Our concrete shed is from Hanson, and we fitted guttering as part of the outfitting process, along with the insulation and lining of the walls and roof. The raw building had a terrible resonance, so there had to be some kind of lining or you couldn't hold a conversation. The insulation means you can still work in it in winter, and the machines don't freeze or get covered in condensation. Sealing took a while, until we managed to find and eliminate all the tiny ingress points. If you don't seal, and don't install guttering then it'll be permanently damp, at least around Manchester!