Slate Roofing: Applications and Maintenance
Evan Landis, MAE/BAE Architectural Engineering, Penn State University 2013


Introduction


Slate roofs have been around for centuries and have proven to be a great roofing medium. Although slate roofs are not as popular as they once were, their appearance is timeless (Figure 1). Older homes and churches that date back to the late 1800's have remained watertight with their original slate roofs intact. In the past, families built homes with the idea that the house would be passed down from generation to generation and were built from materials that would withstand this amount of time. Slate roofs were chosen for this purpose due to their longevity. Slate roofs in most cases have been shown to easily last 100-150 years given that they are properly maintained (Jenkins, 2008). This document will provide information about the pros and cons of slate roofing, the system's components, correct tools, and performance failures.



EJL - Slate Photo.jpg
Figure 1: Slate on a small church in Virginia Credit: Leeland's Slate Roofing

Pros and Cons of Slate Roofing

(Goering, 1999-2012)

Pros

  • Very fire resistant
  • Environmentally friendly
(storm water run off, disposal, recyclable, etc.)
  • Longevity (100-150 years)
  • Architectural aesthetics

Cons

  • System weight
  • Difficult installation
  • Large initial investment
  • Maintenance Cost


System Composition


The slate roofing system consists of 2 or 3 primary layers depending if there is a secondary water proofing membrane installed under the slate. The secondary waterproofing membrane is not necessary on a properly installed slate roof, but can prove useful if there is rain during construction. When choosing materials for a slate roof, it is important to use materials that will last as long as the slate. Dimension lumber of at least 3/4" thickness should be used for decking. Plywood and particle board are popular decking materials but have proven to delaminate over time if exposed to any amount of moisture. The use of non-corrosive fasteners is also required so that a connection failure does not occur over time. Copper, stainless steel, and hot dipped galvanized nails are the types most commonly used. (Jenkins, 2004, 1). (User note: Due to the rigidity of galvanized nails, they are difficult to remove if a mistake is made.) The choice of slate is also important. Make sure to use slate that has been tried-and-proven. Not all slate is created equal. It is important to choose a slate that has been used before and have been proven to last. Most North American and Canadian slate has been tested through use and has been shown to reach a 150 year roof lifetime. This cannot be said about some foreign slates, for example, Spanish Black Slate tends to have a high pyrite content that will stain the roof with a rust color (Jenkins, 2008, 2). The National Slate Association's website explains some of the differences in types of North American slate located here.

EJL - Slate Layout.jpg
Figure 2: Slate roof layout diagram. Credit: Joesph Jenkins, Inc., 143 Forest Lane, Grove City, PA 16127 USA, ph: 866-641-7141.

These system elements are then laid out in a pattern that induces water shedding redundancy. Each piece of slate, except the starter course and first course at the edge of the roof, is laid on top of two other slate. Looking at Figure 2 the keys to notice are that the slates do not line up in ranks, they are organized in an alternating patter from course to course. Figure 2 does not include the secondary water proofing membrane. The membrane would be located under the slate directly against the decking. It should be lain perpendicular to the roof's slope starting at the bottom edge of the roof with positive headlap of 6" under the next course. A minimum of 3" horizontal over lap of the course below and 3" of headlap underneath the slate 2 courses above the slate in question is required. The headlap requirement is further explained in Case 1 later in the document. Also, the "cant" strip is installed so that the slate does not lay flat against the roof. This creates an air space under all the slate to dry out any dampness that may enter the system.

Another important element of a slate roof is the step flashings that are used at the interface of the slate roof to a vertical surface. Flashings should be made from a long lasting material such as copper or stainless steel so that its lifespan is comparable to the slate. Every course of slate needs its own step flashing. The flashing should be placed on top of the last slate in each course, extend at lease 4" up the wall, 4" under the slate and overlap the flashing below it by at least 2".

It is important to punch the nail holes from the back of the slate. Do not use a drill. This creates a break out on the face side of the slate so that the nails can be countersunk into the slate. When nailing the slate, be sure not to over nail the slate. This will cause the slate to crack easily when shifted. Also, it is important that the nail is countersunk into the slate to keep the nail from punching through the slate above it when a load is applied.


Correct Tools


As for any job, having the right tools is key to completing the job correctly. Not only having the tools is important, but it is also important to utilize them correctly. The main tools necessary for a slate roofing project are shown below in Figure 3.

  1. Ladder Hook - attaches to the end of a chicken ladder and is then hooked over the peak of the roof to secure the ladder. The ladder can then be used to traverse the roof helping to distribute the worker's weight and minimize cracked slate.
  2. Slate Hammer - is similar to the everyday claw hammer but the claw has been replaced with a pick to punch nail holes into the back of slate.
  3. Slate Ripper - used to remove cracked, damaged, or broken slate. Its design allows it to slide under the slate and hook the nails attaching the slate to the roof. The rippers mushroom-like head allows it to hook the nails for them to be pulled out.
  4. Slate Cutters - used to cut slate to size or into decorative shapes. It is important that the blade contacts the back of the slate to achieve the decorative chipped edge of the slate tiles.

EJL - Slate Tools.JPG
Figure 3: Slate tools. Credit: Joesph Jenkins, Inc., 143 Forest Lane, Grove City, PA 16127 USA, ph: 866-641-7141.


Roof Maintenance


Maintenance is crucial when prolonging the life span of a slate roof. Slate roofing is not a common a trade, therefore,it is important to find a contractor specializing in slate roofing to perform roof maintenance regularly. If a specialized contractor is not found, the roof may be repaired with caulk and patches that are only a temporary fix and will only lead to more repairs in the future. To the untrained eye, a slate roof can seem as it it does not have any issues, while in reality, it may have various leaks that if left unattended can escalate into major problems. Figure 4 below depicts a roof edge that had water infiltration issues that were left unattended for some time. In the pictures, one can see that the wood is deteriorating and starting to pull away from the building. This is also a good example that it is important to hire the correct contractor first. When looking closely, one can see that the corner was already rebuilt previously, but the problem was not fully corrected and proceeded to get worse. Figure 5 shows the corner being rebuilt as well as the deterioration that has occurred around the valley at this location. If the roof was maintained properly, the leak that started the decaying of the wood would have been caught and fixed before any permanent damage was done. Conditions around valleys are susceptible to problems and need to be examined thoroughly due to the probability of snow build up and abundance of running rain water in this area.

EJL - Deep Penetration.jpg
Figure 4: Damage at roof gable edge. Credit: Leeland's Slate Roofing






EJL - Corner Fix.jpg
Figure 5: Rebuilt corner and surrounding damage. Credit: Leeland's Slate Roofing






















Performance Issues

Slate roofing is not a common trade, and many contractors are not familiar with the material. This creates performance issues in areas that were previously fixed.

Slate Roofs: Avoid These 21 Contractor Errors by Joeseph C. Jenkins has been found to be a good reference for those who plan on repairing or installing a slate roof. The article gives instructive information by informing the contractor of common mistakes made while working on a slate roof. The following cases describe a few of these errors as well as how they should be fixed.

EJL - No headlap.jpg
Figure 6: Photo demonstrating a fix that was completed incorrectly and now needs to be replaced. Credit: Leeland's Slate Roofing



Case 1

Figure 6 is a good demonstration of what not to do when replacing slate tiles. The first issue is that the slate in the bottom course in the photo does not have headlap. It terminates at the the bottom of the course directly above it. The replaced slate should extend up until the top is located 3" underneath the course that is located at the top edge of the photo. The crack shown with the arrow is an unprotected path to the wood underlayment.

The second issue with Figure 6 above is the face nailed slate. The sealant used over top of the nails will only be watertight for a short time in comparison to the lifetime of the slate. As shown, one nail has already penetrated the sealant creating a path for water to enter the system. These nails should be centered underneath the course directly above in the top one-third of the 3" horizontal overlap.










EJL - Pipe Boot.jpg
Figure 7: Pipe booth with no positive lap. Credit: Leeland's Slate Roofing

Case 2


Figure 7 is an example of a repair done by someone who was inexperienced with slate work and was afraid to remove the slate surrounding the rooftop pipe in order to properly install a pip boot. This is a very common situation. Most contractors will work around slate rather than remove it. In this case, the contractor spread sealant around the area in question and made no additional repairs. A minimum of 2 courses should have been removed in order to install this pipe boot properly, the course running horizontally across the pipe and the one directly above it. The pipe boot would then have been located underneath the two courses and appear to be lying on top of the course directly below the pipe to create a positive lap.

Also shown in Figure 7, notice that many slate are hanging lower in comparison to other slate in their respective course. This is a good indication that connection failures are beginning to occur. This could be caused by corroded fasteners or water infiltration decaying the wood underneath so that a solid connection is no longer possible.







EJL - Snow Guards.jpg

Case 3


Snow guards are a very important element to every slate roof. Generally, slate roofs have a steep pitch, causing snow to slide off the roof more easily. This is not only a safety concern but also the force of the sliding snow could separate the gutters from their supports rendering them useless. Figure 8 does not show failed snow guards but is a good demonstration of what snow guards should look like on a slate roof. The strength of the snow guard is not what is important. The guard requires proper spacing. The system will work as long as the strength of the snow guard is proportional to its spacing, but many times one will find snow guards folded flat to the roof with the roof's gutter dangling to the ground. This is due to the snow and ice load exceeding the capacity of the snow guards. The wire snow guards located on the lower portion of the roof shown in Figure 8 should never be used as primary snow guards. Their load bearing capacity is almost negligible with the amount and spacing shown, and their longevity is short in comparison to the slate. It is also good practice to connect the snow guards with screws rather than nails. This decreases the chance that there will be a connection failure before the snow guard fails.









Conclusion


Slate roofs are a complex system of intertwined tiles that all work together to create a water shedding surface when installed properly, but as with any system, when it is installed improperly, it can lead to a catastrophe. The key to success with a slate roof is maintenance. It is important to discover small leaks early before the problem escalates. When problems are caught early, decay as shown in Figure 10 is unlikely to happen. The longevity of a slate roof is largely dependent on the expertise of the contractor building it. Be sure to hire someone with experience in both building and repairing this type of roof. And remember, proper repairs can only be accomplished when using the proper tools for the job. An experienced contractor will use the proper tools to accomplish the task.


EJL -Rotted Wood.jpg
Figure 10: Deteriorating wood decking. Credit: Leeland's Slate Roofing



A special thanks to Leeland's Slate Roofing for providing their photos and expertise!


Related Wiki's


Roofs - Collapse and Performance Failures
  • Gives an outline or types of commercial roofing mediums that are available as well as issues that can arise with each type.

Hubert H. Humphrey Metrodome Roof Snow Collapse of 2010
  • A wiki explaining and evaluating a roof collapse due to snow loads.


Slate Roofing Associations


Slate Roofing Contractors Association
  • Develops installation guides and training programs as well as holds conferences and workshops to educate contractors on slate roof installation and repair.

National Slate Association
  • "...dedicated to promoting excellence in slate roofing practices through the development and dissemination of technical information, standards, and educational resources on the materials and methods used in the manufacture, design, and construction of slate roofs and associated flashing systems (NSA)."



Bibliography


Goering, Matt. (1999-2012) "Pros, Cons & Costs: Slate Roofing Tiles." pg. 1-14. http://www.servicemagic.com/article.show.Pros-Cons-and-Costs-Slate-Roofing-Tiles.16025.html.
This is a short article discussing the pros and cons of using slate roofing tiles.

Jenkins, Joseph C. (2004) "Installing a Slate Roof - The Basics." pg. 1-6. http://www.jenkinsslate.com/downloads/installation_basics.pdf.
This article has sections dedicated to each item necessary to construct a successful slate roof and a description of what materials to use and how to install them.

Jenkins, Joseph C., (2008) "Slate Roofs: Avoid These 21 Contractor Errors." Traditional Roofing, 2008. pg. 10-16.
http://www.traditionalroofing.com/downloads/TR7_21_errors.pdf.
This article states and explains the top 21 most common slate roofing errors made by contractors.


Jenkins, Joesph C. (October 2008) " Slate Roofs: Why Learn the Hard Way." www.rci-online.org/interface/2008-10-jenkins.pdf
This document is a source of slate roofing history and describes the types or slate that are good for roofing and slate that is not. It also includes some dos and do not's of slate roofing



Additional References


Berdahl, Paul; Akbari, Hashem; Levinson, Ronnen; Miller, Willian A. (April 2008) "Weathering of Roofing Materials - An Overview." Construction and Building Materials Volume 22, Issue 2. pg. 423-433. http://dx.doi.org/10.1016/j.bbr.2011.03.031 (accessed December 5, 2012).
This article documents the effects of weathering on many different roofing materials and compares them to one another.

Edwards, J.W.(Jun 01,1896) "Roofing Slate and its Advantage." Stone (1888-19190 13 begin_of_the_skype_highlighting 1888-19190 13 end_of_the_skype_highlighting, no.6: p. 46, http://search.proquest.com/docview/913063933?accountid-13158 (accessed November 2, 2012).
This except from a book gives reasons one should choose a slate roof over other conventional roofing materials.

Jenkins, Joseph C. (November 2003). Slate Roof Bible.
This book describes the proper procedure for installing slate roofing,, a great reference if you are installing, repairing, or assessing a slate roof.


Jenkins, Joseph C., (2008) "Slate Roofs: Avoid These 21 Contractor Errors." Traditional Roofing, 2008. pg. 10-16.
http://www.traditionalroofing.com/downloads/TR7_21_errors.pdf.
This article states and explains the top 21 most common slate roofing errors made by contractors.

Long, Brett; Clark, Shirley E.; Baker, Katherine H.; Berghage, Robert. (2008) "Selecting a Green Roof Media to Minimize Pollutant Loadings in Roof Runoff." Low Impact Development. http://dx.doi.org/10.1061/41007(331)24
Roof run off polluting community drainage systems is an issue with many roofing materials made from artificial products. Slate consisting of natural stone minimizes this issue.

Sharara, Luke M., P.E., M.ASCE; Jordan, James W., S.E., P.E., M.ASCE; Kimble, Ross A., P.E., M.ASCE. (2009) "Residential Roofing Evaluation." Forensic Engineering 2009: Pathology of the Built Environment. American Society of Civil Engineers, 2009.http://dx.doi.org/10.1061/41082(362)19 (accessed December 5, 2012).
This document looks at different types of damages to roof coverings due to storms. The paper also details a process of how to go about this analysis in the field.