DIY Deck Greenhouse
A greenhouse is essentially an area with a glass covering over it where plants can grow in a regulated temperature. The glass above allows suns rays in but doesn't allow heat to get out easily (hence the term "greenhouse effect" though that applies to something different these days). So turning a deck into a greenhouse is a reasonable idea, as the existing deck is basically an existing floor for the potential greenhouse. Now all you need to do is install glass overhead and close it off.
Closing Off the Railings and Floor of Deck
Most decks are made out of wood or composite wood, so when converting them into a greenhouse, the air within the greenhouse itself must not be able to easily escape. This means sealing off any and all cracks in or between the wood (or whatever material the deck is made out of) and covering the openings in the railings, if there are railings on the deck to begin with. Close off the cracks in the deck itself with caulk or thick plastic sheeting that can be laid out on the floor and secured with screws or tape or a layer of gravel. To "wall off" the railings, install thick pieces of hard plastic along them, by screwing them into the railings themselves. These large pieces of hard plastic sheeting can be bought and cut from home gardening stores or industrial supply stores.
Once the existing deck has been closed off from the outside by covering the floor and putting plastic along the railings of the deck, a frame needs to be built for the ceiling of the greenhouse. A rudimentary frame can be built simply out of PVC pipe; wooden and even metal frames can be constructed off the basic metal and wood of the existing deck with a bit more construction effort and money. The frame must be solid enough to hold, at the very least, heavy plastic sheeting which will be draped or attached over it. If thick plastic sheets or pieces of glass are going to be used for the greenhouse's roof, then a wood or metal frame should be constructed to ensure the roof holds up in the event of strong winds or rain.
Greenhouse Roof Material
The greenhouse roof--the area that allows heat and sunlight into the greenhouse itself to help the plants within it thrive--can be made out of a variety of materials, all based upon how much light will work within the greenhouse itself and how much you can afford to spend on materials. As the deck foundation has probably already saved a good deal of money in the construction process, then building a real frame for the greenhouse out of wood or metal and installing real sections in the walls and roof for plastic or glass sheeting is a permanent idea that will yield better results for both your plants and the aesthetic look of your greenhouse deck. If it is too expensive to do this, thick, clear plastic sheeting will have the same effect as these other materials, but may break down over time with heavy rain and wind from outdoors and require more maintenance.