A reader's Roofs and roofing question: Roof rise?

Craig Mc.   from   cumnock, United Kingdom   had this rafter depth to roof rise question.

Bill's reply

Quick answer NO!

• At the top on the right is a triangle.
• Let's call it the roof triangle.
• That triangle may have been worked out from design information that is given.
• In the UK and Aus. it may be the half span of the roof and the angle in degrees.
• In the US it is more likely to be the ratio of the rise and the span.
• Once you have the triangle, whether it is a drawing on a sheet of ply, some calculations from a scientific calculator or a setting on your roofing square, you do not change it.
• What you may change is it's position in the building as a whole, so it will indeed be higher with deeper rafters.
• BUT.. the rise, the angle, the ratio the half span and the rafter length will not change.
• The thing that does change is the depth of the rafters and maybe whatever depth you select for a birdsmouth, if any.

Craig, closely related to this is another reader's question about rafter lengths. Don't get trapped into measuring say from the top of the ridge to the wall plate and thinking that it is the rafter length. Look at this page.