Carpenters have suggested t1-11, James Hardie panels and pine plybead panels which are 4x8 sheets, about a quarter inch thick, with the look of bead board. Tongue and groove is too expensive, so I have to go with something that won't look as good. All are OK looking as far as I'm concerned, so my decision will be based on which will perform best in a porch ceiling application.
Porch is L shaped with one section 5 feet deep 16 feet wide, which will have the sheets of whatever is used running parallel to 5 foot "dummy" 16 inch OC joists, which are for appearance only, not structural. Should nailer boards be inserted between the "joists" to provid perpendicular surfaces to nail to, or will the 16 inch intervals be sufficient for sections that will only be 5 feet long?