How to determine and evaluate the repair history of a slate roof
How to inspect, find, & evaluate roof leaks & repair slate roofs
List of slate roof suppliers for roofing slates & slating tools
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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.
Slate Roof Repair history helps diagnose the condition of a slate roof
Tar or roofing mastic is normally an indication of improper maintenance. It should be
considered a temporary, unreliable repair. Tar or roofing cement should
not be found at chimneys, sidewall flashings, plumbing vents, or elsewhere.
Our photo at left shows tar patching in the dormer valley and below the dormer. But further loose slates are falling off of this roof, possibly damaged during application of the valley tar.
The tar itself usually remains effective as a sealant only for a year
or so. While in place, tar traps moisture within the slates, causing
the slate itself to deteriorate. When flashings leak on a roof they
should be repaired or replaced as necessary, without using roof cement.
Copper tabs and fastener failures on slate roofs:
Watch for numerous copper tabs at the center of replacement shingles.
These indicate that repairs have been made and they may be a clue
about the overall condition of the fasteners. In a first-class slate
repair you should not be able to see the copper tabs. The Old-House
Journal April 1984 roofing issue describes an alternative method
for securing slates which will leave a copper tab exposed. Expert
roofers have a less visible method of fastening the replacement slate.
[See "Repairing Slates" sidebar article.]
Our page top photo shows a perfectly good repair made by sliding a piece of copper flashing up under slates to cover a hole that may have been made by a nail pop from below.
Asphalt shingles applied over slate roofs:
Using asphalt over slates is surprisingly common "re-roof" procedure
as it avoids labor and disposal costs for the slate material. [In
some areas of the Northeast it also represents the popularity of lower-cost
asphalt and the dearth of experienced slaters who might have repaired
the slate roof.]
When nailing the asphalt shingles it is common
to find only a poor bond of new nails to original roof decking. The
slates below tend to chip and bulge the new material resulting in
a "peanut brittle" or "popcorn" effect. Since the comparatively large
thermal mass of the slates retains heat, the asphalt shingles are
"cooked" from their backside as well as from their front when exposed
to sunlight. The life of such shingles is estimated by some to be
half the normal span.
Leaks in Slate Roofs
All slate roofs probably have at least some flashing, slate, or other
damage or mechanical installation errors. Some conditions such as
a side lap error, damaged slate, or even flashing error may leak only
in certain weather conditions such as windy rain storms from a particular
direction, water backup behind ice dams, or prolonged rains.
Where
slates are missing near valleys the adjoining slates may be damaged
as well.
Openings may cause leaks or water to pass below the valley
flashing even if the flashing looks intact. Where there were previous
repairs it's common for the felt underlayment to be torn as well.
If there are porous slates or openings above the tears in the felt,
water may leak through.
Ice dams at roof eaves can be a serious leak source on slate as most
other roof systems. Traditionally 30# felt was used at eaves as "insurance"
against this problem. Some slate suppliers recommend this heavier
felt for all underlayment, not just at the eaves. However two components
conspire to reduce the effectiveness of felt as ice-dam protection:
every nail at the eaves punctures the felt, and with age felts often
dry and disintegrate before the slates have worn out.
Ice
dam protection is improved in new or re-roof applications using the
newer sticky membranes such as WR Grace's Ice and Water ShieldTM.
However the preferred solution to this problem is proper attic ventilation.
Good venting avoids the ice-dam problem and adds reductions in winter
moisture and summer heat problems. Old houses whose attics have been
converted to living space, particularly with un-vented ceilings following
the underside of fully-insulated roofs are likely to be serious moisture
and heat traps.
Slope requirements for slate roofs
In conventional roofing design slates are used on roofs with a slope
of at least 4" of rise in 12" of run, that is, on 4 in 12 roofs. A
3" head lap is used, often 4" when the slope is less than 8 in 12.
So a 20" long slate, with a 3" head lap, would have an exposure of
8.5". For 18" slates the exposure is 7.5", and for 16" slates, 6.5".
Roofs with less head lap or more exposure may be more leak-prone.
But slates have been used even on dead flat surfaces such as balconies and decks, as we show here.
When we inspected this slate covered deck in Newburgh, NY in 1991 we found that the slates had been set in a bed of tar over canvas to form a flat roof that lasted without leaking for nearly 50 years.
But as you can see in our photo, the slates had now come loose from their bed of tar.
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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.
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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