Water – How to tie a new roof in to an existing roof with a pitch transition

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I added a room to the side of my house, and the new roof goes onto the existing roof. I am almost done roofing it. The image below depicts the current status. I put roofing felt down under the two rows of the existing shingles as shown in the image. I have a roll of aluminum flashing that I plan on putting over the felt at the roof intersection where there is about a 1/2" drop from the new roof OSB to the existing roof. After that I have just one more row of rolled roofing to install. Here are my questions.

  1. Does the new felt go far enough under the shingles?
  2. Should the flashing cover the end of the new felt as well as the intersection?
  3. How far under the shingles does the rolled roofing need to extend?
  4. How do I attach the flashing and rolled roofing under the shingles?

I'm thinking that I would lift up the second or third row of shingles and nail under them with the rolled roofing in place.

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Best Answer

  1. The new felt should go under the old felt.

  2. The new flashing should go under the old felt and over the new felt and new roll roofing.

  3. Depends on the slope of the new roof. The lower the slope the further it should extend.

  4. Roofing nails to the existing structural sheathing.

The big idea is that layers of the lower roof should sit below corresponding layers of the upper roof. That is:

  • Everything should lap just like shingles.

  • Nothing should back-lap.

  • The flashing should double lap the roll roofing.

  • Even though the higher roof was installed first, the end result must be the same as if you started at the eave and worked all the way to the ridge.

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