Slate Roof Wear, Aging & Durability InspectAPedia®  -     Slate Roof Wear, Aging & Durability - how long do slate roofs last? Can a slate roof rot? After all, isn't slate stone? 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.

The abandonment of 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.

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Aging and Durability of Slate Roofing - How Long Will a Slate Roof Last?

Close-up Inspection of Slate Roofs is Necessary to Evaluate Their Condition

Does slate "rot"?

We were warned by an experienced slate roofer and we have confirmed by field inspections of slate roofs that determination of slate condition other than by direct up-close inspection is highly unreliable. Slates may look fine from the ground, but be found soft and at end of life on close inspection. By direct inspection we mean looking at slates from a few inches, either from a ladder or some other point of view such as an attic window. We strongly advise inspectors not to walk on slate roofs. Risks include falling - very slippery, and extensive damage to the slates. [Trapasso, personal communication.]

The photographs provided here show how soft roofing slates can become. These slates might, from a distance, look pretty good, but we was easily able to crush the slates with my fingers. we do not agree that we should call this "rotted roofing slate" as did Trapasso, but on seeing how soft and delaminated this slate can become, we can understand his loose use of that term.

It is useful to contrast the above "soft crumbling roofing slate" with the surface delamination of the roofing slates shown at left. The slates in our photo (left) may look questionable from the ground but a closeup inspection may show, as this one did, that the slate body is still thick and hard, and it is surface delamination of the slate that was visible from the ground. These slates are not yet at end of life.

These photographs make clear that it can be tough to accurately evaluate the condition of some slate roofs from ground level - a close-up inspection is needed to distinguish between the slate roofs in our photos shown here. The first is or really past end of roof life and the second slate roof has good life remaining.

Roofing Slate Life Depends on From Where Slates were Quarried

Life expectancy of Virginia Buckingham roofing slate

Darker gray "Buckingham" slates from Virginia are considered a durable high-quality material which, properly maintained, can have a near indefinite life. Average life of these slates is 175 years.

The Old House Journal, April, 1983 p. 57

Life expectancy of Vermont roofing slate

Vermont slate roofs, with proper maintenance, can last indefinitely.

The Old House Journal, April 1983, p.58, and Slate Roofs, National Slate Association, p. 71.

By other sources, Vermont slates have life expectancy of 100-200 years. Our photo at left shows a slate roof in good condition at Vassar College in Poughkeepsie, NY. We do not know the origin of this particular slate, but it's wear condition and age indicate a durable roof.

Steven Trapasso, personal communication. Also the Old House Journal April 1983 rates Vermont slates as having a 175 year life.

Such roofs should be maintained, not replaced.

With proper maintenance such an asset may be continued for an indefinite period. Inspectors who do not know the composition and source of slates would not want to condemn, except in the worst of circumstances, a Vermont slate roof.

Life expectancy of Pennsylvania roofing slate

Typical life for Pennsylvania slate roofs is 40-50 years. These life expectancies are general experience with the material and do not consider installation or environmental factors which vary from one building to the next.

Life expectancy of British and Welsh roofing slate

Earlier British experimentation on slate durability and composition emphasized that variations in the tendency of slates to absorb water may be the principal cause of deterioration seen by modern inspectors. Slates which are soft, delaminating, and thus are at or near end of their useful life may have been damaged by frost. "That sort of slate", said Dr. Watson, the bishop of Llandaff, "other circumstances being the same, is esteemed the best, which imbibes the least water; for the imbibed water not only increases the weight of the covering, but, in frosty weather, being converted into ice, it swells and shivers the slate." Watson continued to suggest that slates, being less porous than clay roofing tiles of the day, were more durable.

The New Practical Builder and Workman's Companion , Peter Nicholson, 1823, reissued 1861, republished with companion modern opinions in Handbook of Building Crafts in Conservation , 1981.

Watson's opinion that slates are more durable than clay tiles is arguable as both materials are found with considerable variation in composition. Indeed unglazed clay absorbs more water (up to 10%) than slate (usually 1%) but tiles are not necessarily damaged by these freeze-thaw conditions. Watson's comments pertain to use of slate in Britain.

The extent of color change in Pennsylvania [and possibly other] slates often indicates the extent of deterioration. More white, more efflorescence probably means more deteriorated.

What is the normal rate of deterioration of roofing slates?

The rate of slate deterioration on roofs depends on the source quarry and the depth from which the slates were cut. These determine the density and composition of the slate. Site conditions, such as the roof pitch, exposure to sun, building moisture and ventilation and exposure to mechanical damage also affect slate life. It is interesting to note that several references indicate that slates actually harden after being quarried and exposed. We'd presume that observation pertains to the early or initial state of use of the slates, and we found no reference which claimed some chemical or molecular process of permanent or continuing hardening over life.

Signs of deteriorating roofing slate

Deteriorating Pennsylvania slate shows a mottled appearance, with white faded patches resulting from exposure to sunlight and moisture.

The white areas generally begin at the three exposed sides of a slate, growing towards the center of the slate as a function of time and exposure.

The white visible in Pennsylvania slate is evidence of efflorescence - lime deposited on the slate surface. If the whole slate is white in color and scaling you should consider the slate to be at the end of its life.

However as long as the slates are physically intact the roof is serviceable. If a roofer recommends tearing off the whole roof you should ask to be shown examples of failed slates.

Roofing slate fading versus mottling

Fading versus mottling: It is important for an inspector to distinguish between the mottled edges-in whitening (efflorescence) and fading. By contrast to the efflorescence process, fading slates generally fade monotonically across the slate rather than from edges in. Faded slates will not show an obvious whitish powder of surface deposited mineral salts as thick as that left by efflorescence. All slates will fade from weather exposure. Those which have only minimal color change are classed as "permanent" or "unfading". Those which change more markedly are classed as "weathering".

Monotonically means that the color change is even across the material rather than showing up as a splotch, or a ring of white around darker color, or in other variations. It's a nice word, not recommended for inspection reports.

We've heard several explanations for the mechanism by which Pennsylvania slates deteriorate, of which the most interesting was the opinion that lime in the slates in the presence of moisture reacts to break down organic components in the slate. For slates higher in carbon or carbonaceous materials this may be indeed the most accurate explanation. Pennsylvania "Black Bed" quarried slates were high in carbonaceous impurities and were recommended by the Slate Association only for temporary buildings or for chalkboards. By the 1920's this slate was recognized as not as durable; you'd not expect to find it surviving on a building in 1991.

The National Slate Association reference does not detail the mechanism of slate deterioration. It does point out that the slate roof on the Saxon Chapel at Stratford-on-Avon was built 1100 years ago - still in good condition.

By Mr. Trapasso's account the decomposition of Pennsylvania slate may be understood as a process similar to decomposition of wood. The lime which was in the slate, when the slate was mined, had not caused deterioration because the slates were protected from sunlight. When the slate is split, cut, punched for use, the lime is in an inactive or dormant state. Following application on a roof surface the slate material is exposed to ultraviolet rays (sunlight) and moisture. These components cause the lime to break down the organic materials in the slate (shale), making the slate increasingly porous and causing the white efflorescence. Lime and other mineral salts are dissolved and deposited on the surface of the slate when moisture evaporates.

Slate Roofs, National Slate Association, p.71-76.

Calcium Oxide - Sources of white minerals and oxides in roofing slates

Examining quantitative analysis of slates from nineteen quarries we observe that while oxides of silica (SIO₂) and aluminum oxide (AL₂O₃) are the dominant components of slates, almost all have measurable levels of calcium oxide (CaO) and other carbonates and oxides. There are dramatically higher levels of these materials in Pennsylvania slates (3%) than in slates from Vermont (.3%-.7%). Clearly there is more material available to form calcium carbonates and efflorescence in the shorter lived materials.

Slate Roofs p. 73

Carbon, which would be broken down in the process described by Trapasso, occurs in slates from only a few quarries, and then in usually small amounts of less than 1%. This would seem to argue against the "slate rots" explanation of deterioration.

Brown staining in roofing slate

The brown color of some slate may be from oxidizing iron content in the slate which is following a similar reactive path. we did not find references to brown in Virginia slates.

The known chronological age of slates may be available from an owner or other documentation. As with all materials, experienced inspectors rate the apparent wear age of the roof, not the actual age.

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ROOFING INSPECTION & REPAIR
ASBESTOS CEMENT & FIBER CEMENT ROOFING
ASPHALT ROOF SHINGLES
FIRE RETARDANT PLYWOOD
FLASHING MEMBRANES PEEL & STICK
FLASHING SIDING DETAILS
FLASHING WALL DETAILS
FLAT ROOF MOISTURE & CONDENSATION
Green House or Solarium Roof Leaks
HAIL DAMAGED SHINGLES
HEAT TAPES & CABLES on Roofs for Ice Dams
INSULATION IDENTIFICATION GUIDE
INSULATION INSPECTION & IMPROVEMENT
LEAD POISONING HAZARDS GUIDE
LEED GREEN BUILDING CERTIFICATION
LOW SLOPE ROOFING
MASONITE WOODRUF FIBERBOARD ROOFING
MEMBRANE & SINGLE PLY ROOFS
METAL ROOFING TYPES
MODIFIED BITUMEN ROOFING
PLASTIC ROOFING TYPES
ROLL ROOFING, ASPHALT
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF COLOR RECOMMENDATIONS
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ROOF INSPECTION SAFETY & LIMITS
ROOF VENTILATION SPECIFICATIONS
ROOF VENTING ENERGY SAVING DETAILS
ROOFING FELT REQUIREMENTS
ROOFING MATERIALS, Age, Types
ROOFING TILE SHAPES & PROFILES
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
SOD ROOFING
SIDING, Sheathing Identification - Photo Guide
STAIN & BIODETERIORATION AGENT CATALOG
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on ROOFS
STAIN DIAGNOSIS on STONE
STANDARDS for ROOFING
STONE CLEANING METHODS
STONE ROOFING
TEST LABS - ROOF SHINGLE
THATCH ROOFING
THERMAL EXPANSION of MATERIALS
THERMAL MASS in BUILDINGS
TILE ROOFING
UNDERLAYMENT REQUIREMENTS on ROOFS
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & WIND DAMAGE

• 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.
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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.

• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
• Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
• The Journal of Light Construction has generously given reprint permission to InspectAPedia.com for this article. All rights and contents are ©Journal of Light Construction and may not be reproduced in any form.
• 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.
• Asphalt Roofing Residential Manual,
• Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
• Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
• Built-Up Roof Systems, Manual, C.W. Griffin, Mcgraw-Hill (Tx); 2nd edition (July 1982), ISBN-10: 0070247838, ISBN-13: 978-0070247833
• 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),
• Concrete Dome Roofs (Longman Concrete Design and Construction Series),
• Concrete Roofing Tile, History of the, Batsford, 1959, AISN B000HLLOUC (availble used)
• Copper Roofing, by CDA
• Copper Roofing, Master specifications for copper roofing and sheet metal work in building construction: Institutional, commercial, industrial, I.E. Anderson, 1961 (hard to find)
• Corrugated Iron, Building on the Frontier, Simon Holloway
• Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
• Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design Construction methods for new and existing buildings Information on insulation, drainage, detailing, irrigation, and plant selection Details on optimal soil formulation Illustrations featuring various stages of construction Best practices for green roof maintenance A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality Tips on the LEED design and certification process Considerations for assessing return on investment Color photographs of successfully installed green roofs Useful checklists, tables, and charts
• 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
• Low Slope Roofing, Manual of, 4th Ed., C.W. Griffin, Richard Fricklas, McGraw-Hill Professional; 4 edition, 2006, ISBN-10: 007145828X, ISBN-13: 978-0071458283
  • Roof failure causes in depth (and specific methods for avoiding them)
  • Roof design fundamentals and flourishes, based on voluminous industry research and experience
  • New technologies and materials -- using them safely and correctly
  • Comprehensive coverage of all major roofing systems pecifications, inspection, and maintenance tools for roofing work
• Metal Roofing, an Illustrated Guide, R.A. Knowlton , [metal shingle roofs],
• Patio Roofs, how to build, Sunset Books
• 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
• Solar heating, radiative cooling and thermal movement: Their effects on built-up roofing (United States. National Bureau of Standards. Technical note), William C Cullen, Superintendent of Documents, U.S. Govt. Print. Off (1963), ASIN: B0007FTV2Q
• Tile Roofs of Alfred: A Clay Tradition in Alfred NY
• "Weather-Resistive Barriers", 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)
• ...