Gray, Green or Slick Wood Decks and Fences
Wood outside turns gray, then green, then slick. What is happening to the boards, why too much pressure ruins them, and how a deck or fence gets cleaned safely.
The short answer
Gray boards are the weathered surface of the wood, and green or black on top of it is mildew and algae from boards that stay damp. Wood is soft, so it has to be cleaned at low pressure with a wide spray. Too much pressure leaves the boards fuzzy, splintered or scarred.
What is happening to the boards
Bare wood left outside changes color in stages. First it goes from its new color to a silvery gray as the surface weathers. Then, on boards that stay damp, green and black growth moves in on top of the gray.
Our own deck page describes how this plays out under Louisville tree cover: a deck in heavy shade spends most of the year damp, the boards go gray, and then they get genuinely slick underfoot. Fences that sit along a tree line or behind shrubs go the same way, often on the side that never sees sun.
Why pressure is the real risk
Wood is far softer than concrete, and pressure is where deck cleaning goes wrong.
TreatedWood.com, published by the wood treatment company Viance, ran guidance from a deck restoration contractor: use no more than 800 psi, a fan tip of at least 30 degrees, and keep the tip at least 8 inches from the board through the whole stroke. Too much pressure or poor technique can etch, splinter or fuzz the wood fibers, or even scar the wood (TreatedWood.com).
The article is honest that experts differ on the exact numbers and calls its own figures conservative. Its point holds either way: wood cannot be cleaned like a driveway.
How we clean decks and fences
Pressure is matched to the surface. Wood is not treated like concrete. Wood and composite decks, concrete and stamped patios, and pool decks each get the pressure and technique that surface can handle, so mold, algae and dirt come off without damage.
Fences get the same care, on wood, vinyl and chain link, and we clean both sides, so the green film comes off the neighbor’s side too.
After it is clean
A cleaned deck is a good time to decide about protection. We do not stain or seal, so if you plan to, follow the product’s instructions on how long the boards need to dry first.
If a deck is slick in the shade, trimming back branches and keeping leaves swept off between cleanings helps it dry faster.
What cleaning will not fix
Cleaning brings back the look of the wood but does not seal or protect it, and we do not stain, seal, sand or repair decks. Boards that are rotten, split or loose need a carpenter. A deck under heavy shade will grow back over time, faster than one in the sun.
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