I tell my neighbors.  I tell my clients.  I tell everybody who will listen.  NEVER pressure wash a deck!  When the pressure-wash companies send their college kids to knock on my door to "wash" my deck, I tell them why they should not be doing that to decks!  When I see their trucks*, they advertise cleaning decks, patios, siding, roofs - you name it!  Don't do it!

The older the deck, the worse the wood reacts to pressure washing.  The big mistake is that companies and homeowners have a tendency to set the pressure way too high.  But even at low pressures the deck can become damaged.

What does pressure washing do?

• It removes loose material and leaves a gafillion dangerous splinters and gaps.  Those gaps open up further letting in more damaging sun and water.
• It removes the natural oils in the wood that are not replaced with sealants.
• It causes wood to dry quickly causing cupping and warping.
• It causes damage.
• It loosens nails as the wood expands.
• It can cause water to enter the house.

So what to do instead?

There are various, excellent, gentle cleansers out there.  Be sure to select cleansers that do not have caustic lye or acid, or say not to use around children, pets or water features.  And the so-called "oxygen" cleansers, with the active ingredient of sodium percarbonate, are great and don't damage the wood, the kids or pets, the house or the yard.

A couple of gentle and highly-rated deck cleansers can be found here and here.  And they're both essentially the same and work in the same way - put on the deck, wait a while, brush gently to clean, and rinse off with a hose.  It's easy to do.

You MUST seal your deck after cleaning.  The sun does more damage to your deck than rain and snow.  BE SURE TO USE A SEALANT THAT HAS UV INHIBITORS IN ADDITION TO WATER PROTECTION.  Read the label.  It will tell you what the contents are.  Water proofing alone is not enough.

Again, highly-rated and UV protective coatings can be found here and here.  Different colors can be ordered as well.  UV rays do more damage to decks than rain and snow, so after a deck is cleaned it should be well protected.  Each of these links are to products that can be tinted to whatever color you want.

My recommendation:  Keep your deck gently cleaned and protected and it will last many decades.  Not doing so makes it unsafe and age much faster.

* I once gave my spiel to a guy sitting in his pressure-wash company truck in a parking lot (with a smile on my face) and he told me he would NEVER pressure-wash his own deck.  He said he sees what damage it can do later!