How much does pressure-treated lumber shrink after installation and should I leave gaps between boards in Vancouver?

Pressure-treated lumber shrinks significantly after installation — typically 1/8 to 1/4 inch across the width of a 5.5-inch deck board as it dries from its initial moisture content of 19-25% down to 12-15% in service. In Metro Vancouver's humid climate, this process takes 6-12 months, and you should account for this shrinkage when spacing your deck boards.

Initial Spacing for Pressure-Treated Decking

Install pressure-treated deck boards with minimal gaps initially — just enough to slide a thin spacer (like a 16d nail) between boards. As the lumber dries over the first year, these tight gaps will naturally open to the ideal 1/8 to 3/16-inch spacing needed for drainage and ventilation. If you start with wide gaps, you'll end up with unsightly 1/4 to 3/8-inch gaps that collect debris and look unprofessional.

The exception is if you're installing kiln-dried pressure-treated lumber, which has already been dried to 15% moisture content and will shrink minimally. However, most PT lumber sold in Metro Vancouver is still wet from the treatment process and will shrink substantially.

Why Gaps Matter in Vancouver's Climate

Proper deck board spacing is critical in Metro Vancouver's marine climate where decks receive over 1,200mm of annual rainfall. Gaps allow rainwater to drain through the deck surface rather than pooling on top, and they provide essential air circulation that helps boards dry between rain events. Without adequate gaps, water sits on the deck surface longer, promoting mould growth, surface staining, and premature wood decay.

Fastener Considerations During Shrinkage

Use two screws per joist intersection, positioned about 3/4 inch from each edge of the board. As PT lumber shrinks, it can cup (edges curl up) or crown (center rises). Proper fastener placement helps minimize this movement. Pre-drilling is essential with pressure-treated lumber to prevent splitting, especially near board ends. Use only stainless steel or hot-dipped galvanized screws — standard zinc-plated fasteners corrode rapidly in contact with ACQ-treated lumber and Vancouver's persistent moisture.

Timeline and Seasonal Considerations

PT lumber installed in Vancouver's dry summer months (July-September) will shrink more dramatically than lumber installed during the wet season when ambient humidity is higher. Summer-installed boards may shrink enough to require gap adjustment or additional fastening if cupping becomes excessive. Winter installations shrink more gradually but still require the same initial tight spacing approach.

When to Hire a Professional

While experienced DIYers can handle ground-level PT deck installation, hire a professional for any elevated deck, complex layouts, or if you're uncomfortable with the precision required for proper board spacing and fastener placement. Professional deck builders understand how different lumber grades and moisture contents behave during the drying process and can adjust their installation techniques accordingly.