1) The starter course is incorrectly installed face up.
One can tell immediately if a slate installer is experienced or not by looking at the starter course. If the starter course is laid with the face side up, the installer is inexperienced and it’s likely there are additional mistakes in the roof installation. The starter course slate is the only slate on the roof that is supposed to be installed with the face down and the back up (see photo below). All other slates on the roof are installed with the face up in order to expose the beveled edges of the slate (the face of the slate is the side that shows the beveled edge — the back doesn’t). The reason the starter course is laid face down is because the drip edge of the starter slate then merges flush with the drip edge of the first course of slate (see photo below). It just looks better. This is a traditional stylistic procedure that doesn’t have any practical effect on the long term functionality of the roof system.
Furthermore, when ordering roofing slates from the quarry, it is advantageous to order the starter slates separate from the field slates (rather than use the field slates as starter slates). A good slate supplier will make sure the starter slates are punched for nail holes only along the top of the slate and the holes will be punched on the front of the slate (rather than the back as is done on the field slates). This allows the slate to be laid face down and the nail-head countersinking to be on the back of the starter slate.
2) The proper cant or shim strip is missing.
The starter course must be angled on the roof in order to match the angle of all the other slates in the field of the roof (see photo below). No slate lies flat on the roof because every slate is overlapping other slates. Since the starter course does not have any other courses of slate underneath it, something must be installed to create the proper angle on the starter course. Traditionally, this has been done by nailing a cant or shim strip made from wood under the bottom edge of the starter course, usually the same wood that is used to create the roof deck (see photos below). Alternatively, the fascia board can be raised slightly in order to raise the bottom of the starter course. Also, if a metal drip edge is used, the drip edge can be formed with an inverted “V” groove (see video below) that functions in the same manner as a cant or shim (see photo below). Other types of shims have also been employed over the years, such as cedar shakes, plaster lath, etc. In any case, if the cant is left out, the slates will not lie correctly on the roof. This is usually manifested as a gap underneath the bottom of the second to fourth courses of slates (see photo below with circled snowguards).
3) Inadequate side laps are installed.
The side laps, or lateral overlaps between the side-butts of the starter slates and the first course of slates, should be a minimum of 3 inches. On a poorly installed starter course, these lateral overlaps will measure an inch or less (see photos below). When the side-butts of the first course are too close to the side-butts of the starter course, a path is created for water entry into the building. The photo below shows such a scenario, where there is virtually no side-lap. This is a certain leak. The people installing this slate roof in Arizona were inexperienced and are facing litigation. The starters were laid face-up (incorrectly) and no cant strip was used.
4) There is not enough headlap on the slate starter course.
This is a common mistake made when the field slates are turned sideways and used for starter slates and no adjustment is made on the first course exposure to allow for adequate headlap on the starter (see photos below). See video. The height (distance up the roof) of the starter slate must be at least 3 inches greater than the vertical exposure on the first course of slate. The exposure on the first course of slates may be reduced by dropping the chalk line for the top of the second course of slates in order to increase the headlap on the starter course. This little detail is often overlooked by inexperienced slaters. Since the bottom of the roof has more water running over it than virtually any other part of the roof, it is imperative that the headlap on the bottom courses of slate be correct. If anything, the slates on the bottom of the roof should have more headlap than the field slates, not less.
5) The starter course is incorrectly installed ahead of the first course.
It is important that the starter course and the first course of slates be installed at the same time, although many installers tend to run the starter course out ahead of the first course (see photo below). The exception to this rule is when the starter slates are the same width as the field slates (or a factor thereof), and the starter slates are only punched for nail holes on the top edge, then the starter course can be laid out ahead of the first course. In all other circumstances, such as when installing random-width slates or using field slates turned sideways as starters, the starter course and the first course must be installed concurrently. This is the only way you can guarantee correct sidelaps and make sure you don’t have stray nail holes in the starter slates in the joint between the first course slates, creating a leak (see photo below). When you lay the starter and first courses concurrently, you can trim each starter slate to the correct length (or to remove stray holes), if necessary, to avoid these problems.
For example, let’s say your field slates are 10 inches by 20 inches. For a 3 inch headlap, you will need an 8.5 inch exposure on the field slates. However, if you turn the slates sideways and use them for starter slates, you will only have 10 inches of slate going up the roof on the starter course. An 8.5 inch exposure on the first course of slates, then, would only leave a headlap of 1.5 inches on the starter course — a mistake. The exposure on the first course of slate must instead be dropped to 7 inches in order to create the 3 inch headlap on the starter course.
Alternatively, you could use a 12 inch high starter slate, thereby guaranteeing a 3.5 inch headlap on the starter course and an 8.5 inch exposure on all field slates. In fact, when you ordered the 10x20 field slates from the quarry, you should have also ordered 10 inch wide and 12 inch long (high) pre-punched starter slates. That would ensure you an easy, fool-proof job and you could go ahead and lay the starters out ahead of the first course.
In addition, if you use the field slates turned sideways as starter slates, then you have to punch an extra nail hole on one side of each starter slate in order to nail the slate to the roof sideways as all slates on the roof must be nailed with two nails. This will leave you with a “stray hole” (an original nail hole) along the bottom of each slate which can create leaking problems if it falls in the slot between two overlying first course slates. So even though the 20w x10h starter slates are a factor of the width of the 10 inch wide field slates, they cannot be laid out in advance of the field slates without risking a stray hole falling in an overlying joint. The starters and the first course must be laid concurrently in this situation.
The starter course, even though invisible, is arguably one of the most important courses on a slate roof. If it’s done right, you’re off to a good start. If it’s done wrong, you have created a slate roof with built-in long-term headaches. Some new installations even go into litigation because of problems with the starter course. If you do a little homework, educate yourself, and see and understand these potential problems before you start installing a slate roof, you will be doing both yourself and your roofing clients a big favor in the long run.