Ribbon slate on a Poughkeepsie roof (C) Daniel Friedman Types of Roofing Slates & Types of Slate Roofing
     


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Types of slates used on building roofs.

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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Types of Slate used in Roofing

Tarred slate roof dormer valley (C) Daniel Friedman

The abandonment of perfectly good slate roofs which should have been repaired is a financial shame and the destruction of a valued asset.

At the same time, careless optimism about a bad slate roof which is at the end of its life risks an angry inspection client. This article reviews types of slate, common defects, inspection topics, and some repair tips. We also provide slate sources and where to buy slate roofing materials and slate roofing tools and products.

Variations occur in color, thickness, surface texture, graduated sizes and varying widths. Standard slate roofs use slates 3/16" to 1/4" thick of one uniform length and width with square tails laid to a line in a conventional shingling pattern.

Textural style roofs use rough-textured slates with uneven butts and varied thickness. Different colors and varying sizes are often used. The resulting roof has a very rustic look.

Graduated slate roofs use slates which vary in size and often in thickness, with larger slates at the eaves, smaller and with less shingle exposure at the ridge. Original work graduated roofs show smooth transition among the sizes from larger to smaller as each course of slates approaches the ridge line. Where lots of repair work was done this effect may have been lost. This roof style makes the roof and building look larger and taller than it actually is.

Shingle exposure is the portion of the shingle which you can see from outside, or the portion left exposed to the weather. In most shingle roofing systems the exposed portion of the shingle is considerably less than the total shingle length.

In addition to color, graduated and textural patterns, slates are placed in a variety of shingling patterns, of which several are illustrated in sketches throughout this paper and in the Slate Roof Photo Library, such as French, Dutch-lap, and open-lap patterns. The Dutch lap uses a 3" side overlap and a vertical exposure of all but 3" of the slate. The result is essentially a single layer of roofing with only 3" of overlap at the top and side. The open joints are all on one side or the other along any given row or course of slates. This system is extra vulnerable to wind-driven rain.

The French method slate pattern also offers essentially a single layer of roofing with three inches around the perimeter of each shingle.

Finally, slates have a natural "grain" in the material. Normally the slates are cut so that the grain runs the length of the slates.

If the slate is quarried improperly, the nail holes create a perforated effect on the slate and it will break in half at the nail holes after the installation. Watch for this interesting defect.

See this slate edge close up, showing the natural layering of mineral deposits in the original stone.

An earlier version of this article appeared in the winter 1991 issue of the ASHI Technical Journal - the content has been edited and extensively updated for this online version: we've corrected the original text, added extensive explanatory text, and added numerous photographs of the conditions we discuss. Copies of the ASHI Technical Journal are available from the American Society of Home Inspectors - ASHI at ashi.com.

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