You can build a deck on a garage, provided you have a garage with a flat roof, or if you plan to build a garage that will be adjacent to the house but below the level of the main floor. A deck atop your garage may well save money compared to a deck built separate from your garage. Another compelling reason to build a deck on your garage may be because you have limited space or if you have a small garden. There are several issues to consider as you plan to build a deck on top of your garage.
Drainage and runoff
A deck is a flat surface, but roofs usually are peaked -- and for good reason. The angle of a roof allows for runoff of rain and melting snow. If you want to build a deck on a roof, first make sure the roof has a covering that will allow water to drain off. It's best to build the deck on top of that, since the decking will allow some water to pass through. If your roof has a slight pitch, as many roofs do that look flat, you can construct the understructure of the deck to level it off.
It goes without saying that adding a deck adds weight to a roof. Check with a builder, architect or building inspector to make sure your garage has enough support to hold the added weight, especially if this is an existing garage. Make modifications to the garage, if necessary, to support the extra weight. If you're building a garage at the same time as the deck, plan for the added weight of the deck in your garage plans.
Deck safety means railings are installed along all outside edges. Plan the height of these to be adequate after the decking has been installed. Since the decking will be higher than the actual rooftop, this means thinking ahead. The best choice is to attach the railings to the deck structure rather than the edge of the garage roof. Pay special attention to any area where the edge of the deck butts up against an exterior wall of the house (if there is a level of the house on the same level as the deck). Make sure there is not a dangerous gap next to the wall, while safeguarding the integrity of the siding on your house.
Any structure built for shade will add extra weight. A good choice for a roof deck is a patio umbrella, since it is lightweight and can be removed for the winter. A retractable awning attached to the side of your house may be an option if the layout of the house and garage allow for that. Plan ahead for any needed shade, such as a roof or pergola, and make sure the building plans include enough support for that load.
If your garage is attached to your house and if your house is more than one level, it may be that you can access the deck by walking out a door in the side of the house. This is an excellent option, especially if the garage is adjacent to a common area of the house such as the kitchen or living room. If the access ends up being through a bedroom, or if there is no such access, build an outside staircase to gain access to the deck from the ground.