APA The Engineered Wood Association, answers a question on T & G Plywood for decks and roofs

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Help Desk Question: Panel Edge Support – What Role Does Tongue-and-Groove Play?
Posted At : August 31, 2007 1:15 PM | Posted By : Merritt Kline
Related Categories: OSB,Code,Plywood,Help Desk,Floors,Roofs

APA Help DeskQuestion: What purpose does tongue-and-groove (T&G) serve on a roof deck or balcony with inhabited space below? What are the advantages of using T&G in an exterior deck application if, for example, the deck is a subfloor to an exterior balcony with waterproofing and 2-inch topping slab over inhabited space, or a low slope roof deck?

Answer: Tongue-and-groove edges are a type of edge support for wood structural panel sheathing. Other types of edge support appropriate for roof sheathing are edge clips (H clips) and lumber blocking.

The purpose of edge support is twofold.

1. Assure that panels meet minimum load capacity requirements when loads are applied near a panel edge.
2. Limit differential panel deflection when loads are applied near a panel edge.

Roof deck sheathing requires edge support when panels are applied over framing spaced greater than certain specified spans.

Balcony and outdoor deck sheathing must meet floor sheathing requirements. Edge support is required for all floor sheathing applications unless 1/4-inch minimum thickness underlayment or 1-1/2 inches of approved cellular or lightweight concrete, or 3/4-inch wood strip finish floor is applied. Acceptable forms of floor sheathing edge support are: T&G, or lumber blocking.

Is one method of edge support superior to the other? What is the strongest method: T&G, blocking, or H clips?

Although appearance is not typically an edge support consideration, when appearance is important, T&G may be the better choice.

Properly installed, lumber blocking is the strongest of the three methods. Edge clips, the least; however, for roof deck sheathing applications, edge clips are perfectly satisfactory and may facilitate proper panel spacing.