Wood – How do cedar and pine compare as decking materials

I'd start by LIGHTLY power washing with a detergent, or if you don't want to rent a power washer and need a workout, use a stiff bristled scrub brush to scrub down the deck with detergent. That'll remove the black/slimy stuff, and will probably resolve most of the problems with the deck itself. Don't really grind into it with the power washer.

The main thing you're going to need to figure out before you can replace portions of it is precisely what the material is. Did your parents have the deck built, or was it there? If you can find old receipts or quotes, that might tell you what the material is. It also might be under warranty -- most of the early composites had a warranty for 15 or 30 years, and if it's only ten years old, then the spongy boards might be replaced by the manufacturer if they're still around. Hard to tell without actually having a product and manufacturer name.

The difficulty with replacing boards in a deck like this without exposed nails is that you have to take off most of the boards to get to the one you need to replace. You start at the edge and remove the facing board, and then you have to remove the hardware holding each board down in turn to be able to safely remove and slot in a replacement. The replacement also won't match, and unlike wood, the composites don't weather to the same color quickly. If the money's there, you might just consider having the entire deck resurfaced, especially if your parents are elderly and the house is going to be sold as they move on in life.

How old is the wood? If we're talking 70 years or so, definitely don't burn that stuff as that'd be old growth wood which you just can't get anymore (easily). I'd even have a hunch that a 70 year old 2x4 might carry as much of a load if not more than a modern cheap pine 2x6.

All that said, as you state, it's not treated lumber. Unless it is cedar, you likely won't get much life out of it as a decking material--unless you have plans to refinish it each and every year.

The 2x4 also wouldn't make for a sufficient floor joist, so you'd have to use new structural lumber for that anyways.

If it's old lumber, I'd consider saving it for another construction project, or donate it to your local re-use center or habitat for humanity. If it's not old lumber, I'd use it for the fire pit.