Greenhouses and other outdoor structures tolerate some pretty nasty weather and do not enjoy the same protection that we give our homes.
Poly (plastic) covering has a limited lifespan, usually 5 to 10 years. Special tape allows holes and tears to be mended, but eventually the plastic degrades and needs replacing.
Common structural problems are rotted baseboards and hip-boards usually made of hemlock or pine. Wood is often the weakest link in the growing environment due to high humidity and constant exposure to the sun. Cedar will last longer and cost more. In most cases I will recommend metal as a superior alternative. Steel and aluminum might have more up-front cost but they will not rot.
I offer a range of metal repair services and can fabricate any part necessary, structural or otherwise. Aluminum, galvanized steel, and stainless steel are all viable options for exterior applications.