The term 'roof pitch' describes the ratio of the rise to that of the width and to measure that you will have to make some effort as it is not the usual measurement of height.
What you will have to do is to take the measure from the top of the roof by placing a bar of wood from the corner of the roof to that of the other side of the floor. After it, you will have to mark a level at twelve inches. When you mark it, you should measure the mark from the surface of the roof and by following the process, you will get the rise.
But there exists hiccups in this method and if the surface of the roof is not regular, then you may get a result that can not be called precise.
What you will have to do is to take the measure from the top of the roof by placing a bar of wood from the corner of the roof to that of the other side of the floor. After it, you will have to mark a level at twelve inches. When you mark it, you should measure the mark from the surface of the roof and by following the process, you will get the rise.
But there exists hiccups in this method and if the surface of the roof is not regular, then you may get a result that can not be called precise.