If your deck is 20 years old it's likely it was built to a code less stringent (and safe) than now. Like 4x4s instead of 6x6s for posts, or end-nailed joists instead of joist hangers. I'd take the opportunity to upgrade those at the same time. And the PT framing is about the cheapest & fastest part to replace.

Can you post photos so we can help you evaluate?

A 20 year old deck that is failing is failing. Period. The whole thing needs to be replaced. Pressure treated wood isn't eternal. It has a finite life and it is a big waste of time and money to put new decking on an old decaying deck frame. Tear the whole thing out and start over.

Thanks guys. I've crawled under the deck a few times and I have to admit I didn't notice any signs of decay or trouble. I shall go take another look to be sure - thanks for the heads up. I'd really want to be sure it requires replacement before destroying a possibly useful structure.

I bought the house 9 years ago and the deck could well be only 15 yr old. Tell me, if the supports look solid, should I continue with my idea of replacing only the deck or is it possible to miss signs of decay? Is it that easy to replace the whole thing?

Ease of replacement depends on the design. I helped a friend replace a 10x10 PT deck (complete teardown + replacement) and did it in a day. Our 660 sf 3-level garapa deck with pergola took us 3 months to build from the ground up.

Pictures, pictures. Then we can help you decide.

It's always a false economy to put new over old.

Treated lumber has a 50 yr warranty , if they look in good shape use them, one thing I would check is for crowns or dips in each joist. You can use a string and some help and slide the string from outside to outside down the deck to see if they are sticking up or down. You can try to turn them over or just replace the few that need it. For PVC decking this is important as it will show dips and crowns much more than other types of decking.

Where I live, the county requires a permit whenever more than 50% of a deck's materials are replaced. If you want to keep the framing, they require you to tear off the decking and then they come and inspect it.

This may not be a requirement where you live, but it makes a lot of sense. The other thing to consider - if you have any future insurance claim involving the deck (someone falls from it, it gets damaged by wind, etc.), most likely the insurance company will deny your claim if you reused existing framing without having it inspected.

If the substructure is in good shape, you can go ahead and just change the decking. Once you remove the old decking, check for rot. If it's attached to the house, make sure that connection is strong, and that the ledger board is flashed. It should be fastened with bolts, not nails. As weedyacres said, look for joist hangers, and add them if there aren't any.

It would help if you would post some pics.

steve

thanks guys. I've had a couple of contractors bid on the job and neither has suggested the joists need to be changed. Once the deck is torn up,it will be easier to look for rot, improper attachments, etc. Thanks for the heads up on the permit - I shall check.
I'll a little confused by what is meant by 'ledger board flashed.' Is there a picture on the web that shows what that means?
I'm 99% sure I want to replace with ipe or massanduba. The contractor said that since the joists are 16" OC, I can't lay them diagonal as the old PT deck but need to go at 90 degrees. This length is just below 15', the length of the lumber I can buy. To avoid buying 16' and cutting, I was wondering if OK to attach a 2X6 PT piece to the exterior support member to avoid screwing too near the end. Or does predrilling make it OK to screw within 1/2" of the end?
I can post pixs but what exactly so you wish to see? I can take a shot from a corner that shows part of the support network. Or I have exterior shots of the overall design.

Post pics of the guts of the deck. Crawl underneath and take photos of all the framing. How joists are attached to ledger, joists to beams, beams to posts, posts to ground, etc. And a couple overall pics.

My back had some surgery so I couldn't get entirely underneath, but here are some shots from the opening. I hope they answer some questions.

I would add joist hangers as other have suggested on ends of the joists. As fas as the "ledger flashing" is concerned, do a web search. You are essentially protecting the existing structure of your house where the "ledger board" (the board that is attached to the house) attaches from moisture penetration. The flashing depends on the type of siding you have. Look underneath the deck on the house side. Do you see the heads of bols or lag screws that attach the ledger board to the house?

My deck is free floating, and only pinned to the house at a bump-out by the back door.