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Slate Roof Installation Quality
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Slate roof installation workmanship: this article explains various factors that help determine the quality and thus life expectancy of a slate roof. 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 updated for this online version - March 2010.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Slate Roof Installation quality, Fasteners Used & Slate Pattern
The following may all be indications that the roof is failing from
the condition of the slate nails:
- many loose slates sliding down and often, pieces of slate
on the ground around the building
- numerous repairs showing old slates re-secured in place
(could also indicate other problems)
- improper tar and flashing materials used to secure loose
slates
In our opinion, galvanized flashings (or too-thin thin copper or painted steel) are a mistake on slate
roofs. The flashing is very likely to wear, rust, and fail before
the slates. While replacing flashings is quite possible and appropriate,
it's often expensive and if not done by an experienced slate roofer,
there is risk of costly damage to the roof.
We had a client this year
whose slate roof was damaged by the mason working on the chimney.
He accepted a low-bidder for roof repairs. We found asphalt shingles
nailed over and around the area of damaged slates. To say that the
roof leaked was the least aggravating observation concerning this
work.
Inspecting from the attic interior may also give clues to fastener
age, type, and condition if some nails are visible. You may see tips
of nails, depending on the length of fasteners used.
Slate Roof Installation Pattern as a Factor in Slate Roof Life
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 Some slate companies advertised A slate roof that cannot leak,
yet [was] inexpensive, easy to apply, beautiful..., durable as time," using a design which was soon found to be a disaster: 12" slates
were placed with 9" exposure, leaving 3" for headlap and 6" which
was backed only by a cap sheet of 32# felt interlaced with the slates.
Roofs were also installed following this poor design, using 14" slates
with 10"-11" exposure. Felt is not functional as a permanent roofing
material: even where it is not exposed directly to sunlight, as the
organics dry out the felt cracks, disintegrates, and leaks.
We have
reports that inspectors have been the subject of legal
actions following their failure to identify this defect in slate roofs.
See the illustrations above. -- Personal communication, Doug Sheldon, Vermont Structural
Slate, December 1990. |
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Slate installation pattern: as we introduced at SLATE ROOF INSPECTION PROCEDURE, some slate roof installation patterns "stretched" the material by installing slates with minimum head lap or side lap.
The proper minimum side lap for roofing slates is shown in our sketch at above left. An "open lap" slate roof is shown at page top (Vassar College Campus, Poughkeepsie, NY) and in our sketch at above right.
In some climates (blowing rain) this may lead to a leaky roof that was fine on a barn but not so nice on a house, in particular if the side lap of the slates is less than 3". Notice that 3" is called for in the right hand sketch and that a 50% head lap is also shown.
The side-lap pattern of the Vassar College campus slate roof at left shows that the slates are overlapped 50% - this slate roof pattern should be fine provided that there is also adequate head lap (which we cannot see). . |
Slate nails: Proper installations use copper, stainless, or hot-dipped galvanized
nails for fastening slates. Very early slate roofs were secured using
wood pegs.
Later ones used tie-wires in some applications. Slaters'
nails have a thinner head than conventional roofing nails, avoiding
damage to the covering slates.
We've found many slate roofs installed
with steel nails in the Northeast.
We've also found lots of roofs
in that area which are losing good slates from nail failures.
Slate nail holes: When a slate is punched (usually at the quarry) the hole is driven
from the back of the slate, leaving a ragged pit at the front (exposed)
surface of the slate so that the nail head can be countersunk flush
with the top of the slate. This avoids damage to the next course of
slates which overlay the nail heads and which may crack under load.
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 Hand punching of nail holes in slates:
Because most roofs have at least some custom slate fitting (for example
at valleys), some slates are hand-punched and may be fractured around
the nail hole. Usually only two holes are punched, 1.25" to 2" from
either side and about a quarter of the length of the slate down from
the top.
More holes and more nails may be used to hang thicker heavier
slates. Where battens or spaced roof sheathing are used the spacing of the
battens will affect where the holes are punched in the slates. Slates
punched too close to their centers or too low in the slate are more
likely to leak when water seeps down in a fan-shaped pattern from
the vertical abutment of the sides of the slates in the next course
up the roof.
If you are inspecting a roof which frequently leaks following
prolonged rains, and if the slates and flashings look pretty good,
and if leaks are everywhere, you might look for improper punching
or nailing errors. Don't rush to condemn the roof - how often and
how badly does it leak? Under what conditions? What is being damaged?
Where is water going?
Nail tightness: Slates should hang loosely on the nail. Nails too tight may break
the slate as they're pulled through it as wood shrinks. Nails not
driven fully are likely to break the slate above.
Other signs of poor installation include inadequate side laps. Side Lapped slate pattern.
Side laps on slate roofs should be not less than 3" and each side-joint should
be as near as possible to the center of the slate below. Particularly
where improper repairs have been done you may find violations of this
rule. Inadequate side laps risk leaks as a result of wind-driven rain.
Inadequate side laps might be suspected on roofs which use varying-width
or graduated width shingles. However some experienced roofers and
inspectors commented that graduated, random-width, graduated width
slate roofs were more costly than other slate systems and were often
installed by more skilled roofers.
Do not mistake graduated or random
slate sizes for necessarily improper application.
Frequently Asked Questions (FAQs) about how to determine the quality or workmanship of a slate roof installation
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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
 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.
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- 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
- 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
- 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
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