you could get away with 12mm up to 600 but will have deflexion and have to careful when working on the roof also hanging anything off the roof
and if you do opt for the cheaper option off sterling board or osb[orientated strand board] go for a slightly smaller center or thicker board
http://www.pbplywood.co.uk/sterling-board-osb-3.html
and if you do opt for the cheaper option off sterling board or osb[orientated strand board] go for a slightly smaller center or thicker board
http://www.pbplywood.co.uk/sterling-board-osb-3.html