Graduated slate roofs

Traditional Roofs from Historic Roots, by Joseph Jenkins

Not all slate is created equal. Being a natural stone mined from the Earth, slate comes in many types with many different characteristics. 

Illustration above from the Slate Roof Bible, 3rd Edition

In the old days, when roofing stone was wrestled from deep quarry holes and dark mines using hand tools and beasts of burden, the splitting of roofing shingles from rock was an arduous and exacting art. Many slate veins contained very hard, rough textured rock strata that could not easily be split into the uniform, thin sheets required for shingles. For the sake of efficiency, the stone was split into the largest slate shingles possible, creating a supply of coarse shingles that varied considerably in size — some larger, some smaller. In order to make good use of all of these slates, a certain style of roof was developed of necessity — graduated slate roofs.

In this style of roofing, the largest stones, sometimes massive, are installed at the bottom of the roof. This allows for the heavier weight of these large slates, perhaps 30 inches long and an inch thick, to be born by the wall of the building. It also relieves the roofer of having to carry the flat stones, which may weigh 200 pounds, to the very top of the roof. Furthermore, the bottom of the roof is exposed to more water than any other part of the roof and heavier slates are more apt to withstand the excess erosion and weathering that occurs at the drip edge. 

As the roof installer progresses up the roof, smaller and smaller slates are used, with the smallest slates, perhaps only 12 inches long, fastened near the top. The result is a roof that “graduates” in size from large at the bottom to small at the top, yielding an architectural style that is utterly unique and quite pleasing to the eye. Traditional graduated roofs also utilize random width slates. There are still many of these roofs in good condition scattered throughout the United States and Europe, yet the art of installing graduated slate roofs is a disappearing one. This article focuses on this art and is intended to preserve some knowledge and, hopefully, revive some interest in one of our architectural treasures — graduated slate roofs.

Graduated Slate Roofs are Ancient

A good place to look at the long history of graduated slate roofs is in Scotland, England and Wales. Scottish slate tends to be a rugged, coarse, and extremely durable material. Unable to split large uniformly thin slates from the raw material available in Scotland, the Scots created a distinctive roofing style with a rough texture in keeping with the stone architecture so characteristic of Scotland’s traditional buildings. This graduated slating style was also popular in England and Wales for the same reason — it allowed for the efficient use of a stubborn material.

Each country had its own installation peculiarities. Scottish slate, for example, was “head nailed” with a single nail hole at the top center of the slate, and nailed into solid wood board decking referred to as “sarking,” as opposed to strips of wood known as “slating lath.” Although slating lath was popular in Wales and England, Scottish slates were so random in lengths, widths and thicknesses that a solid wood deck was desired in order to make it easier to nail the shingles to the roof. Scottish slates are also “shouldered” — their top corners are knocked off, a practice still in use today in much of Europe. Because the Scottish slates were nailed at the top, there was a risk of gale winds lifting the bottom of the slates and blowing them off the roof. The Scots, however, utilized rather thick slates, the weight of which more than compensated for the weakening effect of head nailing. An interesting characteristic of Scottish slate roofs is that the slates can be swiveled on the nail, to one side or another, in order to clear the way for replacing a slate underneath. This swivel effect is aided by the shouldering of the slate. A slate ripper, America’s foremost slate repair tool, is rarely needed when removing a damaged slate in Scotland.

The English and Welsh also once used a head fastening technique involving the use of a wood dowel instead of an iron nail. The dowel was driven through a hole in the top center of the slate, then the shingle was hung over a thin hardwood lath strip that had been hand split from a log. This practice eventually gave way to what is called “center nailing,” a nailing style used in the USA today in which the slates are nailed with two iron or copper slating nails, one on each side, situated about 1/3 of the way down from the top of the shingle. The center-nailed slates in the UK are usually nailed to sawn lath strips, perhaps an inch thick and two inches wide — a method of nailing still prevalent in Europe today and a carry over from the days when all stone roofs were graduated stone roofs. This slating method differs from today’s standard U.S. techniques in that a Scottish style continuous board deck is preferred in the USA rather than the lath strips more common in England. Incidentally, graduated slate roofs are known as “diminishing course” roofs in the UK, while uniform slate roofing is known there as “tally” roofing.

Tradition is in the Blood

On one of my trips to Wales researching slate, I happened to meet a young man who was installing a slate roof. I found it interesting that there were several steps involved in the slate installation that we here in the U.S. almost never encounter. For one, the roofer was obligated to “hole” each slate, as no nail holes are punched in the slate at the quarry as is typically done here in the USA. This is a carry-over of the days when graduated slate roofs were the norm, and nail holes had to be punched on site after the proper lap had been determined for each diminishing course. The position of the holes in the slate was particularly critical when using lath, or 1×2 strips spaced to allow for the nailing of each course, which left little room for error. Sawn lath strips developed from the practice of using hand split lath, as mentioned earlier, and continues to this day as much from tradition as from a lack of lumber resources in England and Wales. But another practice that surprised me was the sorting of the slate prior to installation. This was a roof of uniform sized slates — not a graduated slate roof. Yet, the roofer, according to custom, sorted the slates according to thickness before carrying them up onto the roof, the thicknesses being termed “very heavies,” “heavies,” “mediums,” and “lights.” The very heavies were installed at the bottom of the roof, and so on until the lights finished off the top. I found the variance in thicknesses to be minimal, yet the roofer, with tradition in his blood, carried on a custom that began with the graduated roofs of old: sorting prior to installing.

Today, the sorting of the slate prior to and during installation is critical to the successful creation of a graduated slate roof. It requires careful advance planning for the job to be well done. The number of courses required on the roof must be determined beforehand, and the number and degree of graduations, both in thickness and length, must also be part of the planning of the roof installation. There is no one correct formula for this. Diminishing lengths can occur with each course, or they can occur only with every several courses. In any case, once the particular formula for your particular roof job has been determined, then the correctly sized slates can be ordered from the quarry. For example, I measured the graduations of three random separate graduated slate roofs and have listed the data at the end of this article.

Graduated slate roofs utilize slates of varying lengths, typically with varying thicknesses, as well as slates of random widths. The installation of slates with random widths is an art in itself, as adequate sidelaps must be carefully maintained. That is to say that the side-butts (where the slates butt against each other at their sides) of each pair of slates should be spaced three inches laterally from any side-butt above or below. If the side-butts are spaced too closely to each other, the roof could leak. A sloppy roofer will install random width slates with close side-butts; a master roofer will not.

Furthermore, when a slate course graduates from one length to a smaller one, a transitional row may be necessary in order to maintain a standard three inch headlap. Otherwise, when a length decrease occurs from one course to the next, a five inch headlap may occur, rather than the standard three inch headlap. This is not necessarily a problem as long as the slates with the excess headlap lay well on the roof, but it can be a problem with the roof scaffold staging and the use of standard three inch slate hooks to replace slates left out to accommodate the staging.

Finally, a graduated slate roof can be made a work of art by blending together a variety of slate colors. A common color scheme involves a mix of Vermont slates, including purples, unfading greens, sea greens, grays, and perhaps Vermont black and/or New York red. Sometimes Pennsylvania blacks, Virginia grays, or imported slates are mixed in as well. The percentage of each color must be determined before the slate is ordered, and with a variety of lengths, widths, and thicknesses to also consider, careful pre-planning is a necessity in order to ensure a successful job when creating a graduated slate roof. Some suggested color combinations by Rising and Nelson Slate Co. include: 1) 70% semi-weathering gray green with 30% variegated purple; 2) 50% semi-weathering gray green and 50% variegated purple; 3) 60% unfading mottled green and gray with 40% unfading green; 4) 50% semi weathering gray green with 20% variegated purple, 20% unfading green, and 10% Vermont gray black; 5) 70% unfading green and 30% unfading mottled green and purple. 

Once completed, a new graduated slate roof can be expected to grace a building and charm a community for at least a century, and maybe two.

Three Examples of Graduated Slate Roofs

It’s obvious that the sizes of slates and number of graduations is entirely a matter of style and/or personal taste. 

Heath Residence (below); Grove City, PA; 30 foot rafter, 3 inch headlaps; VT slates. Unusual graduated slate roof with uniform standard thickness slate.

# courses/ length of slate

[Top of Roof]

13 /16 inch

11 /18 inch

9 /20 inch

6 /22 inch

5 /24 inch

[Bottom of roof]

This 90 year old roof had five slate lengths which graduate in 2 inch increments according to an apparently random scheme. The slates on this 90 year old roof are nearly uniform in thickness (1/4 inch).

Ketler Garage (below), Grove City, PA; 25 feet 8 inches from drip edge to ridge; 39 courses; 3 inch headlaps; VT slates

# courses/ length of slate

[Top of Roof]

9/13 inch

1 /14 inch

6 /18 inch

3 /17 inch

5 /16 inch

3 /19 inch

3 /20 inch

3 /21 inch

3 /22 inch

3 /24 inch

[Bottom of roof]

This 90 year old roof graduates randomly. The bottom slates are 1 inch thick, the top slates are 3/16 inch thick.

Ketler House (below); Grove City, PA; 3 inch headlaps; VT slates

# courses/ length of slate

[Top of Roof]

etc.

1 /27 inch

1 /28 inch

1 /29 inch

1 /30 inch

This 90 year old roof graduates one inch per course from bottom to top. The bottom slates are 1 inch thick, the top slates are 3/16 inch thick.

Source : www.traditionalroofing.com – by Joseph Jenkins