Re-Roof Plank Movement

Tyson from Nova Scotia asks: I am in the process of re-roofing my house. The roof is 20' x 40' (one side)and a 12/12 pitch. the house is 180 years old. the rafters are 4"x 6" and are spaced 36" OC. It was decked in 1x rough sawn boards of different widths. 6" to 16" wide. I had to replace all the boards since they were rotten. I replaced them with rough sawn 1x 12 boards (Pine dried to 15%) nailed with 3 1/2" spiral galvanized. The knots are tight. Gaps between the boards are 1/8 to 1/4. I used Tyvek 120 duraguard as an underlay. My concern is with shinkage of the boards and if it will cause problems with my asphalt shingles. I was considering putting 3/8 OSB down on top of the boards to minimize wood movement. I got advice from 4 different carpenters that it would be fine to roof over the 1 x12 boards without the OSB with no problems. Any advice would be greatly appreciated. 

Henry Staggs from The Arizona Roofer says: Install the OSB over the planks. That's going to ensure the roof will last and reduce nail pops and other issues.