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Slate Roof Tile Installation Options

Properly installed slate roofing can provide decades of service, superior fire resistance and exceptional beauty and elegance.

When providing roofing materials, roof contractors often refer to "squares" when talking about the size of a roof installation. When dealing with slate roof tiles it is important to understand while many roofing products can only be installed in one specific manner and hence a "square" requires a predefined quantity of roofing material, that is not the case with slate roof tile. The number of tiles or square feet needed of roof slate for any given installation will vary depending on the method and overlap of each course of slate roof tile used by the installer. This decision as to which installation method will be used is determined by many factors such as weight limitations of the supporting structure, cost of materials and installation, building codes, climate, desired useful life and other factors.

Slate has been used for centuries as a roofing material to provide decades of reliable service. It is not uncommon for a slate roof to last in excess of 100 years! There are basically two methods typically used to install slate roof tile, traditional and hybrid.

At the turn of the century, slate roof tile was installed in a manner where it was the waterproof barrier. Using this method and a 16 x 10-inch slate roof tile a minimum of 221 tiles per square was required. This translates into a dead weight load in excess of approximately 1200 pounds per "square" for just the roof tile. To support this load the building structure must account for such a heavy material by increasing the strength of the roofing truss system, bearing walls of the building and building foundation. Traditional slate installation methods require planning and expense far beyond the cost of the slate roof itself. Because of the large number of slate roof tiles per square that must be installed installation labor can get quite expensive. Installation will typically exceed the costs of the material itself. When using this method it is common to install over wood lathe as opposed to solid plywood sub-roofing. This allows the slate to "breath" avoiding moisture traps and possible mold accumulation under the waterproof barrier of the roofing slate. The lathe is then typically covered by 30# felt paper followed by the slate itself. Copper or stainless steel nails or screws should always be used and traditionally flashing at peaks and valleys are made of copper in this type of installation.

Advances in modern materials technologies have given rise to a number of installation alternatives often referred to as a hybrid installation method. The objectives of the hybrid methods are to allow for the aesthetic finish of a slate roof without the excessive weight or high costs normally associated with a slate roof. In these methods the slate is NOT the protective moisture barrier. An alternative moisture membrane which a thick, sticky, self-adhering rubberized mat is used over the solid plywood sub-roof prior to the installation of the slate roof tile. The slate roof tile then serves as a protective material for the moisture membrane from ultraviolet and impact degradation. Using these methods the number of slate roof tile can be reduced considerably to as low as 130 slate roof tiles per square…about a 40% reduction in roofing tile and weight! As with all things however there are tradeoffs to be considered. While the hybrid method provides for a lighter and less costly finished slate roof it will not have the longevity of a traditionally installed slate roof. The roof is only as good as the moisture membrane underneath the slate roof tile in this instance. When the time ultimately approaches in 20 – 50 years for the membrane replacement the slate roofing tile is typically removed and reused after the application of a new moisture membrane. Both Carlisle Corp and W.R. Grace make roofing membranes that are widely used in this type of installation.

Another method is the use of 30# felt paper interleaved between each course of slate roof tile. The asphalt paper extends over the preceding course of each row of tile sufficient to cover the nails and nail holes in the roof tile. This method is called the Peters Low Slant system.

The Peter Low Slant Slating System is suitable for all slate roof pitches and is ideal for shallow angles (below 30 degrees). Converting a roof from tiled concrete or a clay roof cover to slate roofing tile is a popular choice because the existing roof support timber specifications are usually compatible.

Roof weight using the Peter Low Slant Slating System is approximately half the weight of the Conventional Slating System. This weight savings reduces the supporting timber specifications and associated costs.

The success of the Peter Low Slating System is due to the reduction of slate overlap, lessening the overall weight of the roof and the number of needed rows. In addition, a Roof Felt Waterproof Underlay is installed, assisting the visible multicolor roof slate in rain and snow resistance.

The life span of the Peter Low roof felt waterproof underlay has been subject to much discussion and research. Recently, the useful life span was extended by using a layer of aluminum foil to protect the underlay from the ultraviolet rays from the sun. With this enhancement, a properly installed roof may last from 10 to 30 years.

This installation data is provided for informational purposes only. No warranties express or implied for the information contained herein are given. Buyers should consult with licensed experts in the field before purchasing roof slate products.


We highly encourage you to order slate tile samples prior to purchase.

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Updated 3/18/2011