Why Tear Off the Old Shingles?

Reasons to tear off the old roof before putting on the new one:

1. Roofing is s system.  There's more to a roof than just shingles.  Roofs consist of underlayment, stack flashing, drip edge flashing, chimney flashing, wall flashing, valley flashing, vents, and ice and water shield. Additionally, there is an underlying structure to consider.  If you just shingle over an old roof, there are many potential areas for leakage that could go unaddressed.

2. Longevity. Roof ventilation is necessary to obtain maximum service life.  Multiple layered roofs are hotter and they deteriorate faster.  It's also difficult to get the new roof to lay flat over the old roof.  This makes the shingles more vulnerable to wind damage.

3. Fastener length. At a certain point, your fasteners aren't going to be long enough to bite into the sheathing. This means the top layer won't be as secure.

4. More expensive tear-off later down the road. "What goes up, must come down." More material on the roof=more labor and more dumpster space needed=more money. You can't keep roofing over old shingles forever. At some point you need to do a tear-off.

As a general rule, I prefer a tear-off for every asphalt roof replacement. When I see two layers already, I consider a tear-off a must.