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Ribbon Slates on Slate Roofs

Types, life, sources, effect on slate roof life

Ribbon slate roofing:

What are ribbon slates, and do they wear out faster than other slate roofs? Slates with diagonal or striped inclusions of varying colors, or ribbon slates, include products with a very long life expectancy and other ribbon slates that are or were a low-priced slate with soft inclusions, short life, and leaks.

Here we illustrate different types of ribbon slates used in roofing and we describe how to distinguish the low-priced short-lived ribbon slate from the longer-lived very durable ribbon slate on a roof.

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Ribbon Slates & Ribbon Slate Roofs: appearance, life expectancy, leak risks

Ribbon slate on a mansard roof (C) Daniel Friedman

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.

[Click to enlarge any image]

In the first two photos shown here 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 and on some more technical details that we'll describe just below.

Below: more ribbon slate with diagonal mineral inclusions is shown mixed with solid reds on other slopes of the same home.

"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. Bowles was a mineral technologist employed by the U.S. Bureau of Mines.

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

Ribbon slates are easily identified from the ground. The stripes are accentuated because the ribbon portion absorbs more water than the rest of the slate. Usually the ribbons are darker, often multi-colored browns and reds.


Ribbons of color in ribbon slates on roofs may be in the entire shingle or in some applications, may be in the upper or covered portion of the slates

White Inclusions in Chapman Slate

In the this photo , we see a mix of "ribbon slates" that have light colored or even white diagonal colored stripes of mineral inclusion, but they do not show the characteristic delamination and wear we see on classic ribbon slates.

What's different about this roof, and why are these "ribbon slates" in apparently good shape.

At a 1990 Metro ASHI class on slate roofs, an Albany NY slate roofer suggested that ribbons were desirable for a pattern effect, and that they were equally durable with other slates from Pennsylvania. -- Capital Region ASHI chapter education seminar, fall 1990

Some roofers consider ribbon slates as less durable material.

There is no doubt that In at least some cases, ribbon slates are less durable than other Pennsylvania slates. -- Trapasso, personal communication.

I [DF] agreed with Trapasso, having both worked-on and inspected quite a few slate roofs in the Northeastern U.S. We found many ribbon slate roofs in which the ribbons were red or tan and where those slates to be wearing faster than expected - noting that the durability of ribbon slates depends on the particular minerals which make up the visual diagonals.

The ribbons proved to be a softer material than the surrounding slate.

More than two decades after we first compiled this material on slate roofs, Tim Underhill (Underhill 2015) explains that the slates in our photo, with light or white diagonal stripes, are probably Chapman slate, a Vermont slate product that is also called "ribbon slate" by many of us, but is morphologically different.

In Chapman slates the white inclusions are not soft stripes of calcium or magnesium but rather quartz inclusions that are in fact harder than the slate itself. These slates are from a different quarry or at the very least, a different slate vein even when mined in the same state as the more colorful and often shorter-lived ribbon slates.

Watch out: don't call out white-striped Chapman slates as defective ribbon slate of questionable life - at least not based on these hard white inclusions.

If the diagonals are comprised of minerals softer than the surrounding slate, early wear is likely.

Ribbon slate roof, Clinton St., Poughkeepsie NY (C) Daniel FriedmanIf some ribbon 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.

What's the bottom line on ribbon slates? Good or bad?

Really, as Florida home inspection expert Mark Cramer loves to intone, "... it depends." Some ribbon slates include mineral inclusions that weather out and lead to a short slate roof life. But plenty of ribbon slate roofs like the Poughkeepsie home shown at left are very durable, lasting many decades.

So the life of a ribbon slate roof depends on the particular ribbon slates installed and thus on the quarry where they originated, and on the minerals that make up the stripes in the slate, and also in where in the slate the ribbons are located - exactly where ribbon bands of softer slate, comprised of bands of greater percentages of magnesium (Mg) and calcium (Ca) occur within the slate body. In other ribbon slates inclusions of higher-carbon material may produce a dark, almost black line in the slate.

Some slates are cut so that the ribbon inclusions of Mg and Ca are placed in the head of the slate where they will be covered by succeeding courses of slate as it's laid up the roof.

If the ribbon occurs right at the nailing location in the slate, then softening and loss of material in this area eventually leads to movement of the slate down-roof and ultimately to loss of the slate. (Underhill 2015).

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.

More ribbon slate photos are in our SLATE ROOF COLORS article

Research on Ribbon Slate Properties & Life


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