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Slate Roof Installation Quality InspectAPedia®  -     Slate Roof Installation Quality - what factors determine the quality of a slate roofing job How to inspect, evaluate & repair slate roofs - the basics

Here we discuss various factors that help determine the quality and thus life expectancy of a slate roof. This series of detailed slate roof inspection and repair articles describes procedures for evaluating the condition of slate roofing. How to inspect, identify defects, and estimate remaining life of slate roofs are addressed. The article series also references slate repair procedures, repair slate sources, and slate quarries. An earlier version of this article appearedin the winter 1991 issue of the ASHI Technical Journal - the content has been edited and updated for this online version - July 2007.

Copies of the ASHI Technical Journal are available from the American Society of Home Inspectors - ASHI at ashi.com. we also provide slate sources and where to buy slate roofing materials and slate roofing tools and products.

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Slate Roof Installation quality, Fasteners Used & Slate Pattern

The following may all be indications that the roof is failing from the condition of the slate nails:

• many loose slates sliding down and often, pieces of slate on the ground around the building
• numerous repairs showing old slates re-secured in place (could also indicate other problems)
• improper tar and flashing materials used to secure loose slates

In our opinion, galvanized flashings (or too-thin thin copper or painted steel) are a mistake on slate roofs. The flashing is very likely to wear, rust, and fail before the slates. While replacing flashings is quite possible and appropriate, it's often expensive and if not done by an experienced slate roofer, there is risk of costly damage to the roof.

We had a client this year whose slate roof was damaged by the mason working on the chimney. He accepted a low-bidder for roof repairs. We found asphalt shingles nailed over and around the area of damaged slates. To say that the roof leaked was the least aggravating observation concerning this work.

Inspecting from the attic interior may also give clues to fastener age, type, and condition if some nails are visible. You may see tips of nails, depending on the length of fasteners used.

Slate Roof Installation Pattern as a Factor in Slate Roof Life

Slate nails: Proper installations use copper, stainless, or hot-dipped galvanized nails for fastening slates. Very early slate roofs were secured using wood pegs. Later ones used tie-wires in some applications. Slaters' nails have a thinner head than conventional roofing nails, avoiding damage to the covering slates. We've found many slate roofs installed with steel nails in the Northeast. We've also found lots of roofs in that area which are losing good slates from nail failures. Slate nail holes: When a slate is punched (usually at the quarry) the hole is driven from the back of the slate, leaving a ragged pit at the front (exposed) surface of the slate so that the nail head can be countersunk flush with the top of the slate. This avoids damage to the next course of slates which overlay the nail heads and which may crack under load.

Hand punching of nail holes in slates: Because most roofs have at least some custom slate fitting (for example at valleys), some slates are hand-punched and may be fractured around the nail hole. Usually only two holes are punched, 1.25" to 2" from either side and about a quarter of the length of the slate down from the top.

More holes and more nails may be used to hang thicker heavier slates. Where battens or spaced roof sheathing are used the spacing of the battens will affect where the holes are punched in the slates. Slates punched too close to their centers or too low in the slate are more likely to leak when water seeps down in a fan-shaped pattern from the vertical abutment of the sides of the slates in the next course up the roof.

If you are inspecting a roof which frequently leaks following prolonged rains, and if the slates and flashings look pretty good, and if leaks are everywhere, you might look for improper punching or nailing errors. Don't rush to condemn the roof - how often and how badly does it leak? Under what conditions? What is being damaged? Where is water going?

Nail tightness: Slates should hang loosely on the nail. Nails too tight may break the slate as they're pulled through it as wood shrinks. Nails not driven fully are likely to break the slate above.

Other signs of poor installation include inadequate side laps. Side Lapped slate pattern.

Side laps on slate roofs should be not less than 3" and each side-joint should be as near as possible to the center of the slate below. Particularly where improper repairs have been done you may find violations of this rule. Inadequate side laps risk leaks as a result of wind-driven rain. Inadequate side laps might be suspected on roofs which use varying-width or graduated width shingles. However some experienced roofers and inspectors commented that graduated, random-width, graduated width slate roofs were more costly than other slate systems and were often installed by more skilled roofers.

Do not mistake graduated or random slate sizes for necessarily improper application.

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ROOF INSPECTION SAFETY & LIMITS
SLATE ROOF INSPECTION & REPAIR
How to Inspect & Repair
Types of Roofing Slate
Slate Colors, Chemistry
Slate Roof Aging and Durability
Ribbon Slates
Slate Roof Installation Quality
Other factors in Condition
Slate Roof Repair history
Slate Roof Leaks
Slate Holding actions
How to Repair Slate Roofs
  Copper Tab Method
  Slate Hook Method
SLATE ROOF PHOTO LIBRARY
Slate References
Slate Sources, Repairs
SLATE ROOF INSPECTION CLASS

More Information on Building Diagnostic Inspections and Repairs

• How to Inspect & Repair Slate Roofs a detailed article about evaluating the condition of slate roofing and repairing worn or damaged slate roofs.
• Slate Roofs: a photographic library of slates, slate roofs, roofing styles
• Roofing: Defect Recognition, Repair, Prevention for Roofing & Flashing main web page
• Inspecting and Evaluating Slate Roofs Hudson Valley ASHI seminar content
• Choosing a Roofing Contractor How to find and select a roofing contractor - (thanks to Jessie Schrader)

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