All shingle replacement projects begin with a complete tear-off of the existing shingles or wood shakes down to the substrate. There are several reasons for this:
1) It allows us to inspect and correct any flaws in the wood substrate such as delaminated and/or rotted plywood from small long-term leaks or issues with the original installation of the decking. (This is particularly important along roof valleys and around chimneys.)
Additionally, on roofs where there is wood fascia just below the roof line, it allows us to inspect and correct and damaged sections. (It is not unusual for water to get behind wood fascia if the tie-in between the edge of the roof and the fascia wasn't done properly.)
2) Even where local building codes permit installation of a second layer of shingles,
the new shingles will never lay completely flat, typically causing slight rippling or ridging of the new shingles no matter how carefully they're installed.
3) In the case of wood shakes, the substrate is typically space sheeting rather than solid plywood. We're not aware of any local municipalities which still permit the use of space sheeting; this requires the installation of 1/2" plywood with the seams between the sheets properly staggered for structural integrity.
With any type of roof system, starting with the smoothest, flattest possible substrate will yield the best finished product in terms of service-life and appearance.
After preparing the substrate, we install a double-layer of staggered 15# base sheet. While the second layer isn't a requirement in most municipalities, the minimal additional cost of materials and labor add to the long-term integrity and longevity of the new shingles.
We install heat-welded APP modified bitumen membrane around the base of chimneys, along valleys and at any other potentially vulnerable area. (See the section on Flat Roofs for more information about this material.) We then install metal "saddle stock" flashings at the upslope side of chimneys and metal flashings along the valleys.
Next, we install drip-edge metal as required and new galvanized flashings around all penetrations (plumbing stacks, dryer vents, etc.) through the roof deck.
Finally, we apply the shingles using galvanized nails, not staples, as three decades of experience have shown us nailing is a better method.
Lastly, the shingles along the valleys are carefully trimmed to help prevent accumulation of debris and furnish a neat, finished appearance; rooftop flashings are painted to resemble the new roof and we apply elastomeric caulking as required to all metal roof components.
This is followed by a thorough final clean-up, and your new roof is ready to perform its job and enhance the appearance of your home for many years.