Technical Services Information Bureau TECHNICAL BULLETIN ASSESSING WOOD-BASED SHEATHING JANUARY 2008 60.150 UPDATED MAY 2017 Wood-based sheathing is commonly used under many INSPECTION: exterior claddings, such as portland cement plaster • All wood-based sheathing should be sound, (stucco). Covering wet wood-based sheathing, plywood properly attached to framing members and or oriented strand board (OSB), is not recommended. installed per APA recommendations. STRESS: • A moisture content below 19% prior to applying Wood products swell when exposed to moisture 2 layers of a water-resistive barrier. or humidity. The amount of dimensional change is estimated at 1% of the width or thickness of lumber for • Wood-based panels should be installed with 1/8 every 5% change in moisture content. This expansion inch gaps at edges and ends to allow for of wood-based products can place stress on cement expansion of the panel to minimize stress plaster stucco. This stress can crack plaster. Even a (cracking) in cement stucco (Code requirement minor amount of movement can cause cement plaster behind portland cement plaster per ASTM that is not fully cured (green) to crack. The same stress C1063). can occur when wood-based sheating shrinks or dries. • Fasteners should be set flush prior to applying MOISTURE: the water-resistive barrier(s). The Engineered Wood Association (APA) recommends wall sheathing and lumber framing “should be allowed to dry (no less than 18%) so that moisture absorbed during construction or induced from other sources is minimized”. The average fiber saturation point for wood is typically 28%. At this percentage, water begins to fill all the fiber cells. Decay can generally only get started when the moisture content is above the fiber saturation point for a prolonged period of time. SHEATHED CONSTRUCTION WOOD FRAMING Framing members Water resistant barrier - Max 24” (609 mm) 2 layers on center Self-furring fabric lath Cement plaster basecoat Sheathing Board note 1/8” (3mm) gap between sheets of plywood and OSB at vertical and horizontal joints Finish coat tsib.org This technical document is to serve as a guideline and is not intended for any specific construction project. TSIB makes no warranty or guarantee, expressed or implied. The most updated version of any said technical bulletin will be on the TSIB website and dated accordingly. .