The Colors & Chemistry of Roofing Slates

The Colors & Chemistry of Roofing Slates
How to inspect, evaluate & repair slate roofs - the basics of slate colors and what they mean for slate roof life & care
Questions & Answers about different colors of roofing slate
References

Click to Show or Hide Related Topics

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Roofing slate colors & chemistry: a quick guide. Because the properties, including durability, of roofing slates vary widely, and because the properties of roofing slates depend largely on the quarry from which the slate originated, it is worth knowing what you can probably expect from a roofing slate by its appearance and color. Appearance and color are not substitutes for a chemical analysis of the slate, but they can often tell us quickly about the slate's probable source and about how long it is likely to last. 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. We also provide slate sources and where to buy slate roofing materials and slate roofing tools and products.

Slate Colors, Chemistry, Durability

According to the National Slate Association slate is a stone material which has an indefinite life.

Slate Roofs, reprinted by the Vermont Structural Slate Co. We caution readers that while this is absolutely the best reference we've found on slate roofing, the language on durability is a bit enthusiastic. You'll have to look closely at the footnotes to identify the less durable materials.

Because the actual composition of slate affects its durability (and cost), the identification of the source of existing slates is useful in evaluating the condition and future life of a roof. Common sources of roofing slates used in the US and Canada are Pennsylvania, Virginia, and Vermont. California, Georgia, Michigan, New York and Maine also have or had slate quarries. In 1924 the remaining active slate quarrying areas were Maine, Vermont-New York, Pennsylvania, and Virginia. Common slate names have British origins. Maine's Bangor slate looks a lot like Bangor blue mined in North Wales.

Identification of the slate source by visual inspection is difficult by other than experienced slate handlers or laboratories. However as the slate source has such a dramatic effect on the expected life of the material, any anecdotal, historical, or visual clues are useful. Readers are cautioned not to assume that slates in their area necessarily came only from the nearest slate quarry. Though that's a reasonable bet in many cases, one might find Virginia slate or Maine slate on roofs as distant from the quarries as British Columbia! Vermont Structural Slate Co. will identify slates if you mail them a 1.5"x1.5" triangular sample.

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.

Basic slate roofing colors, their composition, and their sources

Basic roofing slate colors are black, blue-black, gray, blue-gray, purple, mottled purple and green, green, and red. These color names should be preceded by designations of "unfading" or "weathering" (discussed later). Color is determined by variations in chemical and mineral composition.

The mineral composition of the shale from which slates are cut varies significantly among quarries and from state to state in the US. The primary constituents are white and black mica.

See later details in this article. A detailed procedure for analysis of slates was documented in US Geological Survey Bulletin #700, "The Analysis of Silicate and Carbonate Rocks", by Dr. W.F. Hillebrand.

Virginia roofing slate colors & characteristics

Virginia Roofing Slate is blue-gray in color, tough, durable, and considered a top quality material by most slate sources. This slate is notable for a peculiar luster giving attractive lights and shadows on roofs.

Vermont roofing slate colors & characteristics

Vermont roofing slate is typically green with purple, black, and red available. Vermont green slate is the most abundant, with well developed cleavage (easy to split into roofing slates), with good "fissibility" (relatively easy to work), making it the most common variety used.

Slate from Vermont is lower in lime content than the Pennsylvania variety. Chemical composition of slates varies of course by quarry as does slate color. Slates are available in both fading and non-fading varieties. In Vermont slate fading color is not considered an indication of deterioration. Rather it's an aesthetic factor.

Blue-gray Pennsylvania roofing slate colors & characteristics

Pennsylvania blue-gray slate is possibly the best known and most widely distributed of the common slate colors. Some architects and roofers call these slates Pennsylvania black. But as "Black-Bed" slates, a less durable material, might be confused with the better quality Pennsylvania material, the blue-gray name should be used.

Unfading Pennsylvania gray roofing slate color, appearance, characteristics

This is probably a Pennsylvania Gray slate roof on a NY College Campus in Poughkeepsie. Unfading Pennsylvania gray slate, which weathers or fades less than "weathering" varieties, has soft gray tones and may have additional strength or durability.

Unfading Pennsylvania black, heavier than other slates, has a rougher appearance and does not, as its name claims, fade significantly. [Fading may be taken to mean a general "lightening" across the slate.] Blue-black "hard-vein" Pennsylvania slates are hard material which grows darker on exposure to weather.

Gray and Grey-Black roofing slates, where they are from, and their characteristics

Gray and gray-black roofing slates are quarried in Vermont and parts of New York. The gray is lighter than the Pennsylvania blue-gray. You may find it mixed with purple. Gray-black is quarried in both light and dark grays, some with a mottled effect. Unfading green, weathering green, unfading purple, unfading mottled purple and green, variegated purple have been quarried in Vermont and New York.

Unfading red roofing slate, where it is quarried, and its wear characteristics

Unfading red roofing slate is quarried only in Washington County, New York.

Classic real slates remain on this red slate roof in New York's Hudson Valley. Perhaps slates from the Granville NY quarry - we don't know how long this superb material will last as we've never found one that was worn out on a roof.

A nice red slate roof with white details in Poughkeepsie, NY.

More of this nice-condition red "slate" roof in Poughkeepsie, NY. From the ground this roof looks too uniform - we wonder if it's a synthetic slate look alike? Closer inspection is needed.

Ribbon Slates vs "Cleare Slates"

We did not find references listing sources of ribbon slate other than Pennsylvania's Lehigh-Northampton district quarries. Ribbon slate is identified by diagonal "stripes" visible in the shingles.

We show more examples of ribbon slates at RIBBON SLATE and we discuss ribbon slates in detail at SLATE ROOF TYPES, RIBBON SLATE, with other comments about slates with diagonal mineral inclusions also discussed at SLATE ROOF LIFE EXPECTANCY.

Slates which contain no ribbons are referred to as "clear" slates.

Freak slate colors, what are "freak roofing slates"?

If you find slate which is not one of these colors it's a "freak". It would be rare to find a freak which was quarried thinner than 1/4", usually in greater thickness up to 2". Some are not true slates in every quality and may not be salvageable when worn or on roofs in poor condition.

Frequently Asked Questions (FAQs) the chemistry of roof shingles

...

Ask a Question or Search InspectApedia

Use the search box below to ask a question or to search the InspectApedia.com website.

Ask a Question or Enter Search Terms in the InspectApedia search box just below.

Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
  Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
  Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
  Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
  Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
How to Inspect & Repair Slate Roofs a detailed article about evaluating the condition of slate roofing and repairing worn or damaged slate roofs.
Slate Roof Photo Library photographs of colors, types, patterns of slate roofs, slate roof wear, slate roof conditions.
Slate References page from the ASHI Technical Journal
Slate Sources, Repairs page from the ASHI Technical Journal
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)
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.
Slate Roofing Contractors Association of North America
National Slate Association includes list of slate testing laboratories

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Architectural elements: the technological revolution: Galvanized iron roof plates and corrugated sheets; cast iron facades, columns, door and window caps, ... (American historical catalog collection), Diana S Waite, available used out of Amazon.
Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
Concrete Folded Plate Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author), Butterworth-Heinemann, 1998, ISBN-10: 0340662662, ISBN-13: 978-0340662663
Concrete Shell Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author),
Copper Roofing, Master specifications for copper roofing and sheet metal work in building construction: Institutional, commercial, industrial, I.E. Anderson, 1961 (hard to find)
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.
Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362
Historic Slate Roofs : With How-to Info and Specifications, Tina Skinner (Ed), Schiffer Publishing, 2008, ISBN-10: 0764330012 , ISBN-13: 978-0764330018
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
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.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237,
If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided. The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
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 Roofing in Canada (Studi4es in archaeology, architecture, and history),
Smart Guide: Roofing: Step-by-Step Projects, Creative Homeowner (Ed), 2004, ISBN-10: 1580111491, ISBN-13: 978-1580111492
Tile Roofs of Alfred: A Clay Tradition in Alfred NY
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Wood Shingle Roofs, Care and Maintenance of wood shingle and shake roofs (EC), Stanley S. Niemiec (out of print)
...


	InspectAPedia^®	InspectAPedia
	Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair	Ask a Question or Search InspectAPedia

Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair

Ask a Question or Search InspectAPedia

The Colors & Chemistry of Roofing Slates (function() { var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true; po.src = 'https://apis.google.com/js/plusone.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s); })();