Watersheds or side edges (where shingles butt together from side to side.)  must be offset from the same watersheds on courses above and below 5-5/8’. Otherwise, when it rains and water enters between these watersheds onto the nailing portion of the lower shingle, the water spreading left, and right can intersect a nail within this area used to secure the lower course resulting in a leak through same nail.

The lower the slope, the more water will travel laterally to find that nail before gravity finally pulls it down onto the exposed portion of the lower shingle. It is important to note, that the above-described offset of subsequent coursing assumes the nail location conforms to within the tolerance percent of the manufacturer’s recommendation for distances measured from the left and right sides of each shingle and of course vertically within the specified “nail zone” printed on each shingle comprised of two or three horizontal lines running the entire width of the shingle.

The shank of the nails must also fall within these lines. Nailing below the lowest printed line is an exposed nail head, (shiner) which in turn always results in a leak. This is a nail not covered completely by the next course above. The propensity for this to occur is more likely than most think despite how “good” the nailer is. I have found some manufactures better than others at having consistent distance between the exposure line- the line the next shingle up the roof is to be lined up with- and the printed nail line(s) within which the nails must be located.

These are usually three eights of an inch apart-about the width of a round head roofing nail. With most roofers I can hear nailing five nails per second, this gives only a three sixteenth of an inch grace area for either human error or manufacturing error to avoid a shiner. Unfortunately, almost all roofing companies are compensating the installer piecework- by how fast the roof goes on.  In this scenario the installer is not highly compelled to cross examine his workmanship as if he does this, he stops making money to do his repairs-time that could be spent installing the next dollar.

Fortunately, though a surprisingly small number of shiners are found on an average install. But even these could be avoided by dropping the gun gauge by one quarter of an inch to allow the overlapping course to further cover the lower shingle along with an attendant shiner. For those that use the factory exposure line to line up the next shingle I would just say is more than likely they were never properly introduced to the guns gauge. Its purpose is to assure that each course is exposed to the weather the same, exact distance.

We weld ours in place for 100 percent consistency. Using the exposure line is not a good plan as the manufacturing tolerances are too great to yield straight courses. And I have even seen occurrences where although the nail is within the nail zone, The next course lined up with the factory exposure line exposes part of the head of the nail. Nailing too high- above top printed nail line- will result in compromised wind resistance, as those nails are only securing one course and not the underlying course as well.

Also, over nailing (more than six nails per full shingle) will orient nails too close to these watersheds. Manufacturers recommend four nails per full shingle. “Storm” nailing is six, but most roofers will use five. Just remember though, most installers while enjoying the inherent speed delivered them by a pneumatically driven nail gun are always tempted to be thinking about the next shingle to be installed instead of the one in their hand. This blizzard blinds most roofers to the reality of where the nails wound up as they are very quickly covered up by the next shingle – especially if a helper is “feeding” the installer the shingles.

Gun Maintenance:

Another fine point is “dressing” the driver. Nails are stamped through the shingle and underlayment into the substrate in one motion. On an average 2000 square foot roof area, at five nails per shingle, this amounts to about 6,3000 times this driver is slamming another piece of metal! (Nail.) This activity will round the business end of the driver over time making it impossible for this driver to hit that nail square above the shank – on the top of the head. This is due to the random side slip of the driver as it travels down the length of the driver guide/channel. This is usually not a problem If  the end of the driver is as flat as is the head of the nail it is driving. An even and square relationship between the end of the driver and the head of the nail being driven is critical to the desired result of a round nail head seated flush and even with the top surface of the shingle being secured. Otherwise, the nail can be driven in crooked.

The gun also should have its safety adjustment set properly and while nailing, the gun should be oriented perpendicular to the nailing surface which prevents a crooked driven nail. When a nail is driven in crooked, one edge of the nail (the top or sides) will break through the composition shingle while the opposite edge of the nail will be elevated up off the surface of the shingle preventing the next course from making a proper seal across the lower shingle. Both byproducts affect the overlapping shingles ability to resist wind. If the overlapping shingle is stepped on, the raised edge of the nail can poke through the overlapping shingle resulting in a leak from same nail. In closing, there are a lot of ways to get it wrong but as is the case with so many things, focus first on the fundamentals and the speed will come, as a gun in the inexperienced hand is dangerous – even in roofing.