You will need to calculate the Ridge Adjusted height (rah) if you plan to support your ridge first. The two methods used to affix rafter to ridge are;

• Cutting two sets of common rafters and slipping the ridge between them.
• Fixing the height of the ridge first and then nailing the rafters to it.

  If you use the second method you will need to adjust the height of the ridge so that your rafters meet at the corner of the ridge and not below it.
  
  You must deduct the rise amount for that pitch roof that is equal to 1/2 the thickness of the ridge. In this case we are using an 8/12 roof in our example so;

  1. (rah = (8/12) * 1/2rt)
  2. (rah = .666 * 1.5")
  3. rah = 1"
     
     

  You would need to adjust your ridge by 1" lower to plan the rafters to the corners of the the example ridge.