I'm building a fair size cupola to go atop my garage. It will basically be a large dog house shape. I plan to frame it like a little house, sheath it with plywood and cedar shingles and put a three tab shingle roof on it. I'm debating whether to frame the whole thing with treated lumber, or just the parts that rest on the roof. The problem I see with using treated lumber for framing is getting pieces that are straight enough and/or will stay straight. Any advice?

That would be my concern too!

Buy extra stock, sticker it for a week or so, pick the best pieces and return the rest is what I usually do!

Your little cupola will most likely not have any lengths longer than 4'.
PT lumber should fit your build just fine.
I would try to find lumber that is straight and dry to start with.
Good luck to you.

How about a weather vane on my his new cupola?

Yup, there will be a weathervane! That's the whole thing behind the Project. Wife and I saw a weathervane we just had to have. Now it needs a cupola to go under it.

Why bother with the pressure treated when you have it well protected from the weather.

George

I think if you really expect the framing to get wet it would be worth using treated wood otherwise I would just use SPF. I primarily only used treated wood for parts exposed to the weather or close to the ground where there would be a threat of termites.

Well, it does have louvers that could let wind driven rain in, but I'd think the louvers should also let it dry out.

If there's dry, straight pressure treated out there, I haven't found it yet.

I need to make a deck for someone out of pressure treated wood so I bought the lumber a month ago and stacked up and stickered it to let it dry. You might look for it. There is a wood called KDAT coming on the market. It stands for kiln dried after treated.