Slate Roof Installation Quality - what factors determine the quality of a slate roofing job
How to inspect, evaluate & repair slate roofs - the basics
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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.
Copies of the ASHI Technical
Journal are available from the American Society of Home Inspectors - ASHI at ashi.com.
we also provide slate sources and where to buy slate roofing materials and slate roofing tools and products.
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
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.
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.
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.
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.
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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. 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
"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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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.
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.
