3/4″ wood stock is the most common. 5/8″ stock where called for can be supplied with fencing material from most lumberyards. 5/8″ stock is used on the smaller nest boxes because 3/4″ stock is too thick and makes them look odd.
Make pilot holes slightly larger in the primary work piece (first piece in which the screw is inserted) such that screws can be turned in easily without leaving room for movement.
Screws should be more snug in secondary work pieces so that screws can be tightened, but not so tight as to split the wood or to strip the hole and loosen the screw.
Hinges are suggested for easy access. Other methods for attaching roofs are just as good. However, most nest boxes are mounted out of reach for most people.
Birdhouse mounting, monitoring and maintenance on ladders are awkward chores that require “three hands”. Convenience increases safety when working at heights.
You shouldn’t have to pay for any wood if you keep your eyes peeled. Watch for wood scrap piles at fence and home construction sites. These are good sources for the small sizes usually needed for bird houses.
Robins, Phoebes, Mourning Doves, House Finches, Song Sparrows, Blue Jays, Hawks, Owls, Osprey