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Ribbon Slates on Slate Roofs InspectAPedia®  -     Slate Roofing: how to evaluate ribbon slates on a roof How to inspect, evaluate & repair slate roofs - the basics

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.

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Ribbon Slates

The ribbon slate on this mansard roof slope may have been chosen for appearance, though it was not installed so as to use the ribbon diagonals to form any particular pattern. In the first two photos at left the slates have diagonal colored stripes of mineral inclusion, but they do not show the characteristic delamination and wear we see on classic ribbon slates.

Some ribbon slates were a cheaper and shorter-lived product because the mineral inclusions that formed the "ribbons" of color in the roofing slates were a softer material that weathered out of the slate rapidly. However as numerous slate roofers and other readers have pointed out, ribbon slates were also selected and installed on some buildings for their aesthetic appeal and included ribbon slates from other quarries whose mineral inclusions were quite durable. So the answer to the question of "are ribbon slates more or less durable than other roofing slates?" is "it depends" on which quarry was the source of the slate.



More slate with diagonal mineral inclusions is shown mixed with solid reds on other slopes of the same home.







More rather worn ribbon slates on a New York home. We'll post other(sharper focused and close-up) examples of ribbon slate in this article. "Slate is of medium hardness, very fine grained of low porosity, great strength and consists essentially of insoluble and stable minerals that will withstand weathering for hundreds of years. Some slate in Pennsylvania contains ribbons which consist of narrow original beds usually containing carbon, and darker in color than in the body. There is tendency for some ribbons to contain an excessive amount of the less resistant minerals, and they should not appear on exposed surfaces." -- Dr. Oliver Bowles, Mineral Technologist of the US Bureau of Mines, in "The Characteristics of Slate" , June 1923 paper delivered to the American Society for Testing Materials. ASTM.

By "appear," Bowles meant that inferior ribbon slates which contain fast-weathering mineral inclusions should not be used where exposed to the weather.

Some roofers consider ribbon slates as less durable material. we suspect that the durability of ribbon slates depends on the particular minerals which make up the visual diagonals. If the diagonals are comprised of minerals softer than the surrounding slate, early wear is likely. In at least some cases, ribbon slates are less durable than other Pennsylvania slates. -- Trapasso, personal communication.

If these slates were actually shorter-lived than clear cut materials, why were they used? In the 1940's one square (100 sq. ft.) of Pennsylvania slate cost about $6.00, or about $15.00 installed. Because of these attractively low prices and low anticipated replacement cost [boy were they wrong!] ribbon slates were very popular and were used extensively. As slate and roofing costs rose and as ribbon slates were less expensive than clear slates, some clever roofers used slates which were cut so that the ribbons were only in the upper half of the slate. As the ribbons were covered by the next course, these roofs were more durable.

An inspector may spot this interesting material from attic view or from outside if a slate has fallen out of position, exposing the upper half of its predecessor course. The cost of installing a modern slate roof makes the choice of poor materials illogical.

Attic view means inspecting the underside of the roof surface from inside the building. If open or spaced sheathing was used as nailing base for the slates you'll be able to see the backs of the slate material, or in some cases, you'll see roofing felt, usually damaged or soft, which may provide openings to see the slates. Where closely-spaced board sheathing was used you'll not see slates except perhaps through a knot hole or damaged board.

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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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