How To: Tear Off the Roof Deck and Prepare for Shingle Installation
GAF- Mastering The Roof (chapter 04)

Nothing is more critical in roofing than preparing the surface underneath the shingles. Sometimes, there can be hidden damage under the shingles
With a roof tear off, when the old materials are removed, you’ll have a clean deck that allows you to more easily see any potential problems.

If there is reason to believe that there is damage beneath the shingles, it is generally a good idea to tear them off. If the old roof is still in place, it’s more difficult to know what’s underneath.

There could be rotted wood or cracked boards. Also, if the deck is not sound it may not hold the nails as well.

Any of these problems could telegraph through the new roof, and make the finished job look bad as well as compromise the performance of the system.

Remember, cedar shakes, tile,
and slate roofs must always be torn off before installing asphalt shingles.
They are just not the right surfaces for shingles.
Here are some suggestions for removing old asphalt or wood shingles.

To Perform a roof tear off professionals will use specially designed roofing tear off shovels, a large flat shovel, or a serrated edge shovel to remove the existing roofing materials. It’s better to use good tools because the tear-off will go quicker and easier. Remove all debris and waste materials from the deck, and then sweep it clean.

On most roofs it is best to start removing the shingles at the ridge or the top of the roofing area. Otherwise, the debris may fall through the open sheathing and into the house where it could cause additional clean up problems.

Fill in any empty spaces with new boards to provide a continuous smooth nailing surface for the new asphalt shingles. Another option is to apply a new approved deck sheathing. If the deck needs replacing, use either high quality exterior grade plywood, sheathing boards, or The Engineered Wood Association approved exterior grade, non-veneer structural panels.

When installing plywood, be certain to have a 1/8” space between the panels over the rafter boards. This space allows for plywood expansion movement at the joints in the plywood. Sometimes plywood can move enough to create a buckle that will push up roofing materials.

Make sure the decking you install is dry. Failure to use dry materials can result in deck movement that will distort the overlying roofing materials.

For more information, visit GAF.com