What causes soft spots in subfloors of older New Brunswick homes?

Soft spots in older NB subfloors are almost always caused by moisture damage — either rot in the subfloor sheathing, deterioration of the floor joists below, or a combination of both. New Brunswick's Maritime climate makes this one of the most common structural issues in the province's older housing stock, and it should never be ignored.

The most frequent cause is prolonged moisture exposure from below. Older NB homes often have crawl spaces or basements with inadequate or missing vapour barriers, allowing ground moisture to rise continuously into the floor assembly. Over decades, this moisture weakens the wood fibres in both the subfloor boards and the joists supporting them. The problem is worst near exterior walls, around plumbing penetrations (bathrooms, kitchens), and in areas above unheated crawl spaces where condensation forms during NB's cold winters. Homes in coastal communities like Saint John and along the Bay of Fundy, or in river valley locations like Fredericton and Miramichi, experience persistently higher ground moisture that accelerates this deterioration.

Plumbing leaks are the second most common culprit. A slow drip from a bathroom drain, a weeping toilet wax ring, or condensation on cold water pipes can saturate a localized section of subfloor over months or years before anyone notices. By the time the floor feels soft underfoot, the damage is often extensive. In older NB homes with galvanized steel or cast iron drain pipes, corrosion and pinhole leaks are common as these systems reach the end of their service life.

Delamination of plywood affects homes built from the 1950s through the 1970s when interior-grade plywood was commonly used for subfloors. NB's humidity cycles — indoor relative humidity swinging from 20-30% in winter up to 60%+ in summer — cause repeated expansion and contraction that weakens the glue layers in interior-grade plywood over time. The layers separate, losing their structural rigidity and creating the spongy feeling underfoot.

Insect damage is another factor, though less common than moisture. Carpenter ants and powder post beetles can hollow out both subfloor boards and joists in NB homes, particularly in areas where moisture has softened the wood and created hospitable conditions for insects. If you see fine sawdust (frass) near the soft area, insect damage is likely contributing.

Older homes with inadequate joist sizing or spacing may also develop soft spots simply from structural fatigue. Building practices and code requirements were different 60-80 years ago, and some older NB homes have joists that would not meet current standards for span and spacing. Add in the accumulated effects of Maritime moisture and decades of use, and these floors gradually lose rigidity.

Do not simply cover soft spots with new flooring. The soft area will continue to deteriorate and can eventually become a safety hazard. At minimum, you need to identify the cause (moisture, rot, insects, structural) and address it before any flooring project. Access the area from the basement or crawl space to visually inspect the joists and underside of the subfloor. Look for discolouration, crumbling wood, visible mould, or insect activity.

This is firmly professional territory. A soft spot may be a $200 patch or a $5,000 structural repair — you will not know until someone qualified looks at it from below. A flooring contractor or general contractor experienced with NB's older housing stock can assess the damage, identify the root cause, and recommend the most cost-effective repair before your new flooring goes down.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• 3Tone Construction Ltd
• Thirty Four Renovations
• Modern Epoxy Inc
• First united design & construction inc.
• Forever Epoxy Inc