Diagnosing the Installation of Slate Roofs Joseph Jenkins Slate Roofing Contractors Association of North America, Inc. 143 Roofing Lane, Grove City, PA 16127 Phone: 814-786-7015 • E-mail: joe@slateroofers.org RC I I n t e r n a t i o n a l C o n v e n t i o n a n d T r a d e S h ow • Ma rc h 1 4 – 1 9 , 2 0 1 9 J e n ki n s • 1 0 1 Abstract Slate roof installation diagnostics can be difficult and problematic for an inexperienced roof consultant. Yet, failures of new slate roof systems are much too common and often lead to expensive, drawn-out litigation. Slate Roof Installation Guidelines, published by the Slate Roofing Contractors Association of North America, Inc. (SRCA), point out best practices that help to make a slate roof installation problem free and successful. The SRCA guidelines are used in this presentation as a basis for evaluating a slate roof installation. Informational sections will include: • Contract documents and supporting information • Roofing slate identification, characteristics, and quality assessment • General materials, underlayment, nails, flashings, fasteners, and roof decking • Slate execution, headlap, starter courses, ridges and hips, and flashing installation • Case studies – Multimillion-dollar Pennsylvania home – Private home in Scottsdale, Arizona – Church in New Jersey – Shopping center in Louisiana – Museum in New York – Mansion in Pennsylvania Speaker Joseph Jenkins – Slate Roofing Contractors Association of North America, Inc. – Grove City, PA JOSEPH JENKINS is the author of the Slate Roof Bible, and he also publishes Traditional Roofing Magazine. In addition, he is the founder and executive director of the nonprofit SRCA, which is itself a member of both the International Federation for the Roofing Trades (IFD) and an Affiliate member of the National Roofing Contractors Association (NRCA). Jenkins is also a former board member of the National Slate Association. Jenkins was a slate roofing contractor for decades and has personally worked on over a thousand slate roofs. He started roof consulting in 1998, with Ford’s Theater in Washington D.C. being his first client, and he has since consulted on slate roofs of all sizes and ages across the U.S. and Canada. 102 • JenkiKIns RCI International Convention and Trade Show • MarcRCh 14-19, 2019 WHAT DOES THE CONTRACT SAY? It’s hard to imagine a $400,000 roofing project without a written contract document. But it happens (Figure 1). When there is nothing in writing, and the roof is riddled with questionable details, who’s to say what is right and what is wrong? Is the slate poor quality? Maybe the property owner chose that slate based on looks rather than performance. When the roof fails, the finger pointing begins. If there is a written contract (and there always should be), then the answers are usually in black and white. The contract documents should list the type(s) of slate, the size(s) of the slate, the shape (rectangular, rounded, etc.), thickness, slating styles, nail type and length, flashing materials (including pertinent dimensions and weights), slate headlap, valley styles, ridge styles, hip styles, underlayment, roof ventilation, and any other details that could be subject to debate after the job is complete. When diagnosing a slate roof installation, get your hands on the contract documents and any other important paperwork, such as the slate invoices, estimates, and competing bids. There can be a trove of information there that can help you to understand what’s going on. For example, if the new roof is screwed up, look at all the bids on the project. Did the client hire the lowest bidder, way lower than everyone else? There’s your first red flag. IDENTIFY THE SLATE There are numerous types of roofing slates, each with its own characteristics, including longevity. Virginia Buckingham slate is more of a phyllite than a true slate and can be expected to last a couple centuries on a roof. Vermont slates come in a variety of colors, including purple, red, green, black, and gray, in weathering and non-weathering. Most are good for a century or two. There aren’t many new Pennsylvania slate roofs being installed anymore, even though there were many thousands installed in the past. Most of the Pennsylvania quarries are out of operation, but not all. Canada has the largest slate quarry operation in North America. Lots of roofing slates are being imported from overseas—mostly from Spain, China, Brazil, and Argentina (Figure 2). Some slates have pyrites that leach rust stains down the roof; others have knots, knurls, excess calcium, and other defects. Some imported slates have been disasters, failing within 10 years on huge roofs, with the roofs having to be replaced with domestic slates at great expense (Figure 3). Are you dealing with a fading black Chinese slate that was sold as unfading black? Or what? Diagnosing the Installation of Slate Roofs RCI International Convention and Trade Show • MarcRCh 14-19, 2019 JenKIns • 103 Figure 1 – The roof above was installed without a written contract. It was riddled with errors. The general contractor was slapped with a $400,000 judgment. The entire roof had to be removed and reslated. Figure 2 – Imported slates replaced the original domestic slates on this architectural gem in New York City. Most imported slates are in metric measurements, making them easy to identify. Most imported slates are metric and will measure in centimeters rather than inches. This makes them easier to identify should you not be very familiar with slate types. A multimillion-dollar slate reroof was initiated because a roof consultant misidentified the existing striated Vermont slates as “ribbon” slates. At one time, ribbon slate was common. It’s a smooth black Pennsylvania slate with a carbonaceous band running across it, and it doesn’t look anything like Vermont striated black, also know as “Strata Gray” (Figure 4). When I started roofing in 1968, the first thing my 63-year-old Ukrainian mentor told me was, “Stay away from the ribbon slate. It falls apart!” That was 51 years ago, and it did fall apart, right at the ribbon. Today, nobody is quarrying and selling ribbon slates for roofing purposes, nor have they been for many decades. The notorious ribbon slates are not allowed; the language banning them is permanently tucked into contract boilerplate. If you happen to be scrutinizing the fine print on a contract and run into the ribbon slate ban, you may be tempted to accuse the roofer of having installed ribbon slate in violation of the contract, as happened at a large eastern university in recent years. A torrent of finger-pointing and recriminations led to extensive and costly litigation that could have been easily avoided. Now I will always be able to tell my grandchildren that I spent 12 hours in a room with 20 lawyers, something I never wanted to be able to say, all because a roof consultant misidentified the slate on a university roof. UNDERLAYMENT – LET’S BURY THIS ISSUE Underlayment is an inexpensive temporary roofing paper that is installed underneath the slates to keep the rain out of the building while the roof is being installed. After the roof is installed, the underlayment serves no useful purpose and can just disappear without detriment to the roof. And it usually does over time. Slate has been installed for 1000 years, but underlayment has been relatively recently utilized in the slating industry. The longer a building needs to be weathered in before the roof is installed, the heavier and more durable the underlayment needs to be (think churches). In most cases, slate roofs utilize 30-lb. felt underlayment when the roof is going to be installed in days or a couple of weeks. That underlayment can be doubled (half-lapped) to extend its service life a few more weeks. When the slates are installed, the felt will be perforated with thousands of nail holes, rendering it ineffective as a watertight roofing membrane. Once the roof is on, it doesn’t matter; at that point, it’s the slates and flashings that keep the water out, not the underlayment. The underlayment issue is complicated by the fact that there are now many new synthetic products on the market intended to replace felt. How these affect the long-term performance of slate assemblies is anyone’s guess. Traditional slate roofs have underlayment that disintegrates over time. Traditional slate roof decks are boards that have small spaces between them caused by shrinkage of the wood. This system allows for air to percolate through the roof; moisture is not trapped, and things can dry out if moisture happens 104 • JenkiKIns RCI International Convention and Trade Show • MarcRCh 14-19, 2019 Figure 3 – The roof on this museum building had been replaced with imported slates that failed in ten years. The entire roof had to be removed and reslated—but this time with high-quality Vermont slates with a life expectancy of 150 years. Figure 4 – Although popular Vermont “Strata Gray” slates appear to have ribbons running across their face, they shouldn’t be confused with the notorious Pennsylvania “ribbon slates,” which are not suitable for roofing. to enter the roof assembly. Modern nonbreathing underlayments and plywood roof decks change the way roofs perform, especially regarding ventilation and “breathing.” When the roof can’t breathe, ventilation must be designed into the system to prevent condensation and to allow for airflow. Modern “breather” underlayments can keep a roof watertight and still allow it to breathe. If installers are straying from traditional felt underlayments, they should be using breather underlayments. Roof consultants too often incorrectly tell their clients that their slate roof needs to come off because the underlayment has disintegrated. This author has worked on scores of barn roofs that never had a square inch of any type of underlayment. They were century-old buildings. No underlayment was used because it didn’t matter if it rained inside the barn while the roof was going on. The barn owners also didn’t want to spend the money on something they didn’t need. If the slates were being installed on “skip sheathing,” as is common on barns, felt underlayment covering the slating lath would be impractical and even dangerous because it would be hard to see where to put your feet when climbing on the roof. Some say, “Well, who cares if the roof leaks? It’s only a barn, after all.” The farmers care. If their hay gets wet, a fire can result. Barn roofs won’t leak if properly cared for, underlayment or no underlayment. One recent client, a large church, was informed by a roof consultant that their beautiful, mixed-color Vermont graduated slate roof would have to be completely removed and reinstalled to replace the original underlayment. The price tag was $400,000. This advice was completely ludicrous and revealed an unfortunate misunderstanding of slate roofs. Luckily, the church didn’t bite. This is not an isolated case; there are many others that are nearly identical. RCI International Convention and Trade Show • MarcRCh 14-19, 2019 JenKIns • 105 Figure 5 – Good slates, good fasteners, and good substrate are all that is needed for a roof to last over 165 years. The underlayment is irrelevant to the long-term performance of a slate roof. Although the flashings may wear out over time, they are easy enough to replace. Figure 7 – The entire new church roof above was rejected due to incorrect headlap, among other things. All the headlap was 1 in., and in some places, it had 1 in. or no headlap at all. This project went into arbitration and resulted in a judgment against the roofer of $276,000. Figure 6 – The bottoms of the slates in row three overlap the heads of the slates in row one. This is called headlap, and it is critical to the integrity of a slate roof. NAILS AND FLASHINGS Slate roofs only need four things to be successful roofs for generations: good slates, good fasteners, good substrate (roof decking), and good installation (Figure 5). Will all the materials in the roof assembly last a century or two? If S1 (roof-grade) slates are nailed into solid glue-free boards with long-lasting fasteners, and the installation is done correctly on a roof with adequate slope, you’ve got it made. Correct slating nails are smooth-shank nails, most often copper roofing nails, sometimes stainless steel nails when the decking is too hard for copper, and sometimes hot-dipped galvanized nails. Hot-dipped slating nails—both cut nails and wire nails—were in common usage for the past 100 years, only replaced primarily by copper nails in recent decades. The nails must be smooth-shank so they can be removed should the roof need repair or restoration work. Ring-shank nails tend to break off rather than pull out, making the removal of slates and flashings extremely difficult, tedious, and annoying. Traditional flashings were most often terne-coated steel, which needed to be painted, often before it was even installed. Today, we recommend a minimum of 16-oz. copper flashing (20-oz. copper is better), or 4-lb. sheet lead on slate roofs. Stainless steel flashings also have the longevity needed to complement slate. Stainless steel has a relatively high tensile strength and can wear out your tin snips pretty fast if you have to cut a lot of it. Both copper and stainless steel can be lead coated or terne coated. Both copper and stainless steel can be soldered. Aluminum is not recommended, as it cannot be soldered. HEADLAP—HERE’S THE BIG ONE Headlap is the overlap each slate has on the slate two courses below (Figure 6). If the headlap is inadequate and not up to code, then the roof can be condemned and will have to be removed and installed again (Figure 7). Here’s what building codes and industry standards require: • 4-in. headlap on slopes less than an 8:12 down to 4:12 • 3-in. headlap on slopes 8:12 to 20:12. • 2-in. headlap on slopes 20:12 or greater. • On less than a 4:12 slope, slate is not recommended. Headlaps can be increased at ice-dam-prone or poor drainage areas. Contractors will argue that two inches of headlap will work on many slopes less than 20:12, and they’re right in some respects. If a roof has an 8:12 slope and has no penetrations, no chimneys, no valleys, no dormers, just a straight gable, and is free-standing in a field, with no overhanging roofs, then a 2-in. headlap can perform for quite some time, especially in a dry climate. That is, until the 100-year storm (rain or snow), or until a bad ice dam builds up. Construction standards are designed to apply to 106 • JenkiKIns RCI International Convention and Trade Show • MarcRCh 14-19, 2019 Figure 8 – If the roof has an open gable end, the headlap is easy to view. Here the headlap on this college dormitory starter course is almost nonexistent, when it should be at least 3 in. Note also that the starter slate and first course slate are not laid back to back as they should be, and that there is no cant strip—three immediately obvious errors that suggest more errors are waiting to be found. Figure 9 – If you must remove a slate to observe the underlayment, decking, and headlap, use a slate ripper to pull out the slating nails (the ripper does not cut the nails; it bends them and pulls them out in one piece), slide the slate out, then put the same slate back in with a stainless steel or copper slate hook. In some cases, you may need to remove two slates to get a better view of the substrate and underlayment. a variety of situations, building configurations, and climates. The prescribed headlaps will make sure the roof does not leak under that variety of circumstances. Los Angeles only gets 15 inches of rain a year, but it can get 5 inches in one day. That’s the day when you’re glad you installed the headlap according to code, even though you’re virtually in a desert. The best way to determine headlap is to measure the exposure of 10 slates, vertically, in the field of the roof, then divide by ten to get the average exposure measurement (exposure is what you see). Do this on as many of the roof planes as is practical. Double the average exposure measurement, then subtract that from the length of the slate, and you have the average headlap. If the slate is 20 in. long and the average exposure is nine inches, the average headlap is two inches. You will also need to look at the starter courses. This is where headlap is chronically inadequate, but where it’s most important because all the water on the roof runs over the bottom courses. You can easily see the headlap on any open gable end (Figure 8), although it is invisible on hip roofs (because there is no open gable end). Sometimes you will have to pull a slate out to see the headlap, underlayment, and substrate. Pull the slate nails out using a slate ripper and hammer, make your observations, then put the slate back in using a slate hook and hammer (Figure 9). STARTER COURSE—THE FIRST PLACE TO LOOK When you’re on your ladder or lift approaching the roof, the first place you come to is the eaves, or drip edge. Take a close look. Here is where you will find out whether the roofers were experienced or not. The starter course of slate—also known as the “under-eaves course” because it is underneath the first course of slate and is not visible (except the bottom edge)—often shows specific errors. The first and most obvious is the starter slate being installed front-side up when it should be back-side up. The starter course and the first course are supposed to be back to back (Figures 10 RCI International Convention and Trade Show • MarcRCh 14-19, 2019 JenKIns • 107 Figure 10 – The starter course and the first course are typically laid back to back. When the starter slate is laid front to back, as shown here, it often means that the installer was inexperienced and that there will be more installation mistakes to be found. Figure 11 – The starter course and first course are traditionally laid back to back. When the starter course is laid incorrectly, it’s usually an indication of an inexperienced installer. Figure 12 – The water drainage slots between the slates should be offset laterally at least 3 in. Here the offset is almost nonexistent. This is the starter course and first course of slates, which all the water on the roof must pass over. A 6-in.-wide piece of sheet copper slid in between the slate courses will stop it from leaking. and 11). This is purely for aesthetic purposes and does not affect the functioning of the roof. But if these slates are not back to back, then you can be pretty sure that the installer was inexperienced, because putting starter and first courses back to back is a standard and traditional practice. When you see an error on the starter course, you will likely find errors throughout the entire roof installation. Additional starter course errors can be viewed at the bottom edge, too. Lack of sidelap is a common one (Figure 12). The drainage slot between the slates is supposed to be offset laterally from any other slot above or below it by a minimum of 3 in. Otherwise, water can be drawn into an underlying slot by capillary attraction, causing a leak. There should also be a cant strip underneath the starter course (Figure 13). This tilts the starter slate at an angle to match the angle at which all the field slates are tilted. All slates lie on top of other slates, hence the angle. Since the starter slate has no other slate underneath it, it must have something put underneath it to cause the tilt. The cant strip is usually a strip of wood maybe 3/8 in. thick, but it can also be built into a copper drip edge. The cant can be more difficult to observe, but look for it anyway. Speaking of drip edges, they are not necessary on slate roofs. Drip edges are common on asphalt shingle roofs because they hold the shingles up to keep them from sagging over the edge of the roof. Slates don’t sag, so no drip edge is needed. Copper drip edges are sometimes used on slate roofs for aesthetic purposes, or to protect the substrate, or to act as an interface between the roof and the gutters. RIDGES, HIPS, AND FLASHING INSTALLATION A common problem with new slate roof installations is exposed fasteners on flashings. Roofers will nail through apron and ridge flashings and leave exposed nailheads all over the place. Expansion and contraction of the metal due to temperature changes will cause the nails to loosen up, the nailheads to pop up, and the roof to leak. The best way to 108 • JenkiKIns RCI International Convention and Trade Show • MarcRCh 14-19, 2019 Figure 13 – The cant strip is often just a strip of glue-free wood, as indicated by the arrow. If a copper drip edge is being used, then the cant can be formed into the copper, eliminating the need for a wood strip. Figure 14 – Ridge cleats (top) are nailed underneath the flashing where they can’t leak. They’re folded over the flashing to hold it in place, then riveted for extra durability (middle). Contractors will often just nail flashings with exposed nails. These almost always leak (bottom). deal with this is to use concealed fasteners, such as cleats or clips, on flashings (Figure 14). This includes metal hips and ridges, as well as dormer aprons and roof-to-wall transitions. Copper and Common Sense by Revere Copper Products, Inc. has some good details and illustrations showing how to do this. Also, SMACNA’s Architectural Sheet Metal Manual contains a lot of good information. Another flashing issue involves installing metal valleys between slopes that are not symmetrical. The valley flashings should have an inverted v-groove down the center. When this detail is overlooked, hydrostatic pressure can force water under the slates on one side of the valley, causing a leak. Incompatible metals are a sign of poor workmanship (Figure 15). Steel and copper don’t mix, for example, but steel nails and screws are often incorrectly used to fasten copper flashings. Copper rivets with copper-plated steel mandrels (shanks) are also a no-no. They look like they’re copper, but the steel mandrel breaks off inside the rivet when it’s popped, then eventually rusts away, leaving a hole and a point of water entry in the flashing. If the fasteners are magnetic, they should not be used with copper. There are plenty of acceptable fasteners to use with copper, including copper rivets with brass mandrels, stainless steel rivets, plus copper and stainless steel nails and screws. Solder joints in flashings should always be closely checked. This is one problem with using drone surveillance to examine a slate roof. You just can’t get close enough to see important details like cracks in solder joints (Figure 16). Solder joints crack for two basic reasons: the joints weren’t fully sweated in the first place, and/or there was little or no allowance for expansion and contraction. It’s easier to fully sweat the joints if the metal has been pre-tinned along the edges about 1½ in. deep. This is especially helpful when the solderers are short on experience, or when soldering on slopes. Expansion and contraction issues can be solved by well-placed expansion joints and the use of cleats to allow for thermal movement of the metal. This takes the strain off the solder joints. Although cracked solder joints can often be resoldered, it’s not a fun job, and it’s hard to warranty if you’re repairing someone else’s work. There is a lot to slate roofs, but knowledge of some of the fundamentals will allow a roof consultant to easily evaluate a new installation. Does the slate type have a good track record? Were the individual installers (not just the company itself) experienced specifically in slate roof installation? Were the contract documents thorough and professional? Was there a warranty? Look at the starter course: is it correct? If not, there will probably be more installation errors, such as headlap deficiencies and flashing mistakes. If the headlap is deficient, the roof may have to be entirely condemned. If the flashings are incorrect, they can be removed and repaired without having to replace the entire roof. Keep a copy of Revere’s Copper and Common Sense in your library, along with SMACNA’s Architectural Sheet Metal Manual, and of course, a copy of the 3rd edition of the Slate Roof Bible. The SRCA, a 501c6 trade association, has lots of information available on its website at SlateRoofers.org, including a contractor directory, materials source lists, slate roof repair guidelines, and slate roof installation guidelines—all free to the public. RCI International Convention and Trade Show • MarcRCh 14-19, 2019 JenKIns • 109 Figure 15 – Noncompatible metal types are a sign of a careless roofer. Here you see copper rivets with steel mandrels. The mandrel breaks off when the rivet is “popped,” and then rusts away to leave a hole in the center of the rivet. Steel nails or screws on copper flashings will rust rapidly as well. Their deterioration is accelerated by the copper’s galvanic action. Figure 16 – Cracked solder joints are hard to spot from a drone, but they will leak.
