympic oil stain with a toner. Beautiful for awhile.
Strip and brighten to remove the Olympic using Restore A Deck Stripping kits. Stain with TWO 100 Series. EPA registered mildewcide in it to help prevent mold.
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?
Strip and brighten the wood for prep and try the Defy Extreme stain.
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 🙁
Wait until Spring. Strip and brighten for prep with Restore A Deck products. Stain with Defy Hardwood Stain.
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
Thanks
It is mold or rust. It is on top of the stained sikkens?
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.
Sounds like maybe rust. How did you sand? It is not related to the stain or prepping products.
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?
Is the mold on top of the stain and does it wipe off with a wet towel?
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)
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 ….
Make sure to prep first with a cleaner and wood brightener. Stain with either will work but the Armstrong Clark may penetrate your new wood a little better.
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
I would suggest bleaching the framing to help remove the mold. Try apply the Defy Extreme Clear to prevent future mold.
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?
Could be rust as that turns black. Fell free to post some pics in our forum area.
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?
Defy Extreme and TWP 100 Series.
I just cleaned off the black mold and mildew with Dawn and a deck brush. It works great! We had 10 days of rain!
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?
Yes and you should strip and brighten for the prep.
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.
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?
Strip and brighten for prep. Might want to then wait until next spring to stain.
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?
Post a picture please in the forum area.
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?
You have to remove the Sikkens. Stain stripper and wood brightener after the stripping. What state are you located in?
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?
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.
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.
Greetings,
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.
Is there any product that will kill the mold on my oiled deck without stripping or damaging the colour?
Probably not. Have you tried soapy water and a light agitation with a very soft bristle brush?
Does a dry below system under the deck cause mold to grow on top deck
Sorry but not sure on this one. I would assume it would not.
Hi,
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.
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)
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.
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!,
Depends on what the black spots are. If mold you could try the wet and forget. If rust from sanding, you need a wood brightener that is part of the RAD kit.
What do you mean rust from sanding? It looks like mold.. We will try the RAD and wet and forget. Thankyou!
Rust turns black in wood. Looks like small black spots and can look like mold.
Does the aluminum oxide sandpaper get embedded in the wood and rust? Or where does the rust come from? I can't find any info on the internet about it..
Rust can come from anything metal. Feel free to post a picture in our forum and we will let you know if rust or mold.
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?
Go with Azek or Timber Tech. Yes that will help with the mold.
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?
If bleach did not get our the green algae and the black mold, nothing probably will. You might just need to go ahead and finish the process of staining.
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.
Not if it turns a dark brown or black color as we have seen Penofin do numerous times. You would need to strip it.
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?
Samantha, you cannot seal in splinters. I would lightly buff the deck with a floor buffer and sanding pad to smooth the surface than stain the deck.
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.
Clint, we would use a stain stripper first and pressure wash off. Brighten when done with the stripper. This will remove the mold as well. Best to have a tint so you can prolong the Uv graying. Honey colors typically give the wet look you want.
Thanks.. I plan on using an oil base…right?
Correct!
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?
For this it would be better to use a stain stripper, pressure wash off, than brighten.
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.
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.
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. :<(