I am building a timberframe porch (not heated). I plan on using tongue & groove pine (1.125" thick) nailed directly onto the trusses and purlins. On top of the t&g, 3/4" plywood, roofing felt, then shingles. I need to span areas as large as 48" x 72". Roof pitch is 12-12. Is there a table on maximum span for roofing materials or does anyone have any recommendations?

For porches we usually space the rafters or purlins at a maximum of 4'-0" O.C., and then apply 2x6 T&G decking and then 3/4" T&G exterior sheathing over that.