Slate roof deterioration example = soft flaking slates (C) Daniel Friedman Slate Roofing Guide: how to inspect, troubleshoot, & repair slate roofs

  • SLATE ROOF INSPECTION & REPAIR - home - CONTENTS: A detailed guide to slate roof inspection, leak diagnosis, repair methods, and slate sources. How to inspect, evaluate the condition of slate roofs - how much slate roof life is remaining, and when is a slate roof beyond economical repair. Standards, specifications & methods for installing, inspecting, repairing slate roofs. A guide to the types, colors, and durability of different types of roofing slates. A guide to slate roof installation and flashing. Where to buy slate for roofs, where to buy slate roofing tools
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Slate roof inspection, diagnosis & repair guide:

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.

This slate roofing article series also references slate repair procedures, repair slate sources, slate tool sources, and slate quarries around the world.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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A Detailed Guide to Slate Roof Inspection, Diagnosis, Installation & Repair

Slate roof repair and access ladders (C) Daniel FriedmanSlate roofing is considered a high-quality, durable, and aesthetically pleasing material which receives considerable focus in both the sale of properties and in the evaluation of building condition.

Our photo (left) shows a slate roof being replaced in Duluth Minnesota.

[Click to enlarge any image]

Like many building topics, opinions run heavier than actual data. 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.

Watch out: before walking on a slate roof,

A Very Short History of Slate Roofs & The Composition of Slate

Slate church steeple (C) Daniel FriedmanAccording to the US NPS "Roofing for Historic buildings" article series:

Slate is a fine grained crystalline rock metamorphosed from bedded deposits of clay and silt. It can be worked into shingles readily because it has two lines of breakability: cleavage and grain. These occur generally at right angles to each other and are independent of the original bedding planes. [See Slate Colors, Chemistry for details.]

A specification for one of the designs included this direction for the French roof: "Slate the sides of the roof with slate 5 x 12 inches, nailed with galvanized nails; all hips and valleys to be flashed with tin in the best manner." the bedding remains in some slate deposits as visible bands running across the cleavage.

Known as ribbons [see Ribbon Slates], these bands may be either weaker or harder than the surrounding slate. Slate's durability as roofing is due to its high strength, low porosity and low absorption rate. Specific mineral components are responsible for the various colors: carbon (black), hematite (red and purple), chlorite and ferrous iron oxide (green).

Other minerals, considered impurities (calcite and iron sulfides), are slowly transformed by weathering into gypsum, which expands and causes the slate to delaminate.

Slate has been used as a roofing material in Europe for hundreds of years [we believe for more like a thousand or so, or for millenia if stone slabs are included - Ed.], with surviving examples dating to the 8th century.

From the 17th to the 19th centuries, most of the roofing slate used in America was imported from North Wales, where slate quarrying was a major industry.

Although the first commercial slate quarry in the United States was opened in 1785 in Peach Bottom, Pennsylvania, the industry was limited and local until the second half of the 19th century.

At that time the industry grew and matured in response to a growing population, advancements in quarrying technology, an expanding rail system, and the immigration of Welsh slate workers to America.

The United States became a slate exporter after the Civil War, as quarries opened in Vermont, New York, Virginia, and Pennsylvania.

Architectural styles of this period emphasized prominent roof lines and decorative patterns, details that were well suited to the varied colors and shapes available in slate. Properties such as nonflammability, durability, minimal maintenance costs, and aesthetic value made slate all the more desirable. Its primary drawback was its weight, making shipping costly and requiring substantial roof framing.

Between 1897 and 1914 production peaked. Later use of slate often employed different thicknesses and colors, and unevenly cut or aligned butts to produce picturesque effects suitable for English revival styles popular in the early 20th century. After 1915 widespread use of slate roofing declined in the United States, due in part to a lack of skilled labor, but more importantly, due to the development of modern, mass produced materials such as asphalt shingles, which seemed the more economical alternative.

A slate is typically attached to wooden sheathing with two nails driven through prepunched holes, though as with tile, it may be wired or screwed to steel angles on a steel framed roof.

At the end of the19th century asphalt saturated felt laid over the wood sheathing became a standard part of most slate roof installations. Slate is installed with an overlap that depends on the slope of the roof and requires a minimum pitch (generally 4 inches of rise per 12 inches of run) to effectively shed water. Particularly in coastal areas slate can be found laid in mortar, providing extra protection against wind driven rain.

Today slate continues to be quarried domestically as well as being imported from Europe, the U.K., Canada, China and South America.

An abbreviated original version of this article appeared in the winter 1991 issue of the ASHI Technical Journal - the content has been extensively edited with corrections to the original, additional in-depth text explaining items cited in the original article and adding new material, as well as adding about hundreds of color photographs of slate types and slate roofs for this online version. Copies of the ASHI Technical Journal are available from the American Society of Home Inspectors - ASHI at


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