Mastering Roof Inspections: Slate Roofs, Part 3

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. 




Standard slates are of standard size as opposed to custom-cut, and are the most common type of slate. Standard slates are ¼-inch (0.64 cm) thick, plus or minus 1/16-inch. They vary in length and width by no more than 1/8-inch and have a relatively smooth face.

In the past, standard slate tiles were available in up to 30 different sizes. Roofs with standard slates should look flat, even and uniform.

It’s not unusual to see slate tiles with large gaps between them on older utility buildings.


Textural slate is 3/16-inch to 3/8-inch thick. With individual tiles, texture and thickness vary.

Roofs with textural slate tend to have a uniform look similar to standard slate roofs, but the coarser texture and greater thickness of the slates result in stronger shadow lines, which tend to give these roofs more character.

Textural slates may be manufactured with the same variations as standard slates.


Roofs with slates of intermingled thickness use at least three distinct variations in slate thickness; for example, one home might use ¼-inch, 3/8-inch and ½-inch slates. 
Intermingled roofs may have a cobbled appearance and may have more thin slates than thick ones.
In intermingled slate roofs, look to see that variations in the slates' thickness are not preventing them from lying flat.
Heavy Slate 

A heavy slate roof uses slates at least 3/8-inch thick.

Thicker slates may be specified for a roof installation, but the thickest slates included in the order can’t exceed the minimum thickness by more than 1/8-inch.  This means that if the order specifies ½-inch minimum thickness, the slates may vary from ½-inch to 5/8-inch. 

In heavy slate roofs, slates up to 1 inch thick are not unusual, and these slates are heavy and rough. Slates this size may require framing that is heavier than what is typical for a roof, and you should be looking for signs of failure, such as rafter sagging, or cracking of interior or exterior wall coverings. Heavy slates seldom have cut butts.


Graduated slate roofs were typically installed on large institutional structures and churches. Slates vary in size, thickness and exposure, with the largest, thickest tiles laid along the eaves, and the smallest, thinnest installed near the ridge.


Salvaged slates are slates which have been re-used, such as in a repair. Because slates from different quarries have different aging characteristics, salvaged slates may weather or fail to change color at the same rate as the rest of the roof.

Special Order

Special-order slate roofs may use slates that are longer or wider than usual, are of a special color or cut, and/or have special fastening requirements.

Examples of special-order requirements might be lengths of more than 24 inches, or widths of more than 14 inches.

Manufacturing Variations

Depending on the style of the slates installed, you may see variations that are the result of the manufacturing process.


The typical length for a slate is between 10 inches and 24 inches. The length of the slate determines the amount of exposure.  It also affects the cost in different ways. Shorter lengths mean more slates per square, which increases the labor cost for installation.  Longer slates are more difficult to make, which increases their cost of manufacture.


Slates are available in single and random widths. Single-width slates are all the same width, from half the slate's length to 14 inches, although you may see exceptions. Single-width slate should be obvious immediately; vertical joints align in every other course.

Random-width slates vary in width, but most will be about two-thirds of the slate's length. A smaller percentage will be of larger widths. If you’re looking at a roof with many joints that don’t align at alternate courses, it’s probably a random-width design.

Staggered Butts

If you’re looking at a roof whose slate butts don’t align in clean, horizontal lines but instead jog up and down at each slate, then you’re looking at a staggered-butt installation. These roofs require slates of two different lengths. The tops of all slates in each course must align for the slates to lie properly.

Ragged Random

Ragged random slates align along their tops so that the slates all lie in the same plane, but their butts vary randomly, with different lengths, angles and lines. Some may appear to be broken. The same headlap and sidelap limitations apply to all slate installations.


Here, slates of different colors have been installed to form patterns.

All photos are courtesy of Slate Roof Central of Grove City, Pennsylvania.


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

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Mastering Roof Inspections
Roofing Underlayment Types
Inspecting Underlayment on Roofs
Fall-Arrest Systems
Roofing (consumer-targeted)
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