Let’s start with what slate is. It is a stone, mined or quarried in large chunks then split into individual “tiles” by hand. They are then punched and sized, usually at the quarry. Slate roofs have been used in the US for almost 200 years, some still holding strong. 
Standard Slate Installation

We can break down a slate roof into 3 different categories: slate, fasteners, and substrate. When these 3 categories are in harmony a slate roof can last over 150 years. Most of the slate roofs over 100 years old use no substrate whatsoever because the slate is the weather barrier, but the substrate does play an important role while the roof is being installed. It keeps rain out until the slate is installed but after that the slate is the barrier.
Copper nails are the recommended fasteners but hot dipped galvanized as well as stainless steel roofing nails are acceptable. A slate roof needs to be at least a 4:12 pitch, but a greater pitch is desired as it will allow for quicker drainage. A correct headlap (the overlap of the slate on two courses below) is a critical part of the longevity of any standard slate roof installation. Without proper headlap, the water will get in. The pitch of the roof will determine the correct headlap of a roof. At eaves and other areas where ice dams can be a problem, proper headlap is critical. A correct sidelap (the overlap of the adjacent slate staggered above the lower slate) is another essential aspect in keeping water out. Depending on correct nail hole placement, the sidelap should be a minimum of 3”.
Hybrid Slate Installation

This method is a lot like standard installation, with one major exception – there is an extra layer of substrate and it is the weather barrier. A hybrid slate roof allows you to have the look of slate without the excessive weight and high cost normally associated with a slate roof. Ice and water shield is still used at critical areas and #30 felt is used in all others as in a standard installation but in between each course is a membrane that acts as the weather barrier for the roof. The slate is used only for aesthetics and to guard this membrane from UV light and impact degradation. The membrane used is usually 30 and 60 mil thick. Using this method the amount of slate used per 10‘x10’ area (1 square) is reduced by 40%, which also reduces the weight from standard installation greatly. The trade off of this method is the roof is only as strong as the membrane, once the membrane degrade the roof needs to be replaced. This is done by remove the old slate, changing out the membrane and reinstalling the removed slate.