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

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

Bill can you tell me that if i build a roof with 75x50 and the the roof is at 30 degrees then i build a roof with 150x50 at 30 degrees, will the two roofs have different rises because of different size timbers i'm using? will the 150x50 roof be slightly higher?

Bill's reply

rafter depth

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.

Cheers Bill

If you didn't find exactly what you are looking for try this search tool that will search the site and the web.

"What can be added to the happiness of a man who is in health, out of debt, and has a clear conscience?
Adam Smith 1723-1790

"When we build, let us think that we build for ever."John Ruskin 1819-1900

coates hire
Hire Equipment
Furniture Fittings - Architectural Hardware - Electronic Locking Systems - Technical Hardware
BuilderBill sponsorship

BuilderBill Books

building maths
Building Maths

Stair Design
asbestos ebook
Asbestos Book

Calculator Pages.

Concrete yardage calculator

Reader's Questions.

Questions and answers.



Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
You must check your own local conditions.
Copyright © Bill Bradley 2007-2012. All rights reserved.
Page copy protected against web site content infringement by Copyscape