Mastering Roof Inspections: Slate Roofs, Part 7

by Kenton Shepard and Nick Gromicko, CMI®



The purpose of the series “Mastering Roof Inspections” is to teach home inspectors, as well as insurance and roofing professionals, how to recognize proper and improper conditions while inspecting steep-slope, residential roofs. This series covers roof framing, roofing materials, the attic, and the conditions that affect the roofing materials and components, including wind and hail. 

INSTALLATION (continued)


The sidelap is the separation between vertical joints in adjacent courses. The minimum sidelap between slate edges is 3 inches. Slates should also have a minimum sidelap from the centerline of nail holes of 1½ to 2 inches. Ideally, vertical joints should center on slates in the adjoining courses.

Headlap or Exposure by Slope

The headlap is the amount by which alternate courses overlap, as you can see in this illustration.

The formula for slate tile exposure is based on half the shingle's length. The standard minimum headlap is 3 inches, so the exposure for a 20-inch slate is:
20 - 3 = 17 ÷ 2 = 8½ inches.

According to modern building codes, if the roof is 4:12 or greater but less than 8:12, the headlap should be a minimum of 4 inches.

The exposure for a 20-inch slate is:
20 - 4 = 16 ÷ 2 = an 8-inch exposure.

If the roof is 8:12 or greater but less than 20:12, the headlap should be a minimum of 3 inches.

If the roof is steeper than 20:12, the headlap can be reduced to 2 inches, and the exposure is:

20 - 2 = 18 ÷ 2 = 9 inches.

These are modern headlap minimums and may vary by area. In regions where snow is not an issue, it may be common to find 2-inch headlaps, especially on older homes.

Gable Slates

At gable ends, slates should also overhang the roof edge by about 1½ to 2 inches. This means that the pre-punched nail hole nearest the roof edge will be too close to the edge. The roofer should punch another hole that will be about 1½ inches in from the roof edge once the slate is in place. You should check that gable slates are securely fastened.


The methods used to install hips and valleys will give you a good idea of the level of expertise of the installer.  A variety of methods can be used, and some require more expertise than others. An assessment of the installer’s level of expertise is helpful because there are some parts of the system that will be hidden from view. If the visible work is done well, there’s a good chance that the hidden work was also done well. If the visible work is poor, the chances are that the hidden work is also poor.


You may see both open and closed valleys.

Closed valleys are not closed in the sense that the slate forms a continuous moisture barrier across the valley, but in the sense that there is no gap down the center of the valley. Beneath the slate, the valley should be lined with heavy-duty underlayment or metal.


Open valleys are preferable. The preferred alternative to continuous flashing is to use short sections of flashing that are installed with each course. This is simply another form of step-flashing. Here are some tips on inspecting valleys.

Open valleys should be installed with a taper so that they’re wider at the bottom than at the top, which helps keep them clear of debris. Valleys that don’t taper are not defective.  It’s more of a quality issue.  A taper is desirable.

Field slates that meet with the valley -- called valley slates -- need to be cut to the angle of the valley. These cuts should be chamfered and the chamfer should face up, just like the rest of the slates.

Before the valley slates are fastened, they each need an additional hole punched to avoid nailing through the valley metal.  Heavy slates should have two extra holes. These holes should be punched above an existing nail hole, rather than in the center of the slate. If they’re in the center of the slate, they’ll align too closely with the vertical joint in the course above, increasing the potential for leaks.

In the past, valleys were occasionally built rounded, but these are rare and require a great deal of expertise to install correctly. If you see a rounded valley, you won’t confirm proper installation.  Just look for signs of leakage.


Learn how to master a roof inspection from beginning to end by reading the entire InterNACHI series: Mastering Roof Inspections.

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Inspecting Underlayment on Roofs
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Roofing (consumer-targeted)
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