I think a problem you might have with the single pitch roof is drips from condensation. Not sure of the answer to that, but good ventilation will help.
I think ou have an excellant idea though. I paid mucho money for a large polytunnel, which is great, but have access to timber, round or square and could have done something like you have very cheaply.
I have found that the polytunnel easily keeps moist compared to a greenhouse.
What are you planning to grow?
I usually try to cram to much in. I grew some great strawberries, which I know would grow outdoors but I had no problems with birds, and I cant have em outdoors as we free range hens and ducks.