I'm about to convert my deck into a greenhouse. It is 20x15, so it will be a great size. The deck is two feet off the ground and well supported. My plan is to remove the railing, construct a frame and mount 75" by 72" doubled-paned windows on a three foot base wall. For the roofing I'm going with 10mm Polycarb. The back wall, the one closest to the house, will be a solid wall painted black to absorb the heat. Although it will be attached to the house by a door, I won't be running venting into the greenhouse, instead I plan to install a Natural Gas heater to suppliment the air transfer heat in the winter. In the summer I'll close off the door in the day and use an automatic fan to vent the warm air. I also plan to have two roof vents that I can open.

I have several questions. First, should I place some water mass under the deck to act as a heat sink? Or, should I seal off that bottom part completely with a small brick wall all the way around the deck? I plan to cover the decking floor with a rubber matting, then put indoor-outdoor carpeting down. The room will be used as a reading room as well as a greenhouse.

Are there things I should look out for in this conversion? Weight issues? The footers and supporting deck subsystems are in great shape and although the deck is fifteen years old, it shows no sign of wear.

I plan to run electrical into the room, but not water. Our home is a log home and getting to the pipes would be a problem. Also, I try to use rain water as much as possible, so I have to haul it from barrels anyway.

My plan is to grow year-round vegetables and flowers. We start our large outdoor garden in our smaller detached greenhouse now, so we might expand that operation as well. Mostly, I just need to work around growing things all year long, especially in the cold dark winter and my current GH (8x10) is just too small. I keep it, of course, as well, but the attached one will be more of a "luxury" room.

I also plan to use 55 gallon barrels as bases for tables so that I get water mass. I also will probaly line the walls of the black wall with 2 liter bottles painted blakc. That works well in my smaller house. I want to get about 1500 pounds of water in the new GH. My engineer friend assures me that my deck can withstand that, especially after we add several new base posts underneath.

I'm looking for advice, suggestions and warnings please.