Roof-Slates-Guide

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 nails should always be used and traditionally flashing at peaks and valleys are made of copper in this type of installation.

warm roof tiles

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. There are over 200 manufacturers of roofing membranes that may be used in this type of installation.