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How to Inspect and Repair Slate Roofs - The Basics Detailed ASHI Technical Journal Article, 1991, Updated 2007 InspectAPedia®  -     How to inspect, evaluate & repair slate roofs - the basics Dutch lap and other slate roofing patterns that may affect slate roof life

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 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.

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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.

Inspectors should be cautious in evaluating roof condition to avoid failing the roof material itself when leaks are confined to flashing areas. 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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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ROOFING INSPECTION & REPAIR
ASBESTOS CEMENT ROOFING
ASPHALT ROOF SHINGLES
FIRE RETARDANT PLYWOOD
ROOF INSPECTION SAFETY & LIMITS
ROOF VENTILATION SPECIFICATIONS
ROOF VENTING ENERGY SAVING DETAILS
SLATE ROOF INSPECTION & REPAIR
How to Inspect & Repair
Types of Roofing Slate
Slate Colors, Chemistry
Slate Roof Aging and Durability
Ribbon Slates
Slate Roof Installation Quality
Other factors in Condition
Slate Roof Repair history
Slate Roof Leaks
Slate Holding actions
How to Repair Slate Roofs
  Copper Tab Method
  Slate Hook Method
SLATE ROOF PHOTO LIBRARY
Slate References
Slate Sources, Repairs
SLATE ROOF INSPECTION CLASS
SOD ROOFING
SIDING WOOD
STAIN DIAGNOSIS on Building Exteriors
STAIN DIAGNOSIS on Roofs
STANDARDS for ROOFING
STONE ROOFING
THATCH ROOFING
THERMAL EXPANSION of MATERIALS
THERMAL MASS in BUILDINGS
THERMAL MASS in UPSTAIRS
TILE ROOFING
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & WIND DAMAGE

• 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 Roofing References

• SLATE ROOF INSPECTION & REPAIR home page
• SLATE ROOF PHOTO LIBRARY Our online photo dictionary of roofing slate colors, types, and conditions has moved to its own web page
• SLATE ROOF INSPECTION CLASS presentation notes and illustrations
• 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.
• 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.
• Slate Roofing Contractors Association of North America
• National Slate Association includes list of slate testing laboratories

Slate Replacement & Repair Sources

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.

• Vermont Structural Slate Co., PO Box 98 Fair Haven, VT 05743 802/265-4933 802/265-3865 FAX (Green, purple, unfading red, and Spanish imported black.)
• Rising & Nelson Slate Co., West Pawlet, VT 05775; 802/645-0150 (all VT colors plus Buckingham Gray from VA and blacks from PA.)
• Evergreen Slate Co., 68 Potter Ave., Granville, NY 12832; 518/642-2530. Vermont slates in 10 colors.
• Hilltop Slate Co., Middle Granville, NY 12849; 518/642-2270 (all VT colors plus imported Spanish Black)
• Structural Slate Co., Pen Argyl, PA 18072; 215/863-4145 (PA black)
• Buckingham Slate Co., 4110 Fitzhugh Ave., Richmond, VA 23230; 805/355-4351 (VA Buckingham Grey)

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. Village Center Drive, Reading, PA 19607 800/233-3155
• BritSlate^TM 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

More Information on Building Diagnostic Inspections and Repairs

• How to Inspect & Repair Slate Roofs a detailed article about evaluating the condition of slate roofing and repairing worn or damaged slate roofs.
• Slate Roofs: a photographic library of slates, slate roofs, roofing styles
• 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)

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