Inspecting Roof Coverings Checklist Part 3: Slate

In our ongoing look at roof coverings and the problems reported on by home inspectors, we’ve already explored issues associated with asphalt shingles and cedar shakes/shingles. If you’ve been reading closely, a common theme has emerged when discussing these critical elements of a home: Nothing lasts forever.

This adage rings true for every part of a building, but it’s especially applicable when dissecting the many defects that can plague a roof, which is exposed to wind, rain, ice, snow, aggressive critters, fallen limbs, brutal sun, and wide temperature variations. Add poor installation practices to this list of factors and it’s no wonder that countless roofs need repairs or complete replacements long before their intended lifespan. Today, we turn our attention to a more expensive and far less frequently seen option than asphalt shingles—slate.

Because of the nature of slate, your inspector will not be walking on the roof, as it poses a risk of falling as well as the possibility of breaking roofing tiles. Other inspection methods, such as viewing the roof from a ladder propped up against the roof’s edge, observing from a balcony, or using binoculars or a drone, will be employed for safety reasons and to prevent damage.

As many homeowners with slate roofs discover, even this most durable and long-lasting of coverings (from 75 to 200-plus years) can have its share of problems, eventually giving in to normal aging and Mother Nature. The lifespan and properties of the slate (e.g., porousness and the ability to retain its natural color) will be largely contingent on the quarry, or even the section of the quarry, where the stone originated, with denser varieties being less prone to water absorption and faster deterioration. Because the quality of slate from even the same quarry can vary widely, it is not possible through observation to unequivocally identify what kind of slate is on a roof or its lifespan, although a history of when the roof was installed will provide the inspector with some useful clues in this regard.

Here is a brief checklist of what the certified inspectors at A-Pro Home Inspection have found when checking slate roofs over the last 27 years.

Weathering: As noted in previous posts, the weather plays a significant role in shortening the lifespan of a roof covering. Slate roofing materials will become more porous over time. Evidence may be observed of delamination of the surface cleavage plane as well as damage to internal layers. Delamination invites moisture into the tile that will eventually lead to loss of strength, softening, crumbling, chipping, cracking, and breaking at the corners. A close examination will help the inspector determine if the delamination is limited to the upper surface of the tile (less of a concern) or if the entire body has been affected (a much bigger problem). Your inspector will also note a significant presence of a white powdery mineral deposit known as efflorescence—a red flag that the roof has moisture-related issues. Other defects, such as a deterioration of tiles due to mineral inclusions, will also end up in the home inspection report.

Cracks may also be the result of impact damage from, say, a fallen limb or an individual previously walking on the roof. While it will be difficult for the inspector to assign blame to either manmade or natural factors, all such defects will be highlighted.

Underlayment: Slate roofs will likely outlast their underlayments, which will deteriorate over time and may contribute to roof leaking, especially where slate tiles have not been professionally installed. Your inspector will note if they were able to view the underlayment, the condition of the underlayment when possible, and absence of an underlayment, which is not uncommon on older slate roofs. Another red flag for an inspector is the use of an interlayment installed between courses of slate. Like the underlayment, this will probably lose its usefulness long before the slate, increasing the chances of leakage.

Fasteners: Nails driven incorrectly into the manufacturer-provided holes can be a cause of problems, with those hammered in too tightly leading to cracks over time. If not driven in far enough, moisture penetration may result. Your inspector will zero in on the condition and nailing method of the slate fasteners as a possible culprit if there is leakage in the home but no other evidence of roof deterioration/missing tiles. Protruding nails can also accelerate delamination and lead to splitting.

Like with underlayments, even properly driven nails will likely not outlive the slate roof itself. The same goes for flashing of the chimney, roof penetrations, headwalls, and sidewalls (step flashing), which will need to be checked for missing, damaged, or incorrectly installed pieces during the roof portion of the inspection. The inspector will also keep an eye out for the quality of previous repairs made to the slate roof.

Other Issues: Lack of a cant strip, which can lead to tile cracks; poor attic and rafter ventilation that can cook the underside of slate and expedite deterioration; and exposed fasteners and use of sealants—two common repair mistakes.

Inspecting a home’s roof covering is part of A-Pro’s 500-point home inspection. Ask your local A-Pro Home Inspection team about its complete foundation-to-roof inspection in Boulder. To schedule one, call 1-720-438-4595 or visit the link below.