Mastering Roof Inspections: Slate Roofs, Part 13

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. 



You should be sure that you’re looking at real slate. A number of companies produce artificial slate tiles that are cast using a mold made from stone slate. The industry term is composite slate. Composite slates from different manufacturers have different installation and maintenance requirements, as well as wear characteristics that differ from each other and from natural slate.

Unless the seller provides documentation, you’ll have no way of knowing the manufacturer’s installation requirements, and you should make that plain in your inspection report. The best you can do is to look for obvious defects, such as poor flashing methods and inadequate sidelaps. Sidelaps for many types of tile, wood shingles, slate and shakes are 1½ inches, so if you see less than that, recommend confirmation by a qualified roofing contractor.

From a distance, composite slate can be mistaken for real stone.

Here’s a composite slate. The clue to recognizing composite slate is the straight, outside edges of the tile. Stone slate has rough edges.

You can also spot composite slate by looking at the underside. Composite slates are cast to look like slate only on their top face.

These particular slates are from Global Manufacturing and are made from recycled materials. The advantages to composites are that they’re much lighter than slate, have Class 4 impact rating (the highest rating), and can be installed over a standard felt underlayment using a pneumatic gun.

Composite slates have been made from many different materials, such as:

  • recycled plastic;
  • thermo-plastic olefin (TPO);
  • rubber;
  • fiber-reinforced cement;
  • extruded concrete;
  • clay;
  • limestone; and
  • slate particles.


Although composite slates typically have a 50-year limited warranty, their manufacturers have not been in business that long yet. A few have been in business for over 20 years. Many have entered the business but lasted only a few years due to numerous warranty claims.

When composite slates fail, it’s likely to be in one of the following ways.

  • Failed Coatings

At least one company uses a coating to color their slates, and there have been some coating failures. Most manufacturers mix the pigments with the material so that the color goes clear through the tile.

  • Curling

A few of the thinner slates will curl if they’re not pre-bent when they’re installed. Unless they’re hollow, this can often be corrected using double-sided tape.

  • Inadequate Headlap

Leaking is often caused by improper installation.

The headlap for composites is 3 inches minimum, which is the same as for stone slates. You may see 2-inch headlaps on roofs steeper than 12:12, but any slope less than that should have at least a 3-inch headlap.

You’ll have no way of knowing during an inspection what the actual minimum headlap required by the manufacturer is, but if you see less than 3 inches, you should recommend evaluation by a qualified roofing contractor.

  • Excessive Brittleness

Many manufacturing companies that have closed created composites that became excessively brittle over time. After a while, the slates just cracked and fell apart.

Look before you walk!

It’s crucial that you examine these roofs carefully before you try to walk them. Some types of failed composite roofs are impossible to walk without causing damage.  So, from a window, deck or ladder, check out composite slates closely before you step on them.

Some types of solid, polymer-based composites can be walked as long as the sun is shining on them, but get very slippery as soon as the sun goes down. These tiles are typically smoother and stiffer than rubber composites that are safer to walk.


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

 Take InterNACHI’s free, online 
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Mastering Roof Inspections
Roofing Underlayment Types
Inspecting Underlayment on Roofs
Fall-Arrest Systems
Roofing (consumer-targeted)
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