Choosing Roof Decking and Sheathing

Roof decking or sheathing for slate roofing must be adequate to support the loads involved, but not less than nominal 1 inch lumber or 5/8″- inch plywood sheathing. When selecting, one must consider that an S-1 grade slate will have an expected service life of over 75 years. There are some slate roofs that have lasted over 150 years. A prudent designer will specify a roof decking material that can be anticipated to provide a service life equal to the slate roofing it supports. Here are some considerations when selecting:

• Nominal 1-inch and thicker lumber  – This decking will proved life spans of over 150 years when used in conjunction with slate roofs. These commonly are of tongue-and-groove, shiplap, or square-edged boards, commonly 6 inches to 8 inches wide and usually not more than 10 inches wide.
• Spaced, or skip, sheathing boards – 1 inch or thicker lumber of varying widths installed with gaps, or spaces, between the boards is another possibility. The predominant use of this type of system was on barns and other outbuildings. Primarily due to heat loss and the possible infiltration of fine grained snow, these types of systems were not recommended for residences except in favorable climatic conditions.
• Plywood panel sheathing – In the later half of the 20th century this became the most commonly used roof sheathing product for slate roofing. This is true in today’s market as well. While plywood sheathing has not been long enough to be tested for 100 years of service life, it has not shown any common cause for failure at the half century mark. Be aware, however, that there are many different types of plywood and their characteristics (including thickness, plies, glues, wood species, grade, and finish) vary widely.
• Oriented Strand Board (OSB) – OSB wood panels are a relatively new product, since the early to mid 1990’s, they have attained a dominant share of the market as roof sheathing for many other types of roofing products. OSB sheathing is rarely used for slate.
• Structural Insulated Panels (SIPs) –  Constructed from OSB and plywood panels with rigid insulation sandwiched between the panels in a pre-engineered structural system which free spans over roof purlins / beams or structural walls without the use of additional rafters, thus forming both the roof structure and roof deck in one. The relatively large overall thickness of some of these products demands that proper attention be paid to the manner in which they are secured to the roof structure. The steeper the roof slope, the higher the lateral load on the tops of the fasteners used to secure nailboards.

NSA Recommendation

The National Slate Association recommends the use of roof decking that has significant history without failure when used as a component of a slate roofing assembly, such as 5/8″ to 3/4″ APA rated Exterior plywood, 1″ nominal lumber, or plywood clad nailboards and plywood clad SIPS’s with proper shear blocking. Lesser thicknesses, and newer composite design materials may meet minimum code requirements for other products. These material have not however built a reliable record of accomplishment to match the longevity of a slate roof system.

Other thoughts are that the roof decking should be of a consistent thickness and securely fastened per code requirements to prevent wind uplift, cupping, or curling and to provide a smooth and planar nailing surface for the slate shingles. Where extreme roof loads or long spans between rafters or framing members exist, a qualified engineer must provide calculations to arrive at a roof decking or sheathing capable of meeting the required structural loads