How to Inspect and Repair Slate Roofs - The Basics InspectAPedia® -
How to inspect, evaluate & repair slate roofs
Detailed ASHI Technical Journal Article about slate roofs for home inspectors & home owners, 1991, Updated 2012
Dutch lap and other slate roofing patterns that may affect slate roof life
Questions & answers about late roof installation, maintenance, repair, inspection, life expectancy, and common problems
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.
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An earlier version of this article appeared in the winter 1991 issue of the ASHI Technical Journal
- the content has been edited and updated with extensive text corrections and additions, and numerous photographs have been provided for this online version - July 2007, updated April 2009. Copies of the ASHI Technical
Journal are available from the American Society of Home Inspectors - ASHI at ashi.com.
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.
How to Inspect & Repair Slate Roofs - Detailed Procedures, Repair Methods, Slate and Replacement Sources
by Alan Carson & Dan Friedman
Every slate roof looks terrific when it's wet - unless you're seeing
leaks inside. There are, fortunately, some better ways to look at
and think about this material. Slate roof failures result from breakdown
of the material itself, from poor installation, or from poor maintenance.
Photographs of an actual slate roof installed using the pattern shown at left are are at the top of this page and included in
our slate photo library. This was, in keeping with the spirit of the Dutch, an economical slate roof but not necessarily a better slate roof. Some slate roof installations such as that sketched at left and installed in a
Dutch Lap or Dutch Side Lap slate roofing pattern, relied on the roofing felt underlayment - material much less durable than the slate itself.
Evaluation of the condition of slate roofing, and estimating its remaining life considers at least these factors:
Type of slate installed on the roof - durability of the slate material itself depends on the quarry from which the slate was cut and even the location within the quarry
Age and condition of the slates - where are we in the expected life of the material and what damage is visible
Condition of the roof flashings - rusted leaky roof flashing may require removal of some good slates in order to repair the flashing
Slate Roof Installation quality, fasteners used to secure the slates to the roof; the slate installation pattern used may determine the resistance of the roof to leaks and wind-blown rain
The slate roof's repair history, quality of maintenance work - if improper or poor repairs have been done the slate roof may have been damaged or its future life reduced
Roof Leaks - are a telltale which could point to any or all of the above problem sources
Roof Structure - on uncommon occasions slate roofing may be installed on a structure not designed to carry its weight
This article assists readers in evaluating the condition of slate roofs and planning for the maintenance, repair, or
replacement of slate roofing as we discuss these concerns. Other very important factors in evaluating the condition of a building roof such as the condition
of roof decking or sheathing, and roof structure and framing, and condition of valleys and other flashings. Many of the slate roof defects discussed
below are illustrated in our online Slate Roof Photo Library included below in this document.
Advice About Walking on & Inspecting Slate Roofs
Inspectors should be cautious in evaluating any roof condition to avoid failing the roof material
itself when leaks are confined to flashing areas.
Watch out: our own experience is that it is absolutely impossible to walk directly on slate roofs without damaging them, particularly if the slates are worn, loose, damaged. And walking on such surfaces is unsafe.
On some other fragile but not totally fragile roofs such as slate roofs, cement tile, cement-asbestos, fiber cement, and hard-fired ceramic clay tile roofs, contractors suspend a ladder over the roof surface, hanging it from the ridge (as shown in our photo of slate roor replacement underway in Duluth, MN), and cushioning it off of the roof surface using foam or insulation padding, or contractors work from scaffolding.
See ROOF INSPECTION SAFETY & LIMITS where we describe roof safety and roof damage issues when inspecting, repairing, or otherwise walking on other roof materials such as asphalt, slate, wood roofs. As stated in stated in Best Practices Guide to Residential Construction (printed text), and in our online article at FLASHING, CLAY TILE ROOFS, to prevent breakage, walk on clay and similarly fragile roof tiles with extreme caution or best, not at all.
We've found good Vermont slate roofs which have been "roofed over" with asphalt (and
ruined) when the leaks were located in and only in metal valleys. Metal flashings may be deteriorating from slate particles washing
off of the roof.
Our photo at left of an open lap slate pattern (a Vassar College slate roof) and at page top of a Dutch Lap slate roof found in Vermont, and the sketch at above left show what was sold as a "better" roof but was actually a leak prone slate installation pattern that
covered as much area as possible with as little slate as possible. The minimal head lap and side lap invite slate roof leaks in windy rainy weather.
Don't confuse the open lap pattern (left) with the minimal head and side lap roofing slate pattern in the sketch above.
We read repair suggestions in the Old-House Journal
involving use of roofing cement and felt for temporary purposes, as well as the preferable soldered or replacement repairs.
We've found
severe corrosion, particularly on copper, when asphalt roof cement has been used for temporary repairs. In our opinion this is a poor
short-term repair which causes increased damage.
On an historic restoration project involving slate roofs with copper flashings the worst corrosion
we found on the copper was where roof cement had been used to patch leaks. Other patches and materials over similar leaks caused much
less apparent damage.> are not considered in this discussion but must indeed be evaluated by the inspector.
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Alan Carson (Carson Dunlop, Associates,) 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
Thanks to 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
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.
If less than 25% of the slates on a roof are damaged or sliding down from worn out nails,
slate repairs are in order. Luckily replacement slates are available from a variety of
slate quarries, slate suppliers, and slate roofing companies. Slate "look alike" products
are also available using cementious materials. If the building owner cannot afford to
repair a slate roof where most slates are intact, holding action slate roof repairs should be
followed, such as simply sliding metal flashing up under broken or missing slates on the roof.
In addition to these slate sources there are several slate look-alike
substitutes. While the application techniques, flashing and nailing
concerns are similar, the wear characteristics of these materials
may be quite different and were not evaluated for this paper. This
paper is not an endorsement, nor a critique of any of these materials.
Eternit, Inc., rigid fiber reinforced cement roofing slates
(and board products). Unfortunately as of July 2011 it looks as if Eternit fiber cement products are no longer distributed in the U.S. Eternit products are available in other countries. In 2005 Eternit merged with Marely to form Marely Eternit, part of the Etex group. Etex Group Headquarters
Avenue de Tervueren 361, 1150 Brussels, Belgium, Tel: +32 2 778 12 11, Fax: +32 2 778 12 12, Quoting: "Formed in 2005 after the amalgamation of two market leading companies, Marley Roofing and Eternit Building Materials [www.marley.co.uk/ ], our unrivalled product portfolio includes all types of concrete and clay tiles, double and single lap slates, decorative cladding, profiled sheeting and a range of building boards." Note: the original Eternit Fiber Cement Products Contact we had was: Village Center Drive, Reading, PA 19607 800/233-3155 - now obsolete. This Etex web page lists Eternit product distributors: http://www.etexgroup.com/companies/results#/continent/country/235
BritSlateTM are made from 100% quarried slate particles
combined with an adhesive (epoxy?) and no fillers with no questionable
side effects according to the manufacturer. According to the manufacturer,
they can be cut, drilled, etc. and come with a 50-year guarantee.
They're less prone to breakage than slate, fireproof, and do not absorb
water (like natural slate), and do not encourage organic growth (moss
and lichens) because they are not porous. BritSlate North America,
Inc., 647 West Boylston St., Worcester, MA 01606 508/852-4888 508/852-7224
FAX 2/90.
Reinforced-cement shingles which "look like slate but are
lighter and one-third the cost" are available from Atlas International
Building Products, 5600 Hochelaga St., Montreal, Quebec, Canada H1N
1W1. [$180/square loose or $400./sq. installed, 30-year guarantee.]
Supra-Slate "looks like slate and is manufactured in colors
of real quarry slate." Tegusol is a double interlocking clay roof
tile. Available from: Supradur Manufacturing Corporation, PO Box 908,
Rye, NY 10580 800/223-1948 or 914/967-8230. or 122 East 42nd St.,
NY, NY 10168 212/697-1160
Lifetile(R) Boral Concrete Products, Inc., produces high
density extruded concrete roofing tiles meeting Class "A" requirements.
Dallas, TX 214/544-2227
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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