For centuries, slate has been utilized as a stunning roofing material due to its natural occurrence. Slate rock is made up of sedimentary particles of clay and silt, which form when minerals react with water. Roofing Slate is dense, durable, and non-absorbent: it does not absorb water or moisture like wood, so it lasts much longer than wood shingles.

The color of slate depends on its chemical and mineral composition. The most common thickness of slate roofing is 3/16″ to 1/4″. Because of the relatively high cost of slate, however, the Chicagoland market for this material has decreased.

Slate Weight

The weight of a square of slate roofing varies significantly depending on the thickness of each slate. Slate is available in numerous thicknesses and types from different quarries, so when you add up the weight of felt, nails, and standard 3/16″ slate your total will vary considerably. When all is said and done, however, a square of slate roofing has a dead load of approximately 800 pounds per square.

Rooftop Terrace

NRCA recommends that slate roofing be applied over continuous or closely spaced wood decking. When plywood is used, NRCA recommends that exterior-grade plywood be used with a minimum thickness of 5/8 inch.

When you build a roof deck out of the following materials, you should take extra precautions:

The NRCA has raised concerns about potential issues with fastener-holding and dimensional stability of oriented strand board (OSB) and other non-veneer materials when used as roof decking, due to the impact of moisture.
Several manufacturers of roofing materials suggest that wooden roof decks should be made with non-oil preservative pressure-treated wood or non-treated air- or kiln-dried lumber.
Consideration should be given to the use of fire-retardant-treated wood panel decks due to the potential premature activation of fire retardants caused by heat history in service, which may result in the deterioration of some panels.

Roofing Underlayment: Before slate is applied, underlayment (also known as “felt paper”) is installed over the roof deck. Its primary functions are to provide temporary weather protection until the slate is installed and to provide a secondary weatherproofing barrier if moisture penetrates the slate roof Chicago covering. It is important that an underlayment’s service life is comparable to the design service life of the slate roof Chicago covering, as many slate roofs have outlived their underlayment felts.

Common underlayments include asphalt-saturated, non-perforated organic felts, designated as Type 15 and Type 30 or referred to as No. 15 and No. 30. Synthetic underlayment is another type, characterized by its lightweight, water-resistant properties, high tear strength, and ease of walking on. While appealing, there are significant issues to consider before using synthetic roofing underlayments. To date, there are no applicable ASTM standards for these products, and many do not meet current building code requirements.

The NRCA’s guidelines state that for roof decks with a slope of 8:12 (34 degrees) or more, a single layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion is sufficient. However, for roof slopes ranging from 4:12 (18 degrees) to 8:12 (34 degrees), a minimum of two layers of No. 30 asphalt-saturated felt is recommended. Slate roof Chicago systems should not be installed on roof slopes less than 4:12 (18 degrees).

For regions that undergo harsh weather conditions with strong wind-driven rains, the NRCA suggests utilizing at least two layers of No. 30 asphalt-saturated felt applied horizontally in shingle style. For locations such as the Chicagoland area, where the average temperature for January is 30º F or lower, the NRCA suggests the installation of an ice-dam protection membrane. This membrane should be applied at the roof’s eaves and extend upslope a minimum of 24 inches from the building’s exterior wall line.