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Types of Roofing Slates & Types of Slate Roofing InspectAPedia®  -     Types of Roofing Slates & Types of Slate Roofing: How to Inspect and Repair Slate Roofs 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.

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.

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.

© Copyright 2010 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Types of Slate used in Roofing

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.

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ROOF INSPECTION SAFETY & LIMITS
ROOF VENTILATION SPECIFICATIONS
ROOF VENTING ENERGY SAVING DETAILS
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

About The Authors

Alan Carson and Dan Friedman are ASHI Members, home inspectors, educators and writers in Toronto and Poughkeepsie respectively. Information in this article is taken from the references shown below, from inspection experience, from opinions shared at ASHI and other education seminars, from the authors' personal, limited experience repairing and rebuilding of slate roofs, and from very helpful Bill Markcrow and Doug Sheldon at Vermont Structural Slate Co. we also paraphrased from a presentation to NY Metro ASHI Members by Mr. Steven Trapasso in October, 1990. Mr. Trapasso has 40 years experience in working with slate, and has a clear love affair with the material, particularly when it comes from Vermont.

Slate Roofing References

• SLATE ROOF INSPECTION & REPAIR home page
• SLATE ROOF PHOTO LIBRARY Our online photo dictionary of roofing slate colors, types, and conditions has moved to its own web page
• SLATE ROOF INSPECTION CLASS presentation notes and illustrations
• Slate Roofs, National Slate Association, 1926, reprinted 1977 by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It has gone in and out of print on occasion.)
• The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com, 143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
• Slate Roofs , Steven Trapasso, presentation to NY Metro ASHI, Inc. chapter seminar, November 1990.
• The Old-House Journal , Special Roof Issue , April 1983, The Old-House Journal, PO Box 50214, Boulder, CO 80321-0214
• National Slate Technology Center courses for architects, builders, etc.
• Handbook of Building Crafts in Conservation, Jack Bower, Ed., Van Nostrand Reinhold Company, NY 1981 ISBN 0-442-2135-3 Library of Congress Catalog Card Nr. 81-50643.
• Slate Roofing Contractors Association of North America
• National Slate Association includes list of slate testing laboratories

Slate Replacement & Repair Sources

If less than 25% of the slates on a roof are damaged or sliding down from worn out nails, slate repairs are in order. Luckily replacement slates are available from a variety of slate quarries, slate suppliers, and slate roofing companies. Slate "look alike" products are also available using cementious materials. If the building owner cannot afford to repair a slate roof where most slates are intact, holding action slate roof repairs should be followed, such as simply sliding metal flashing up under broken or missing slates on the roof.

• Vermont Structural Slate Co., PO Box 98 Fair Haven, VT 05743 802/265-4933 802/265-3865 FAX (Green, purple, unfading red, and Spanish imported black.)
• Rising & Nelson Slate Co., West Pawlet, VT 05775; 802/645-0150 (all VT colors plus Buckingham Gray from VA and blacks from PA.)
• Evergreen Slate Co., 68 Potter Ave., Granville, NY 12832; 518/642-2530. Vermont slates in 10 colors.
• Hilltop Slate Co., Middle Granville, NY 12849; 518/642-2270 (all VT colors plus imported Spanish Black)
• Structural Slate Co., Pen Argyl, PA 18072; 215/863-4145 (PA black)
• Buckingham Slate Co., 4110 Fitzhugh Ave., Richmond, VA 23230; 805/355-4351 (VA Buckingham Grey)

In addition to these slate sources there are several slate look-alike substitutes. While the application techniques, flashing and nailing concerns are similar, the wear characteristics of these materials may be quite different and were not evaluated for this paper. This paper is not an endorsement, nor a critique of any of these materials.

• Eternit, Inc., rigid fiber reinforced cement roofing slates (and board products. Village Center Drive, Reading, PA 19607 800/233-3155
• BritSlate^TM are made from 100% quarried slate particles combined with an adhesive (epoxy?) and no fillers with no questionable side effects according to the manufacturer. According to the manufacturer, they can be cut, drilled, etc. and come with a 50-year guarantee. They're less prone to breakage than slate, fireproof, and do not absorb water (like natural slate), and do not encourage organic growth (moss and lichens) because they are not porous. BritSlate North America, Inc., 647 West Boylston St., Worcester, MA 01606 508/852-4888 508/852-7224 FAX 2/90.
• Reinforced-cement shingles which "look like slate but are lighter and one-third the cost" are available from Atlas International Building Products, 5600 Hochelaga St., Montreal, Quebec, Canada H1N 1W1. [$180/square loose or $400./sq. installed, 30-year guarantee.]
• Supra-Slate "looks like slate and is manufactured in colors of real quarry slate." Tegusol is a double interlocking clay roof tile. Available from: Supradur Manufacturing Corporation, PO Box 908, Rye, NY 10580 800/223-1948 or 914/967-8230. or 122 East 42nd St., NY, NY 10168 212/697-1160
• Lifetile(R) Boral Concrete Products, Inc., produces high density extruded concrete roofing tiles meeting Class "A" requirements. Dallas, TX 214/544-2227

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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