How do I fix gaps between deck boards that have shrunk in Bathurst NB?

Gaps between pressure-treated deck boards after installation are completely normal — pressure-treated lumber is saturated with preservative solution at the mill and will shrink as it dries over its first one to two years, with gaps up to 1/4 inch being typical and expected. In Bathurst and along New Brunswick's north shore, the Maritime climate means this drying process can take longer than in drier regions, but the shrinkage will still occur.

When pressure-treated wood arrives at the lumber yard or job site, its moisture content is often 40 percent or higher. As it acclimatizes to Bathurst's environment over the following 12 to 24 months, it will lose much of that moisture and shrink across the width of each board. A standard 5.5-inch-wide deck board can shrink by 1/8 to 1/4 inch, and if the boards were installed tightly together with no gap — which many builders do intentionally, knowing the wood will shrink — you will end up with noticeable gaps once the drying is complete.

If your gaps are 1/4 inch or less and relatively uniform across the deck, there is nothing to fix. This spacing actually benefits your deck by allowing water to drain through and air to circulate, which reduces the risk of rot and mould growth. Decks in Bathurst face heavy snow loads through winter and significant rainfall in spring and fall, so good drainage between boards is genuinely important for longevity.

If the gaps are excessive — wider than 1/4 inch — or if they are uneven with some boards tight and others widely spaced, the best solution is to re-space the boards. This involves removing the screws from each board, shifting the boards to create even spacing, and re-fastening them. While this is labour-intensive, it produces the best result. You will likely end up with a gap at one edge of the deck where you have redistributed the extra space, which can be filled with an additional narrow board or a trim piece.

For gaps that are moderate but bothersome, there are deck board gap filler products available at building supply stores in the Bathurst area. These are flexible strips or inserts that press into the gaps. However, these are cosmetic solutions that can trap debris and moisture, potentially causing more harm than good in New Brunswick's wet climate. They also tend to pop out over time as the wood continues to expand and contract seasonally.

One approach to avoid entirely is filling gaps with caulk or wood filler. These rigid materials will crack and fail as the wood moves with seasonal moisture changes, creating an ugly mess that is difficult to clean up. Bathurst experiences significant humidity swings between the damp coastal summers and the dry heated winters, so your deck boards will continue to move slightly every year even after the initial shrinkage period is over.

If you are planning a new deck or replacing boards, ask your builder to install the pressure-treated lumber with a 1/8-inch gap between boards. This accounts for the fact that the wood will shrink further, and you will end up with spacing in the 1/4-inch range once drying is complete — the ideal gap for drainage and aesthetics. Some experienced builders in northern New Brunswick will also let pressure-treated lumber acclimate on site for a few weeks before installation, which reduces the amount of post-installation shrinkage.

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