Wrens will use a house that has an entry hole dimension of 1 1/8 inches, any larger and house sparrows and other less desirable birds will use them. The floor should be 4 inches square and 3-6 inches below the entry hole. Do not add or buy any birdhouse with a perch. The birds don't need it, whereas preadators and other less desireable birds will have easy access if perches are used. The roof should be slightly slanted to shed water and keep the contents of the nest dry.