How to Evaluate Slate Roof Condition - other factors InspectAPedia® -
Other factors can assist in evaluating the condition of a slate roof - we discuss them here
When is a slate roof really worn out?
When to replace a slate roof
How to inspect, evaluate & repair slate roofs - the basics
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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. Copies of the ASHI Technical
Journal are available from the American Society of Home Inspectors - ASHI at ashi.com.
Roof slope, as with all roof systems, is a big factor in shingle wear.
The steeper the slope, the more durable the roof. Slates have been
used even on flat roofs, as ballast, and slates were used to line
the reflecting pool at the US Capitol in Washington DC.
Condensation in attic interiors can be a problem. All slate roofs
need ventilation to equalize the temperature between inside and outside
the roof surface. Elimination of unwanted moisture in winter and heat
in summer will both extend roof life. Serious damage from interior
moisture is likely to be to roof sheathing and framing, but high interior
moisture will shorten the life of most roof coverings as well.
The open lap slate pattern (left) was probably intended
for use on buildings with high interior moisture - perhaps a cow barn.
Most normal slate roof installations, as originally built, may have
had adequate ventilation. In our opinion, serious moisture problems
may begin when modern renovations insulate between rafters and add
interior finishes in attics. For those designs special provisions
may be needed to cool and dry the roof cavity.
Mineral deposits found on the interior surface (attic view) of slates,
when none are visible outside, are a sign of possible attic moisture
problems. Where no outside staining or efflorescence match inside findings,
the inspector may suspect that sun exposure is drying the outer surface
leaving longer moist conditions inside. If these conditions are not
consistent on all roof surfaces you may find the most mineral deposits
on the north or shaded roof surfaces.
Moss on slates (see our photo at page top) should be considered as damaging as on any other type
of shingle. The moss retains moisture against the slate and its roots
may actually penetrate and damage the material. On older roofs with
heavy moss the growth can actually lift and separate the shingles.
Chemical treatments may help with moss as it does on wood roofs. Extreme
care should be used in working with chemicals.
Our photo at page top shows a mossy and lichens covered slate roof on Phillips Road in Poughkeepsie, NY - this slate barn roof is at the end of its life and is leaking.
Roof framing for slate roofs is often stronger than similar framing
for houses of the same age for which lighter roof materials were used.
A slate roof can weigh as little as 700 pounds per square
up to 8000 pounds where 2" thick slates were
used on a graduated slate roof. On a residential building the maximum
weight per square you'd probably find would be 2500 pounds. The addition
of slate to a house not framed for this purpose will require an analysis
of the roof framing system.
(One roofing square is 100 sq. ft.)
Clues about the quality of a slater's work may be found in hip and
ridge design. This topic is not discussed here.
Flashing details, valley flashing methods, weather exposure, mechanical
damage from falling limbs, foot traffic or improper workmanship, are
examples of other factors which affect roof life. They are not discussed
here. -- See Alan Carson's "Slate Roof" presentation notes used at several ASHI Seminars.
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Additional technical contributors & reference sources for this article are listed below.
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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
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, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book 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. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
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.
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
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).
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
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