I'm right across the water from you on Vashon. Two suggestions:
#1-If you build a building or a car port use standing seam roofing: waterproof, doesn't wear out, easy to repair.
#2-I have a couple of those steel frame 'hoop' garages with a fabric cover. If you buy the good ones they can really last - I have one that's been up for 13 years and shows no sign of deterioration - if you go that direction for some relatively cheap cover make sure that you keep water from wicking into the building from the bottom. They make a fabric floor cover. If you just put it over dirt or gravel the cover will keep it all inside and you will find everything covered with moisture. We use the waterproof fabric floor and leave the roll-up door open all the time
PM if you want details.
