Why is my deck bouncy or springy when I walk on it in Moncton NB?

A bouncy or springy deck is caused by undersized joists, excessive joist spans, or both, and it is a structural issue that should not be ignored. While a little flex might seem harmless, that movement indicates the framing is being stressed beyond its comfortable capacity, and over time it leads to fastener failure, cracked boards, and potentially dangerous structural problems.

The most common cause in Moncton-area decks is joists that span too far without intermediate support. The National Building Code of Canada and New Brunswick's adopted code set maximum allowable spans based on joist size, spacing, and wood species. For SPF lumber at 16-inch on-centre spacing, a 2x8 joist should span no more than about 10 feet, and a 2x10 tops out around 13 feet. Many older decks in Moncton were built with 2x8 joists spanning 12 or even 14 feet without a mid-span beam, which felt fine when the wood was new and stiff but becomes progressively bouncier as the lumber ages and softens over years of Maritime moisture exposure.

Diagnosing the Problem

Get underneath the deck and measure your joists. Note the lumber size, the spacing between joists, and the total unsupported span from the ledger board to the beam or from beam to beam. If the span exceeds the allowable maximum for your joist size, you have found the problem. Also look for joists that have developed a twist, a crown in the wrong direction, or visible cracking along the grain. Moncton's humidity, which regularly exceeds 80 percent in summer, accelerates the softening of SPF lumber that was not kiln-dried before installation.

The most effective fix is to add a beam at mid-span, supported by properly footed posts. This cuts the effective span in half and eliminates the bounce immediately. For a deck with 2x8 joists spanning 14 feet, adding a beam at the 7-foot mark brings each span well within the safe range and transforms the feel of the deck entirely.

If adding a beam is not practical due to grade conditions or access limitations, sistering new joists alongside the existing ones is the next best option. Bolting a new full-length 2x10 against each existing 2x8 with 3/8-inch carriage bolts at 16-inch intervals effectively upgrades the joist to a much stiffer composite member. This approach requires temporarily removing some decking boards for access but does not require any new footings.

A lighter intervention is adding solid blocking between joists at mid-span and at third points. Cross-bracing or solid 2x blocking forces adjacent joists to share the load, reducing the bounce felt at any single point. Blocking alone will not fix a seriously overspanned deck, but it makes a meaningful improvement when the span is only slightly over the limit.

Do not simply add more screws to the deck boards or ignore the bounce. A deck that flexes excessively is working its fasteners loose with every footstep, and in Moncton's freeze-thaw climate those loosened connections deteriorate rapidly through winter.

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