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Traditional slate roofs are fabulously successful roofing systems that can easily function as a waterproof covering on a building for a century - or, if properly constructed, for even 150 or 200 years. Some slate roofs in Europe are still in good functioning condition after 400 years.

Slate Roof

Beautiful to look at, slate roofs are simple, low-tech roofing systems made of natural materials: primarily stone (slate) with wood boards or battens and metal fasteners (nails). When the slates need to be replaced, they can be discarded as clean fill, unlike the toxic waste of petrochemical roofs; as such, slate roofs are sought after by the ecologically minded.

Four Main Reasons Why Older Slate Roofs Fail

  1. Type of slate
    Some types of slate wear out sooner than others, and once they wear out, they can’t be saved. There are many types of roofing slate, each with its own particular qualities and idiosyncrasies. The 120-year-old cathedral roof mentioned above was installed in an ornate pattern of black and green slate. The green slate originated in Vermont, the black in Pennsylvania. The black slate had a life expectancy of about 120 years, and having reached that age, it was showing a lot of delamination, softening, and crumbling. The green slates, however, remained hard and 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, it would not need to be replaced. But because half of the slates were of this softer black variety that had reached the end of its life, the roof was not repairable; it had to be replaced (with new slate, of course).

    It is imperative that people who own or work on slate roofs know the different types of roofing slate — their origins, longevities, characteristics, and qualities — and be able to identify the slate on the roof in question. If sight identification is not possible, then they must be able to send a slate sample or photo to someone who knows slate to have it identified. In the U.S., roofing slate is still being quarried in Virginia, Pennsylvania, New York, and Vermont; a century ago there were hundreds more American slate quarries, including such states as Maine and Georgia, and the differences between slates from the various quarries were sometimes striking. Knowing the history of slate quarries is important for people who work with traditional slate roofs.

  2. Flashing failures
    Even if a slate roof is composed of very long-lasting slate, the metal flashings can wear out and leak before the slate begins to fail. These flashings are sheet metal joints installed to prevent water entry between the various planes of the roof, such as in the valleys, along dormer walls, and around roof penetrations such as chimneys. The most common older flashings were made from terne-coated steel, which is steel coated with a lead/tin combination, erroneously but commonly called “tin.” Terne-coated steel must be painted regularly to avoid corrosion. Copper flashings (either plain copper or lead-coated) were used primarily on institutions and upper-scale residences; sheet-lead flashings were often used on older buildings, especially around plumbing vent pipes. Terne flashings could last 90 years or longer, if they were kept painted. Copper flashings, because they typically are not painted, begin to corrode, pit, and leak after about 60 to 70 years, in areas of high wear such as valleys. For this reason, older copper flashings should be painted to extend their effective lives.

    Only the flashings should be replaced, not the entire roof. This work is routine for slate-roof-restoration professionals. One of the extraordinary characteristics of slate roofs is that they are designed to 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 replacing 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.

    Then repairing or restoring a slate roof, individual slates are removed to expose the existing flashings, which can then be removed and replaced. After the removed slates are put back into their original positions, the repaired roof should look much the same as it did before the repair, except with new flashings. A good repair is invisible to the layperson.

  3. Slates are broken or missing
    It is not uncommon for a century-old slate roof to have 50 or more slates fail from simple attrition. Slate is a natural stone with faults and hairline cracks, and slates will eventually break here and there on a roof. A typical 20-square roof (2,000 sq.ft.), with a typical 10-in.-x-20-in. slate, will have about 3,400 slates. If 50 fail after a century, then the roof’s failure rate is less than 1.5% per 100 years — phenomenally small. One missing slate, however, is all it takes to create a leak, which in turn may cause someone to shout, “Tear off the roof and replace it!” More often than not, people unknowingly lose a good, repairable slate roof when they could have easily repaired or restored it.

    Faulty slates should simply be removed and replaced. Replacement slates that match the originals in size, shape, and color must be used whenever possible. Replacement slates must never be fastened in place with visible straps or face (exposed) nails. Instead, there are two generally accepted methods of fastening replacement slates into place: the nail-and-bib method and the slate-hook method (see diagram below).

    The nail-and-bib method is perhaps the most widely used. It involves nailing the replacement slate with a nail in the slot between the overlying slates (see illustration) and then sliding a “bib” flashing under the overlying slates and over the nail head. The bib is often bent slightly to friction-fit it into place. It can be composed of aluminum, copper, or other non-corrodible metal, but the bib should not be shiny and reflective like stainless steel, as it may then be visible from the ground on a sunny day. Instead, copper or brown-painted aluminum (coil stock) are preferred. A common-size bib is 4 in. x 7 in.

    A slate hook is a hard wire hook made of galvanized steel, copper, or stainless steel, approximately 3 in. long. A small exposed loop hooks the replacement slate in place (see illustration). In this instance, an exposed repair device is acceptable because the tiny hook is almost invisible from the ground. Stainless-steel hooks are stronger than copper. Slate hooks are preferable to the nail-and- bib on particularly hard slates and on new slate roofs, especially for repairs.

    The tool required to remove slates from a roof is the slate ripper: a sword like object that slides up under the slate and pulls out the two nails which hold it in place. A slate hammer, another important slate-roofing tool, has a hole punch at one end, used to punch holes in slates for nailing.

    Some slate hammers also have shanks designed to cut slates, which is done by a chopping motion against a straight edge, typically a Slater’s stake. Salvaged slates readily punch without breaking and leave a clean hole with a “countersunk” characteristic, into which the nail head sits. New slates can be hard and brittle, and require some practice for easy punching with a slate hammer. Standard-thickness slates (3/16 in.) are readily cut with a simple handheld device, a slate cutter.

    Contractors should 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 not proper to work on slate roofs by walking on them using ropes, as walking on slate roofs breaks the slates, which is the primary reason low-slope slate roofs fail prematurely.

    Slate roofs can, in some circumstances, be carefully walked on by a qualified slate roofer — which means someone who will repair any slates he or she may break while moving about. It is improper to tar or coat the surfaces of slate roofs, or to use surface tar for repairs. Not only is it unsightly, but it won’t stop leaks permanently and will ruin the slates.

  4. Abuse, bad repairs, amateur work, neglect
    One of the biggest problems facing older slate roofs today, and a cause of many leaks, is not natural attrition, flashing failures, broken slates, or global slate failures. It is, put plainly, bad work. There are many unqualified persons attempting to repair slate roofs, who don’t know what they’re doing. In my own slate-roof-restoration business, fully half of the work we do is the removal and replacement of faulty repair work. Slate-roof owners pay good money to have their roofs abused, then they have to pay good money again to have it repaired correctly. Abused roofs include the ones that are walked on by “Bigfoot” and the ones that are face-nailed, tarred, repaired with non-matching slates, coated, or re-flashed incorrectly.

    Furthermore, roofing contractors who have little or no expertise in slate roofs may advise a client to replace a slate roof which could have many decades of life remaining - and a client will listen to bad advice if it is the only advice that can be found. All these factors combined can make a frustrated client want to forever remove a slate roof, no matter how much longer it could last if repaired properly.

    Additional Considerations
    Low-slope slate roofs will fail prematurely because people will walk on them over the years and break the slates. The resulting leaks are often repaired by non-professionals because these roofs are readily accessible. These repairs tend to be done poorly: The roof will still leak, resulting in more traffic on the roof. A downward spiral of deterioration thus begins, ending with the demise of the slate roof. The lowest slope advisable for a slate roof is 4:12; however, for the roof to last a long time, the slope should be too steep to walk on, which would bring it up to about 8:12 or steeper.

    Nails are sometimes said to be the cause of slate-roof failure, but they are often not at all to blame. It is true that nails will corrode on an older slate roof, but this problem 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 and 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 U.S., 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 1-in. thick) or into horizontal wooden strips (slating lath or battens, usually 1x2s or 1x3s), depending on the predilection of the installer. Lath roofs are common in Wales, England, and Europe, so immigrants from those countries often copied their traditional styles of slate installation once they arrived here at the turn of the last century. Traditional Scottish roofs use solid boarding, as is more common in the U.S. Most of the older slate roofs in the U.S. 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 seen many a hot-dipped or cut-steel nail that has been on a slate roof for 100 years and still in quite serviceable condition. The exceptions are as mentioned above: poor nails to begin with (not hot-dipped), or a roof on its last legs due to slate deterioration.

    The need for felt underlayment on slate roofs is another exaggerated “urban myth,” so to speak. The most common underlayment on older slate roofs is 30-lb. felt, used to prevent leaking during installation. After about 75 years, the felt deteriorates almost to a powder under the slates, but this is not a cause for concern. Many slate roofs in the U.S. 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 and so no felt was used when the roof was installed. A felt underlayment is essential only during installation on a structure where rainwater can damage the interior. It is very bad advice to tell someone that they must replace a slate roof because its felt has worn out, yet such recommendations are often given by roofing contractors or consultants who don’t know what they’re talking about.

    Nowadays, the trend is to install slate roofs as if they are simply asphalt roofs with slate on them — which they are not. Traditional, tried and proven methods of slate roofing are being abandoned and replaced by methods that cater to the convenience of the contractor and/or architect. Consequently, architects are now specifying new slate roofs with plywood roof decks and ice- and water-shield (to preserve the plywood), as they do for asphalt roofs. Although slate can be installed on plywood, you cannot expect a plywood roof deck to last as long as a natural wood deck, which will easily endure 150 years, maybe much longer. I stayed in a house in Scotland last year with a 215-year-old, original, 1-in. board roof deck, and, of course, a slate roof in excellent condition. Natural roof decks do not need ice- and water-shields, a fact that has been proven by countless century-old slate roofs with natural wood decks and no shields. There is no acceptable reason to downgrade proven, simple, natural, and fabulously successful traditional slate-roofing methods. The trend toward membrane-covered plywood decks under slate will create a whole new set of problems for slate roofs in the future. When smart roofers and architects stick with traditional roofing methods, they create for future generations one of America’s most overlooked treasures: a beautiful, long-lasting slate roof.