Preparation

• Remove the roof and top half of the walls of the garage. Consider keeping the framing intact to more easily install the windows. Keep the garage door as the entry for the greenhouse or remove it and build up a half wall with windows at the top , and a door. Although keeping the garage door is a cheaper choice, removing it and installing a traditional door makes entering and exiting more convenient.
  
  If you do not have a drainage system in the garage consider hiring a plumbing company to install one. Plants benefit from a watering that runs through the soil and drains out of the bottom of the pot. Although most pots have a saucer underneath to catch drainage water, it's tough to guess the proper amount of water. Accidental overwatering can extend beyond the saucer, therefore a drainage system will help prevent standing water. If you are planning to house a large amount of plants, a water draining system is almost a necessity because greenhouse plants benefit from a watering that mimics rainfall and its hard to control the amount of water in that type of watering.
  
  

Installing the Windows

• The windows are the most important feature of the greenhouse. They enable light to reach the plants and provide ventilation so that air can flow through the building. Choose windows for the walls that have a manual open and close ventilation system. The windows for the roof should consist of only glass. If you put windows with a manual ventilation system on the roof you risk getting unwanted water into the greenhouse through leaking ventilation. The manual systems are comprised of slats that may not seal together when they are closed.
  
  If the wood frame slats of the garage are close together, cut out some of the slats to create room for the windows. Measure the openings to order windows so that they fit into the openings perfectly. Install each window into the opening by lining the window edges with a moisture seal and then fitting the windows into place carefully. If the window has holes in the exterior frame for attaching, place a nail or screw in the middle of each hole and secure the window to the frame.