All successful roof projects require that the installers adhere to proper application methods. Improper workmanship contributes to a decrease in the roof systems service life. All types of roof systems - both steep-slope and low-slope applications - are prone to potential workmanship errors. In the case of steep-slope shingle applications, there are a number of common mistakes that should be avoided to ensure a long service life. A successful roofing project and satisfied client can be achieved if these common mistakes can be avoided in the field.
Mistake: Leaving out the starter strip.
Problem: Does not provide proper base for shingle application.
Proper application: Starter shingles should be applied at the rake and continue along the eaves. The starter course shingles should be cut to match the exposure of the existing first course. For remedial applications on three-tab shingles, this is 5 inches; for new construction, it is 9 inches. Trim approximately 6 inches off the length of the first shingle to offset or stagger the shingles from the first full course. Application should be in accordance with the shingle manufacturer's latest printed specifications.
Mistake: Shingles do not overhang at the eaves.
Problem: Could contribute to roof blow-off.
Proper application: The shingles should overhang the eaves and the rakes by a minimum of 1/2 inch. There should be a 1/16-inch spacing between the shingles. Nailing should be completed 3 inches above the eaves using the proper amount of nails for the geographic wind zone.
Mistake: Improper shingle alignment.
Problem: Not aesthetically pleasing.
Proper application: Shingle alignment - both vertical and horizontal - is required. The best way to ensure that the shingles are horizontally aligned in new applications is by the use of a chalk line. On recover applications, the new shingles should be aligned and butted with the existing shingles. Set horizontal chalk lines every 10 inches from the bottom of the first course up the ridge. Set vertical chalk lines every 36 inches from the roof ridge to an end of every shingle along the first course.
Mistake: Improper shingle nailing.
Problem: Potential for roof blow-off.
Proper application: Install the proper amount of shingles required by the manufacturer in the specific geographic wind zone. Most three-tab shingles require four nails in typical applications and six nails in high-velocity wind zones. The placement of the nails is as important as the number of nails. Place nails in the manufacturer's required placement areas. Most dimensional shingle manufacturers now provide shingle placement zones. Nails should be driven straight into the deck. Nailing from an angle should be avoided, and nails should penetrate through the deck a minimum of 3/4 of an inch. Standard roofing nails have barbed shanks and are typically 11-gauge or 12-gauge nails with heads from 3/8 to 7/16 of an inch in diameter.
Mistake: Use of asphalt-based cements for shingle repairs.
Problem: Incompatible materials will contribute to further shingle delamination.
Proper application: Asphalt-based cements should only be applied at the underside of shingles. Deteriorated shingles that show evidence of curling, cracking, splitting or openings should be replaced with new shingles.
Mistake: No underlayment application.
Problem: Roof leaks.
Proper application: Application of underlayment provides benefits to the roof system at the deck and shingle components. These benefits add to the long-term weatherproofing success of the roof system. Another primary reason for their use is that most building codes require the application of underlayments on steep-slope roofs. Underlayments are also required on lower slope (2:12 to 4:12) shingle applications. It is imperative that contractors familiarize themselves with the codes in their work areas. Installations that are not completed in accordance with codes can be very costly to remedy, particularly in this case where the remedy would include the removal of the steep-slope covering.
Underlayments serve as a weatherproofing barrier, protecting the deck from moisture prior to shingle application, and they can be applied to dry in the roof in advance of the shingle installation. This is of particular importance on new construction projects where there can be a lengthy lapse between deck installation and shingle application. Moisture absorption in the deck could lead to delamination or warping of the deck surface, providing an unsuitable long-term substrate.
In the completed system, the underlayment provides secondary protection against moisture infiltration. It prevents moisture from entering into the interior spaces if the shingles are lifted, displaced or torn. The underlayment also provides added protection to the applied shingles by separating the contact of wood resins and the shingles. Some wood resins can have an adverse effect on the shingles, contributing to deterioration over time. Underlayments also serve in leveling the deck by bridging over any slight irregularities and creating a flat and even surface for the shingle application.
Mistake: Improper termination of penetrations.
Problem: Roof leaks.
Proper application: Most residential roofs have soil stacks that require proper terminations. Apply shingles up to the vent pipe and cut an opening in the shingle that is set over the pipe. Set the shingle in an approved asphalt cement. Set the vent flange over the pipe - flat on the roof - in asphalt cement that has been evenly applied over the bottom shingle. Resume shingle application and set the successive courses around the pipe. Upper and side shingles that overlap the vent should be set in asphalt cement. The asphalt cement should be applied in an even application in moderation. Do not apply asphalt-based cement over the top of the shingle. Excessive asphalt cement application may cause blistering.