An increase in ambient moisture causes wood sheathing panels to expand. When they do, buckling may occur if the panelshave not been installed correctly. While swelling or buckling of sheathing panels doesn’t affect their performance or strength, it can cause blemishes in the building’s appearance that will lead to callbacks.
All wood products gain or lose moisture until their moisture content is in keeping with their environment. Solid wood tends to expand or shrink more than wood panels because the ability of each strand in OSB to expand or contract is limited by adjacent strands. This means that the total dimensional change of an unrestrained panel is about 1/8 of an inch. This may increase if panels get really wet and if the panels are thinner. While the structural integrity of the panel may not be compromised, buckling may occur in instances where the required 1/8 gap has not been left between all sides of the panel.
There are three ways to prevent panels from buckling:
The effects of buckling can be minimized by the correct acclimation of panels onsite. This means that panels are properly stored onsite for as long as it takes for them to find an equilibrium with the ambient moisture.
Cut the unit straps to prevent uneven expansion on panels in the event of rain. Store panels under a roof or cover them loosely and keep them flat to prevent damage to edges.
If panels must be stored outside, here are some guidelines:
- Place three 4x4s on the ground as block supports.
- Block supports should be aligned to ensure an even distribution of weight if units are stacked.
- Lay the panels flat on the block supports.
- Place a 2×4 on top of the panels and cover the panels and the 2x4s with plastic sheeting or waterproof tarps.
- Secure the edges of the cover a little way from the panel sides to allow air to circulate around the panels.
- Stack panels in the order in which they will be used to minimize lifting and damage to panel edges.
Always leave 1/8-inch gap on all sides of the panel to allow enough room for panels to expand and contract without buckling.
Proper ventilation of crawlspaces and attics will help moisture to evaporate or escape and reduce the chances of panels swelling and expanding.
Consult your design professional for the correct fasteners to use and for fastener patterns. Do not over-or under-drive the fasteners. Get a guide to correct fastening here.
If you are gluing your sub-floor, select adhesives which comply with APA specifications AFG-01 or ASTM D3498. Clean joists of dust and dirt and ensure they are dry before applying adhesive. Only apply glue to two panels at a time to prevent drying. Use