Things you should know before installing a slated roof.

18 Steps to a Successful Slate Roof Installation

Joseph Jenkins — Joseph Jenkins, Inc.

Slate roofs are arguably the finest roofs in the world — beautiful, natural, and long lasting. Like any facet of the construction industry, however, installations of slate roofs benefit from experience, knowledge and practice. Here are 18 things to keep in mind before tackling a slate roofing project:

1) Construct the roof properly. The roof must have a slope of 4:12 or above (4 inches of rise to 12 inches of run). If the slope is lower than 8:12, then the headlap should be 4 inches. The steeper the roof, the longer it will last. If the roof slope is shallow enough to walk on, someone will walk on it and damage the slates. So if you’re going to design a long-lasting slate roof, make the slope too steep to walk on.

Also, it's prudent to assume that all elements of the roof assembly should function successfully for at least 150 years. This means using roofing slates, fasteners and a roof deck material with known and proven longevities of a century and a half. Roof decking with the required longevity includes solid lumber at least 3/4 inch thick or other durable decking materials such as nailable concrete and gypsum. Avoid any laminated or glued roof decking products, despite the fact that plywood and OSB are the roof deck materials of choice for standard roofers and architects today. Slate roofs are not standard roofs — they’re exceptional roofs which will grace the building they’re on long after the installer is dead and even after his grandchildren have grown old. For this reason, fast and cheap roof decking materials should be avoided.

For example, we recently reslated a roof on a house that was 170 years old. The first 40 years, the house had wood shakes on it. The next 130 years, it had Vermont “sea green” slates on it. The 130-year-old slates weren’t even that bad, but bad workmanship and incorrect maintenance over that period of time had despoiled the roof to the extent that it had to be reslated. We nailed the new Vermont sea green slates to the original roof deck — 1 inch thick oak boards. I have seen slates renailed to a 215 year old original 1 inch thick pine roof deck in Scotland. If you’re going to build your roof out of plywood or OSB, there is no way you will ever be able to reslate the roof over the original deck after the original slates wear out. In fact, the original slates will probably outlast the roof deck. Instead, you will have to replace the entire roof deck as well, an issue that can be avoided by having the foresight to use a long-lasting roof decking material in the first place.

2) Select the correct slates. This means you need to do your homework and understand the differences between roofing slates — they aren’t all the same by any means. Price and color are not the only criteria to go by when selecting roofing slates. You should also consider the size, thickness, type, and manufacturing quality. Some roofing slates will contain pyrites that will run red stains down the roof. Some imported slates can fade badly and even fall apart within a few years. Thick slates are harder to cut and may require more experience during installation. Small slates take longer to install because it takes more of them to cover a section of roof. Some manufacturer’s quality control is lacking. Some drill the nail holes in standard thickness slates (not good) while some punch them (good), some cull out bad slates, some leave them in the mix for the contractor to sort out. Foreign (overseas) slates can be a real gamble if you don’t know where they come from and what kind of track record they have. There is a lot to think about when selecting roofing slates. U.S. or Canadian slates from a reputable manufacturer are usually a good bet. There are a lot of good foreign slates too, but finding a reliable source of them can be difficult. Source lists for new and salvaged roofing slates, domestic and foreign, are available at SlateRoofers.org.

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