I want to install a gas water heater in the corner, under the roof of my covered porch (no walls), but there are two windows that will be within 6" of the outside diameter of the heater. Can I do this and be okay with usual building codes, if I build a fire closet and vent the heater through the roof?

Thanks. Jack

No such thing as "usual" codes. Depends on what code your location uses. Suggest you contact the local permitting authority. Many use the" international fuel gas code".

What city and state do you live?

If it is an exterior Tankless Hot water heater with built-in vent then it is not OK. You have to installing it according to mandated clearances. If it is conventional tank type hot water heater then - in my area of operation - it is OK to install it within 6" of operable wall opening, like window or door. However, it has to be inside water heater enclosure with type "B" vent terminating above the roof

Hope that helps

Milo