As with all gas water heater installations it is very important to have sufficient combustion and dilution air to insure proper drafting of the exhaust products and safe water heater operation.
Gas hot water heater in attic.
In addition to all the regular code requirements for a water heater when the water heater is installed in an attic you must first have an attic access hatch that is a minimum of 24 x36 and even more for larger water heaters.
A transfer grille from the ceiling of the water heater room to another room of the house will allow combustion air to be pulled from conditioned space eliminating the flow of hot air into the attic in the winter and out of it in the summer.
Leaks from a water heater can be major depending on where the leak is coming from damage can be to attic flooring insulation drywall and even flooring if it leaks a long time a 50 gallon water heater will overflow the pan and then some if it bursts or the bottom rusts out mold can form from the moisture in your attic.
As outside temperatures increase the need for proper combustion air and attic ventilation becomes even more critical.
Giant tanks of water happen to take up a lot of square footage and primarily for that reason many builders have opted to place water heaters in the attic of new homes.
Remember the water heater must fit through the hatch to permit future change outs.