Place on gravel with a weed barrier so top of gravel is higher than surrounding ground.

I would go with the metal roof and siding with a vapor barrier. If the garage is meant to be used as storage for your bird, you can also get a "Car Capsule" to park the bird in while it's in whatever garage you decide on. I used a Car Capsule for my 56 Customline while I was building my shop in Maine. Maine is a VERY wet place and I had no issues at all with moisture while my 56 was in the Car Capsule

  Ron Lane,  Meridian, ID

I am also considering a shipping container but the 8 foot width is a problem getting in  and out of the car. Do they make them with a side door? Could I put a side door in one that wouldn't leak?

I think where you live plays into this. I’m outside Seattle, and the best large garage I had was stick frame and plywood siding. First wife kept that . Two buddies have pole buildings with full slabs and metal walls and roof. We use insulation umder the roof, otherwise the condensation will rain on you. Termites and other wood bugs aren’t a big problem here, but I know they are some places. So, location is a big deal. Up here, metal would be the best for initial cost, and full insulation to stop the condensation on the weather changes.

I’ve stored cars here in 8x20 shipping containers with good tops (roof) that didn’t leak. A little exhaust fan cut in the wall, and a desiccant tub for a motor home (checked when needed) did a fine job. Some cars I had to climb out the window with only 8 ft.

Either way, it’s better than my used shipping containers.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

I'm thinking about buying a pre fab garage on skids with a wooden floor to store my T Bird so I have room in my shop to work on other projects. 12 x 20 should be ok. Debating on going with a metal roof with vapor barrier and metal siding or shingled roof and T5 wood siding. I am concerned about moisture condensing with a metal building. Less maintenance with metal. Either roof will outlast me but will have to paint wood siding eventually. Not a huge deal but something to consider Opinions?