It was with great interest that I came across a copy of the new book, Historic Slate Roofs — With How-To Info and Specifications. Although no author is named, it is published by Schiffer Books of Atglen, PA, and was produced in collaboration with the existing National Slate Association, according to the Foreword and Acknowledgements. In a nutshell, the book is a reproduction of the classic, Slate Roofs, published by the original National Slate Association in 1926. Historic Slate Roofs includes a front section of 19 color photographs, “A New Preface to an Expanded Classic.” These photos were included in order to “update this book with images of an actual installation.”
Before I start critically examining this new book, let me say this: should you buy it? Yes. It has a hardcover; the price is good; it’s well made by the Chinese (who are good book producers); the photos are excellent quality; and you get an exact copy of the original 1926 classic, Slate Roofs. If it were only a copy of Slate Roofs, it would have been a better book — most of the misinformation comes with the added material. Having said that, however, the original Slate Roofs had its share of misinformation as well. The publication of Historic Slate Roofs provided an opportunity to correct some of that original erroneous material, but the errors remain intact, which is disappointing.
The original Slate Roofs is a worthy reference resource to have on hand. It’s full of good information. However, I find fault with some of the practices suggested, such as bedding hip and ridge slates in mastic. This practice may be fine for the installer, but when the restoration or repair contractor must come back later and repair the roof, the glued slates won’t come apart without breaking. They weren’t thinking about this when the book was published back in 1926, but now that we’re doing a lot of restoration work here in the U.S., slates glued down with mastic are a big headache. Luckily, most installers did not follow the instructions in Slate Roofs and most hips and ridges are free of mastic and can be taken apart and put back together. Slaters back then were an experienced bunch who could figure out the proper way to do things. What makes me cringe is the thought of the new hips and ridges that will be installed with mastic by inexperienced roofers following erroneous instructions being published today.
There are numerous other examples of questionable information and inaccurate illustrations in Slate Roofs that I could cite, but won’t, due to a limitation in space here. I must however point out, as I have done many times in the past, including at NSA Committee Meetings, that the photo of the roof brackets in Slate Roofs on page 15 of the original publication is upside-down, reproduced without correction in Historic Slate Roofs (Figure 1). The correct orientation for this image would be 90 degrees clockwise (Figure 2).
If an unsuspecting person were to install the roof jacks according to the published illustration as shown at left above, the roof scaffold would immediately collapse. When one considers the inherent danger of the high, steep, slate roofing trade, every effort should be made to ensure safety. There is no excuse for publishing an incorrect image of this nature, over and over again (some quarries also re-publish Slate Roofs, without corrections). OK - now that I have that out of my system, let’s move on.
The use of 19 photographs of a new slate installation in the beginning of the book was a good idea. It’s unfortunate that they show bad practices. For example, the photos clearly show each course of slates being installed with a layer of peel and stick glued over the top of the slates. The peel and stick is then being covered with felt (Figure 3). The slates are then chalked on the slate face to align the next course. Once again, this is an installation technique that assumes the roof will never need repair or restoration, because trying to repair a slate roof that has the slates glued down with peel and stick is impossible. Even just felt paper underlayment installed on top of the slates is an awful practice, one that may be appropriate for cedar shakes, but not for slate. Why not? It's called UNDERlayment because it lays UNDER the slates. Try to slip a slate ripper underneath a course of slates where underlayment lurks between the slates. The paper blocks the ripper — it bunches up and prevents the ripper from accessing the slating nails, making repairs a giant headache. This is the voice of experience speaking here. Laying felt paper on top of slates is a mistake. Installing peel and stick over top of slates is a disaster.
The headlap was shown correctly, but it would have been helpful if the roofers were shown using a slate hammer rather than a drill to put holes in slates, or even a slate hammer being used to install the slates themselves rather than a carpenter’s hammer. It would have been better to show a replacement slate being installed on the new roof with a slate hook rather than with a nail and bib. I would not recommend these photos as reference material for anyone wanting to install a slate roof.
Page xiii is a “Roofing Slate Detail and Specification Sheet.” The specifications call for a cant strip “1 inch in thickness at the lower side and 2 inches or more in breadth...” However, standard thickness slates require a cant that is about 3/8 inches in thickness (1/4 inch to 1/2 inch will work), not one inch. The specifications also call for 40 pound asphalt felt, which is not easy to come by these days (30 pound felt is typical and traditional). They also call for “16 ounce soft rolled copper flashing 24 inches wide fastened at the top only” for open valleys. In fact, the width of the valley copper depends on the exposure of the valley — 24 inches of copper is excessive under most circumstances. Fastening only at the top is not a good idea either; the valleys should be fastened along their length on both sides. Furthermore, 20 ounce partially hardened copper is far more preferable than 16 ounce soft copper in valleys, where longevity and durability are important. Regarding ridges, the specifications state, “that the three top courses of slate shall be laid in elastic cement...,” a slate roof restoration contractor’s nightmare. And for hips, “All hips shall be... embedded in elastic cement...,” an issue I have already discussed, with ample chagrin.
If the reader suspects that I am intentionally trying to pillory this new book, I am not. As a published slate roofing educator, however, I do take umbrage at the thought that the misinformation I have been trying to clear up since I published the first edition of the Slate Roof Bible in 1997, continues to be published and disseminated with the full knowledge of professionals in the field. Buy the book, put in on your shelf as a reproduction of a historical document, but don’t rely on it for correct slate roof installation specifications.