What is the correct gap between deck boards for expansion in New Brunswick's climate?

For pressure-treated wood decking in New Brunswick, leave a 1/8-inch gap between boards at the time of installation. For composite decking, the gap should be 3/16 inch to accommodate greater thermal expansion. These spacing dimensions are measured edge to edge between adjacent boards and are essential for both drainage and material movement throughout the year.

New Brunswick's climate is one of the most demanding in Canada when it comes to deck board movement. Seasonal temperature swings regularly exceed 50 degrees Celsius, from minus 25 or colder in January to plus 30 or more in July. On top of that, humidity levels shift dramatically between the dry cold of winter and the damp heat of summer. Both temperature and moisture cause decking materials to expand, contract, and in the case of wood, swell or shrink across their width.

Pressure-treated lumber, which remains the most popular decking material in New Brunswick due to its affordability and availability, absorbs and releases moisture as conditions change. When boards are installed during the warmer months, the wood is typically at or near its driest dimension, so a 1/8-inch gap is appropriate. If you are installing in the spring or fall when the wood may still be relatively wet from the lumber yard, you can install the boards tight together or with just a 1/16-inch gap, because the boards will shrink as they dry and open up to roughly the right spacing on their own. The key is to understand the moisture content of the lumber at the time of installation and adjust accordingly. Kiln-dried after treatment lumber, which is becoming more common at New Brunswick building supply stores, will already be at a lower moisture content and should receive the full 1/8-inch gap regardless of the season.

Composite decking behaves differently. Products from manufacturers like Trex, TimberTech, and Fiberon do not absorb moisture the way wood does, but they expand and contract significantly with temperature changes. A composite board can grow by as much as 1/4 inch over a 20-foot length between winter and summer. The 3/16-inch gap between boards handles the lateral expansion, but you also need to follow the manufacturer's end-to-end gapping requirements, which are usually 1/8 to 1/4 inch at butt joints depending on the ambient temperature at installation. Most composite manufacturers include specific gapping instructions and sometimes even provide spacer clips in the box.

Getting the gap wrong in either direction causes problems. Too tight and the boards will buckle upward as they expand, creating a tripping hazard and putting stress on the fasteners. This is especially common with composite decking installed during cold weather without adequate gaps, because when summer arrives the boards have nowhere to go. Too wide and the gaps become a nuisance, catching furniture legs, bare toes, and debris that promotes moisture retention and mould growth between the boards.

For consistent spacing during installation, use a framing nail, a carpenter's pencil, or a purpose-made deck board spacer tool. Some hidden fastener systems, which are popular with grooved composite boards, have built-in spacing features that automatically set the correct gap as you install each board. These systems work well for composite products but are generally not used with pressure-treated lumber, which is typically face-screwed.

Proper gapping is a small detail that makes a meaningful difference in how your deck performs over 15 to 25 years in New Brunswick's challenging climate.

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