ooops I said to Doc my last contact would be the last question on this subject it seems I told a porky.
I have made up a template rafer out of scrap wood with the birdsmouth cut into 1/3 or 33mm of a piece of 100mm ply , size the rafters will be, and fixed to 50 x25 to act as a template rafter. The conlusion is the template set [35deg] on a wall plate placed on mock wall gives the height of the roof as 93cm off the wall level marked on the house wall to top of rafter at ridgeboard. From house wall to outside wall of porch will be 1mt app [Using 9" Thermoblocks]
Question does 93cm sound right as I don't want to c.....up good timber rafters. The interlocking tiles to be used give a min of 35deg angle.
My apoligise for those who think I rambled on getting to the question.