Every fall our deck turns into a moss farm and water pools in a few spots rather than running off. I pressure wash it in the spring and by November it looks the same. Someone told me the pooling means the deck was built flat instead of with a slope, which apparently is a construction detail rather than a maintenance problem. Is that fixable without rebuilding, and how much damage is standing water actually doing? We're on the north side of the house with big cedars overhead, so it gets very little direct sun and stays damp for most of the year, which I a###ume makes all of this worse.
Standing water is doing real damage, since decay needs sustained moisture and that's exactly what a puddle provides. Your friend is right that decks should be built with a slight slope away from the house, and a flat build holds water at every low point. Fixing it after the fact usually means adjusting or sistering joists when the decking comes off, which is a repair rather than a rebuild. Also stop pressure washing at high pressure, it erodes the wood surface and makes absorption worse. We had GotRot do Seattle Deck Repair (https://www.igotrot.com/seattle-wa-deck-repair/) with a re-sloped frame and the moss problem largely went with it.