18 years old,that project was done with CCA mateial. Buy now and you will get acq material the life of said is no place near the cca stuff, in fact no one has any idea on what the life span of even .40 acq will be. I do know when I cut into acq the center is still yellow the preasure treatment looks like spray paint. CCA never did this. Composite material on the hype scale is hmmmmm around 10 points with #1 being no hype at all. Ipe is 0 hype or #1 on the hype scale. Will a high $ ipe deck last 20 years with a acq.40 frame under it ? Now that is a hype question. It has nothing to do with ipe, everything to do with the green mafia pushing big money into an area they knew nothing about. Then drifing on over to the wine/pasta bar after calling jane fonda on the cell in Carmel and asking how the grilled shirmp was. I have a Wallet over here with a 20 year old pt cca project fairly big one,same kinda thing splits,splenters, but hey!! the thing still stands and dont look all that bad in a realative sort of way. I did some replacement,very cool ipe benches with allum balusters, couple of coats of twp finish,braced up the tiled bar,things like that. Tina is good for another 20 with some upkeep and furture replacement of the decking but I cant see any problem with the frame. J

I'm not sure why you have never had moisture inside the existing shed given those issues, and changing the roof line so that it increases the amount of water and snow on the uphill side sounds like a recipe for rotting of a wood structure over time. It may be that the existing roof design is a big factor in preventing water damage to date. Moisture will cause expansion and contraction of wood, which means that a caulk/adhesive bond between wood and concrete will likely fail over time, so it's hard to recommend this type of connection. I have three thoughts on these challenges: 1): excavate around the pad so you can lower the soil level to several inches below the top of the pad, and trench down and install 4" drains all around the perimeter. Then you should be able to overlap the sides of the existing pad; or 2): pour a new, slightly smaller, 4" pad on top of the existing one: an 8'X10'X4" pad will take about one yard of concrete. Then you can overlap the siding; or 3): Lay a cinder block perimeter foundation in mortar all around the outer edge of the existing pad and build your shed on top of this. With any of these approaches you should also do two things: 1): Securely anchor the shed to the foundation; and 2): Install gutters and downspouts.

What is the foundation made from?

Read the installation instruction for the PVC deck you decide to work with. From my experience PVC decking feel better on 12" center framing. We always tear out the frames and start with a new, flat, and properly constructed frame. Old frames take significant amounts of work and time to prepare for a quality installation. The frame is a critical aspect of having a good looking end product. Do it right, it will last a lifetime.