Slate roofs are roofs made of rock. The rock is mined or quarried from holes in the ground, pits, tunnels, and deep shafts. It's hand-worked into manageable sizes, then split into roofing shingles with hammers and chisels. The finished shingles are punched for nail holes and the thin slabs of stone are fastened to the roof deck with nails. A typical slate roof will last at least a century and maybe two. Commercial roof slate quarrying began in the US around the mid 1800s.
Above: Camara Slate Quarry, Vermont (left); Shawn Camara hand splitting a slate shingle from a block of rock (center); Brent Ulisky installing the slate shingles on a roof (right).
There are two general categories of slate roofs upon which a roof professional will be called to provide consultation services. They are 1) older slate roofs, and 2) new slate roofs. By far the most common slate roof in the US and in the world is an older slate roof, as such roofs can easily last a century or more in good working order. Therefore, the bulk of this article will focus on older slate roofs. However, roof professionals are also called upon to provide advice when replacing or installing a slate roof. This issue is addressed later in this article.
OLDER SLATE ROOFS
Traditional slate roofing systems are fabulously successful systems that can easily function as a waterproof covering for 100 years, and, if properly constructed, for 150 years, or even 200 years. Some slate roofs in Europe are still in good functioning condition after 400 years. Their longevity, however, is not their only quality. Slate roofs are made of natural materials - primarily stone (slate) and wood (boards or lath/battens), with metal fasteners (nails). They are simple roofing systems, and they're beautiful to look at. When they do need to be replaced, they can be discarded as clean fill, as opposed to the toxic waste of petrochemical roofs. As such, they are roofs that are sought after by those who have ecological concerns.
Finally, slate roofs are arguably the least expensive roof money can buy when the entire life of the roof is taken into consideration. I recently looked at a slate roof on a cathedral in Arkansas that is 120 years old (picture below). The cost to install this 117 square, ornate roof, which has a 220 foot high spire, was $765.00 for the labor and $1,166.50 for the materials in 1881. Even adjusted for inflation, it should be obvious that this was money well spent.
TWO MAIN REASONS WHY OLDER SLATE ROOFS FAIL
1) TYPE OF SLATE
The primary reason older slate roofs fail is because of the type of slate - some wear out sooner than others. There are many types of roofing slate and they each have their own particular qualities and idiosyncrasies. On the 120 year old cathedral roof mentioned above, for example, the slate was installed with an ornate pattern of black and green slate. The green slate originated in Vermont, while the black slate originated in Pennsylvania. The black slate had a life expectancy of about 120 years and was showing a lot of delamination and crumbling. The slates had served their useful life and were now failing. The green slates, on the other hand, showed no deterioration after 120 years. It's anyone's guess how much longer they would last. If the entire roof had been installed with the green slate, the roof would not have needed replaced at this time.
Cathedral of St. Andrew, Little Rock, Arkansas, showing two types of slate: PA black and VT unfading green. The photo below, incidentally, shows the inside of the steeple, looking up.
So it is imperative that roof professionals who deal with slate roofs know the different types of slate, their origins, longevities, characteristics, and qualities, and be able to identify them by sight. If sight identification is not possible, then they must be able to send a slate sample somewhere to have it identified. Presently, in the US, roof slate is still being quarried in Virginia, Pennsylvania, New York, and Vermont. However, a century ago there were hundreds more slate quarries than there are today, including in Maine, and the differences between the slates from the various quarries were sometimes phenomenal. So a knowledge of the history of slate quarries is also important for roof professionals who work with older slate roofs.
In addition, it must be pointed out that roof slate has been quarried and/or mined in Europe for centuries. Wales has perhaps some of the oldest slate quarries in the world. Spain is now the largest producer of roofing slate in the world, and China has entered the market in a big way. Slate is also produced in South Africa, India, central Europe, Italy, and elsewhere throughout the world. These foreign slates are now entering the American market and being used on new slate roofs.
Softer PA black slate, 120 years old, on cathedral in Little Rock (above)
PA black and VT unfading green, 120 years old, on cathedral. Black is worn out, green is still good (above)
VT unfading green slate on 120 year old house. Slate is quite good (above)
PA black slate on house in NJ. Slate is nearly worn out at 80 years old (above)
2) FLASHING FAILURES
Even if a slate roof is composed of very long lasting slate material, the metal flashings will wear out and leak before the slate wears out. These flashings are metal joints that are installed between the various planes of the roof in order to prevent water entry. They are also installed around roof penetrations such as chimneys. The older flashings were usually made from terne coated steel (steel coated with a lead/tin combination, also know colloquially as "tin"), which had to be painted regularly to avoid corrosion. Copper flashings were also used, primarily in association with institutions and upper-scale residences. The terne flashings could last 90 years or longer if they were kept painted. Copper flashings, ironically, because they are typically not painted, will begin to corrode, pit, and leak in about 60 to 70 years in areas of high wear, such as valleys. For this reason, older copper flashings should be painted in order to extend their effective lives.
When flashings begin to fail on a slate roof that is made of sound slates, only the flashings should be replaced, not the entire roof. This is routine work for slate roof restoration professionals. One of the extraordinary characteristics of slate roofs is that they can be taken apart and put back together. Broken slates, worn flashings, rotted sheathing boards, or any element of the roof can be removed and replaced without the need to replace the entire roof. Because of this unusual maintenance characteristic, slate roofs can be made to last as long as the slate itself will last, which could be hundreds of years.
When repairing or restoring a slate roof, individual slates are removed from the roof in order to expose the existing flashings, which can then be removed and replaced. The removed slates can then be put back into their original positions, and the repaired roof will look much the same as it did before the repair. Faulty slates are also removed in order to replace them, and matching slates must be used whenever possible. The tool required for removing slates from a roof is the slate ripper - a sword-like object that slides up under the slate to be removed and pulls out the two nails that hold it in place. A slate hammer has a hole punch at one end, used to punch holes in slates for nailing. Slate hammers also often have shanks designed to cut slates, which is done by a chopping motion against a straight edge such as a slater's stake. Slates readily punch without breaking, leaving a clean hole with a "countersunk" characteristic into which the nail head sits. Slates are also readily cut with a simple hand-held device, a slate cutter. Slate roofing contractors work on slate roofs using hook ladders, which keep their weight off the slate while giving them a safe work platform to cling to. It is improper to work on slate roofs by walking on them using ropes. It is also improper to tar the surfaces of slate roofs. Slate is an excellent material to work with, having extraordinary physical characteristics. It does not shatter like glass, as many people believe.
Typical flashing repair - before, during, and after:
Valley replacement on badly abused residential slate roof (below)
Above: the valley looks completely shot and is heavily tarred (most inexperienced roofers would tear this roof off even though it is still highly restorable).
Above: the old metal is removed, as are the tarred slates. New copper is installed.
Above: The finished valley job leaves the roof looking as good as new - a one day job for one man.
Below: Some of the tools and equipment that are commonly used during slate roof restoration.
ADDITIONAL ASSESSMENT CONSIDERATIONS
There are additional nuances involved with the assessment of older slate roofs, such as type of nails, underlayment, roof sheathing or battens, slope, and previous maintenance. These factors will influence the longevity of a slate roof, more or less.
Low slope slate roofs will fail prematurely because people will walk on them over the years and break the slates. The resultant leaks are often repaired by non-professionals because the roofs are low in slope and therefore accessible. These repairs tend to be done poorly; the roof will still leak, resulting in more traffic on the roof, and a downward spiral of deterioration begins, ending with the demise of the slate roof. The lowest slope advisable for a slate roof is 4:12. However, the slope should be too steep to walk on in order for the roof to last a long time. That would bring the slope up to about 8:12 or steeper.
Nails are often said to be the cause of slate roof failure, however, this is often not the case at all. It is true that nails will corrode on an older slate roof, but this is most likely under two general conditions: 1) the nails were of poor quality when initially installed, and/or 2) the slate has reached the end of its life and moisture is now penetrating the roof, thereby corroding the nails. Originally, in Wales, slate roofs were installed with wooden pegs driven through a hole in the top center of the slate. The slate/peg combination was then hung over a horizontal lath on the roof - no nails were used.
The weight of the slates overlapping each other held the roofing in place. In the US, slates are nailed in place with two nails situated about a third of the way down the slate, along the outside edges. The slates are nailed into boards (roof sheathing - usually one inch thick) or into horizontal wooden strips (slating lath or battens, usually one by twos or threes), depending on the predilection of the installer. Lath roofs are common in Wales, England and Europe, so immigrants from those countries may have chosen to copy their traditional styles of slate installation once they arrived here in the US at the turn of the last century. Traditional Scottish roofs use solid boarding, as is more common in the US. Most of the older slate roofs in the US are nailed with hot-dipped galvanized roofing nails, although most institutional and upper-scale residential roofs are nailed with copper nails. Some older slate roofs are nailed with square-cut iron nails. I have examined many a hot-dipped nail that has been on a slate roof for 100 years and many have been still in quite serviceable condition. The exceptions are as mentioned above: poor nails to begin with (not hot-dipped), or a roof on it’s last legs due to slate deterioration.
Barn roof (above) showing lath roof decking. 100 year old white oak lath is still in good condition despite the totally deteriorated valley flashing. Lath roof (below) being sheathed over with 1x10 green hemlock lumber prior to reslating.
The need for felt underlayment on slate roofs is another "urban myth," so to speak. The most common underlayment on older slate roofs is 30 pound felt. It is used in order to prevent leaking during installation. After about 75 years, the felt deteriorates almost to a powder under the slates. This is not a cause for concern. Many slate roofs in the US have been installed with no felt underlayment whatsoever and they do not leak, even after a century. This is true for virtually all barn roofs, where leaking during installation was not a concern. The felt underlayment is only essential during installation on a structure where rain water can damage the interior. It is very bad advice to tell someone that they must replace their slate roof because the felt has worn out, although this sort of advice is often given by roofing contractors or professionals who don't know what they're talking about.
Finally, one of the most common causes of leaks in older slate roofs is, unfortunately, improper repairs. Bad repairs will still leak, they look ugly, and they can be very costly because the roof owner usually has to pay for them. These factors combined can make a roof owner, in frustration, want to forever remove his slate roof no matter how much longer it will last if properly repaired. In addition, roofing contractors, as well as professionals, who have little or no expertise in slate roofs, will advise a roof owner to replace a slate roof which may have many decades of life remaining. A client will listen to bad advice when it is the only advice that can be found.
Typical bad repairs (above) — wrong type and shape of replacement slate, face nailed through overlying slates - ugly, and a sure leak through face nail holes.
Above: Exposed nail in slot is used to fasten replacement slate, then caulked - another sure leak (should have used a bib flashing to cover the nail head).
Above: An exposed nail in the slot without caulk or bib flashing becomes a "hidden leak" because the nail head rusts away and the remaining hole is almost invisible. This is not uncommon on old slate roofs, but few people can spot this leak. Always use either a bib flashing or a slate hook when replacing a slate (see below).
NEW SLATE ROOFS
Roofing professionals will be called upon to advise on the installation of new slate roofs. This is where the tar will hit the fan in some modern roofing circles. There exists today such a wide gap between natural, traditional roofing techniques and modern, chemically based roofing techniques, that most roofing contractors and architects today are experienced only in the latter. They therefore specify slate roofs as if they are just a variation of an asphalt shingle roof, which they are not. Slate roofs are expected to last a century or two, and the materials used in conjunction with the slate should have that degree of longevity, at least. Asphalt and chemically derived roofing has an average life expectancy of 10-20 years. Therefore, asphalt roofing systems should never be used as a pattern for slate roof installation.
The common steep-slope asphalt roof system of today employs a plywood or other laminated or glued wood product deck, often covered with a self-adhesive underlayment. A properly installed slate roof, however, will avoid any laminated or glued wood product for decking, as these materials have no record of the longevity required for a slate roofing system, and in fact, do have a record of delamination and premature failure, especially under adverse climate conditions. Furthermore, ice and water shield is a product designed to prolong the life of laminated wood decking, and therefore has no real purpose on a properly constructed slate roof.
A properly constructed slate roof, built according to traditional American techniques, will have a board roof deck — either one inch thick rough sawn lumber, or ¾" planed, kiln-dried lumber, or 1 ½" planed, kiln dried lumber (more common on institutions). Tongue-in-groove lumber is also used. The nails used on new slate roofs should be either copper or stainless steel, although hot dipped galvanized nails will last a century or much longer. Aluminum nails and electro-galvanized nails should be avoided. It should go without saying that the design of the roof itself adds much to its effectiveness and longevity. Low slope slate roofs (less than 4:12), for example, should be avoided. For best results, slate roofs should be designed so they’re too steep to walk on. Also, non-corrosive flashings should be used, such as terne-coated stainless steel, or copper. The flashing can be eliminated altogether, in some cases, by designing the roof with rounded (slated) valleys and slate hips and ridges, as is more common in Europe.
A fully sheathed board roof deck with a temporary 30 lb. felt underlayment is a very successful tried-and-proven roofing system used with slate roofs, and should be used with all new slate roofs for best results. The felt is optional — no underlayment is needed for the proper functioning of the roof. The valley replacement on a 104 year old church shown above reveals the 1” rough-sawn hemlock board decking and the complete absence of underlayment. The slate is PA black, still in good condition. No water can penetrate the slate/copper roof system, which is why the underlayment is optional.
Roof ventilation is a big issue today in the roofing industry, and it should be. Asphalt roofing shingles nailed or stapled onto plywood decks are roofs systems that will not breathe – they suffocate the roof, and proper ventilation is imperative. Otherwise the plywood will delaminate and the roof will fall apart. Slate roofs attached to board decking, on the other hand, are breathable roof systems. They are not air-tight – they’re water tight. Ventilation may be necessary to prevent condensation occurring under the roof sheathing from warm inside air leaking into the roof space. This is easily achieved with gable vents or roof vents, air spaces between the insulation and the sheathing, and ventilated soffits. However, most older residential slate roofs had no particular ventilation systems associated with them. After a century, most are still in good working order. Such is the advantage of a roof that can breathe on its own. When these older slate roofs are retrofitted with roof insulation, care must be made to ensure that warm air will not come in contact with cold roof sheathing. Aluminum ridge vents should be avoided on slate roofs. Such vents are designed to be used with asphalt shingle roofs — as such, they’re cheaply made, they do not fit the ridge of a slate roof well, and they interfere with the normal maintenance of a slate roof.
New slate roofs are only as good as the installation. Contractors who are skilled at asphalt roofing are not necessarily qualified to install slate. For example, many new slate roofs today are having serious problems such as scores of slates falling out within the first ten years after installation. This is caused by the installers walking on the slates during installation — a common practice among asphalt shinglers on asphalt roofs, but a mistake on a slate roof. Furthermore, roofing slates are overlapped in such a manner that each course overlaps two courses below. This particular overlap is called the headlap, and three inches is standard. If a proper headlap is not used, the roof can have serious problems later. Over-nailing and under-nailing of the slates can also be a problem. Over-nailed slates are nailed too tightly and the nail can break through the slate, or the slate can crack under the stress. Under-nailed slates are not nailed tightly enough and the nail-head will rub against the overlying slate, eventually wearing a hole through it. Poorly installed slate roofs are unfortunately and unfairly beginning to give slate a bad name. Asphalt shingle roofers make their money in roof replacement. Perhaps they have a vested interest in installing a roof that will fail prematurely. The installation of slate roofs is an art in itself, and, when mastered, it creates a roof to be proud of — a work of practical beauty that can benefit many generations.
One major concern today is the inadvertent selection of bad slates for roofing material, often from foreign sources. There are bad slates available on the market — slates that will fail within as little as ten years. The best way to determine the quality of a certain type of slate is to do a historical analysis. Purple Vermont slates, for example have been historically proven to easily last 150 years. PA black slates may last 100 or more years. Chinese or Spanish slates of an unknown origin are a total gamble, some failing in as little as one year. Any roof professional who specifies or encourages the use of a slate material of unknown origin is asking for a nightmare.
Much more information about all aspects of slate roofs is available in the Slate Roof Bible. Jenkinsslate.com, and Traditionalroofing.com also provide an abundance of information about slate roofs.
SOME SLATE ROOF CONSULTING HIGHLIGHTS
I was called upon to inspect the roof of the Ford Theater in downtown Washington DC (below) in 1998 – the site where Lincoln had been assassinated. The roof was leaking badly in one spot and not so badly in another. The National Park Service, responsible for the maintenance of the building, was considering replacing the roof.
There was good access to the roof from an adjacent building which had a flat roof. You could stand immediately beside the slate roof and have the drip edge of the slate right at waist height. The slates looked old, and, in fact, they were the original 1865 Buckingham, Virginia slates, installed just prior to Lincoln’s death (he visited the theater shortly after it had been constructed). Surely this 133 year old roof was a candidate for a re-roofing project.
To make a long story short, after inspecting the roof it was obvious to me that the roof had already been replaced. The original slates had been removed about 25 years earlier, the good slates salvaged, and then re-installed with copper nails on the side that could be viewed from the neighboring flat roof. The old slates now covered an area which comprised about 1/5 of the total roof. The entire remainder of the roof had been re-slated with new Buckingham, Virginia slates. Essentially, the Ford Theater already had a new roof, a fact not visible from the flat roof below.
A solder seam on a copper coping of one parapet wall had split right where water pooled on the parapet. This was the cause of the primary leak. A metal flashing between the flat roof next door and the theater building had pulled away from the building. This was the cause of the secondary leak. The slate roof did not leak at all! No one at the National Park Service had been employed there for more than about 20 years, so no one remembered when the roof had been replaced, and there were no records of it either. My visit saved the federal government a few hundred thousand dollars.
In 2000, I was called upon to look at an up-scale estate in the New York City suburbs (see below). Some slates had been found on the ground alongside the mansion after the winter ice had thawed, and the owners were concerned. A slate roofing "expert," a "third generation slater," was called in to inspect the roof. He subsequently advised that the entire slate roof needed replaced — to the tune of $450,000. This seemed odd to me, especially considering that the slate roof was only 12 years old. When I went to look at the roof, I discovered that the snowguards — the metal fixtures on the eaves that hook under the slates and protrude up off the roof surface to prevent snow and ice avalanches – had pulled out under the weight of an excessive snow load. When the snowguards let loose, they pulled some slates off with them and they all ended up on the ground. The snow guards failed because there were not enough of them for the size and steepness of the roof. This had been a particularly bad winter for ice and snow, and the sparse snow guards couldn’t hold the load.
The entire problem was repairable by replacing the missing slates and doubling the number of snow guards, or leaving them off completely. And it was all covered by homeowner’s insurance! The lesson to be learned here is that just because someone calls himself an expert on slate, that doesn’t mean he’s not a shyster. Always get a second opinion in a situation of this magnitude. In this case, it save the prudent owners nearly half a million dollars!
Smithsonian roof (with Buckingham slate).
It is very common to run into situations where perfectly good slate roofs have been condemned as beyond repair by either roofing contractors or roof professionals. Usually the contractors are trying to land a re-roof job, or the professionals are selling materials or getting a commission on something. However, a properly informed, honest roof professional can save a client a lot of money as well as preserve one of America’s most overlooked treasures — slate roofs.
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