We are a deck building company.....not deck cleaning or sealing/staining of any kind. For those tasks, we leave it to the experts. When we complete a pressure treated deck, we refer a local deck cleaner for cleaning. sealing or staining. Still, whether pressure treated or composite decking, most homeowners don't seal or stain the underside of their deck. This part of your deck most never sees sunshine or any other kind light, so it has a harder time to dry out after any kind of wet weather, humidity, spills, etc. Therefore, you may experience black or green surface mold and mildew. Most folks think you can just whip the pressure washer out and use any kind of chemical or cleaner, and that's not the case as you will find in the below articles from Bob Villa and Ask The Builder:
Wood is a hygroscopic material, which is a fancy term that means it likes to soak up and retain water (think of a sponge). This makes sense, since wood comes from trees, which soak up water to grow. While this is good for the tree, its not good for your lumber, furniture, or trim. It becomes the optimal place for mold and mildew to grow.
The black and green coatings on the wood surfaces are mildew, mold and algae. While they're fairly harmless to the treated lumber, most people find the organic coatings very objectionable. Constantly cleaning off all of the outdoor furniture adds insult to injury.
You have several options available to you to transform the unsightly underside of the deck back to it's pristine original condition. All but one of them will cause you to get very dirty and wet. The one option will simply lighten the load in your purse or bank account.
If you don't want to do the work yourself, you can hire a deck-cleaning company to transform the underside of the upper deck. My guess is most will try to sell you on using a power or pressure washer to blast away the mold, mildew and green algae. If the workman hasn't been trained on how to use the powerful tool, the concentrated stream of water will also blast away the lighter colored wood in between the darker bands of wood grain.
When the wood dries after being cleaned by a pressure washer, it often has the appearance of a fuzzy peach. There are countless small wood fibers that are hanging on the wood floor joists and the wood decking above. Some people find this fuzzy look to be very objectionable. The only way to remove the fuzzy wood fibers is to sand them off after the wood is completely dry. This can be very costly.
To avoid the peach-fuzz syndrome on your wood deck, I suggest you consider removing the black mold and mildew, along with the green algae, using a hand-pump garden sprayer, some oxygen bleach, a scrub brush and your garden hose with a regular nozzle.
When powdered oxygen bleach is mixed with water, it releases countless oxygen ions that rapidly break apart mold, mildew and algae. The solution is not harmful to any of the landscaping, lawn or trees around your deck. Chlorine bleach, on the other hand, is highly toxic to vegetation. Beware of cleaners that say they contain sodium hypochlorite. That's the chemical name for chlorine bleach.
Gravity is going to work against you when you clean the underside of a wood deck. Water, gunk, dirt etc. are going to drip on you no matter how hard you try to avoid it. You absolutely want to wear your oldest clothes and goggles when you work. Goggles completely cover your eyes. Don't confuse them with safety glasses. Safety glasses will not protect your eyes well enough.
Working with oxygen bleach is easy. You simply add the powder to warm water, and stir it until it's dissolved. Pour the solution into a regular garden hand-pump sprayer and saturate the underside of the wood deck coating all surfaces. You'll notice immediately that the oxygen solution starts to foam and bubble as it attacks the offensive mold, mildew and algae.
I recommend you apply the solution to dry wood surfaces to get the best results. If the wood is wet, the oxygen solution can't soak into the wood to deep clean it. Allow the solution to soak on the wood for at least ten minutes, then use a scrub brush on a pole to lightly clean the surfaces. Immediately rinse the scrubbed areas with clean water. You'll be amazed at how clean the surfaces are.
Be very careful working from a step ladder. It's not hard to tip the ladder over or lose your balance when you're tilting your head back to work. The better method, in my opinion, is to work off an elevated level platform or rent a small rolling scaffolding.
When working with oxygen bleach solution, you don't want it to evaporate before you scrub the wood surfaces. Spray the underside of the wood surfaces ahead of time and keep the wood wet with the solution. Always have some of the wood surfaces soaking with the solution while you're scrubbing and rinsing other parts of the deck.
To prevent the mold, mildew and algae from returning, all you have to do is spray the underside of the deck with an oxygen bleach solution two or three times a season and rinse. If you periodically apply the oxygen bleach so that you don't get heavy deposits, you'll never have to scrub again