Should I put expansion joints between my patio and house foundation?

Yes — an expansion joint between your concrete patio and your house foundation is not just recommended, it is essential in New Brunswick's climate. This is one of the most important details in patio construction and one that is sometimes skipped by less experienced contractors.

Concrete expands when temperatures rise and contracts when temperatures fall. A poured concrete patio is attached to nothing — it is designed to move independently of the house. When a patio is poured directly against the foundation wall without an isolation joint, the patio and foundation press against each other as both concrete masses expand in summer heat. Since the foundation is immovable (it is a structural element anchored below the frost line), the patio has nowhere to go and the force builds until something gives — typically cracking the patio slab, or worse, cracking or staining the foundation wall.

NB's dramatic temperature swings amplify this problem. New Brunswick experiences temperature ranges of 60-70 degrees Celsius between winter lows and summer highs. That thermal range causes measurable expansion and contraction in a concrete slab. A 20-foot concrete patio can expand or contract by 3/8 inch or more across that temperature range. Without an isolation joint to absorb that movement, the stresses accumulate at whatever point the patio contacts the fixed structure.

Installing the joint is straightforward. Use a pre-formed expansion joint filler strip (1/2 inch thick fibreglass or closed-cell foam backer rod, available at any NB building supply store) placed vertically against the foundation wall before the pour. Set the top edge at finished concrete height. Pour the patio concrete up to the filler strip. After curing, apply a bead of self-levelling polyurethane or silicone caulk at the top of the joint to seal against water infiltration — this is the part that needs maintenance every few years as it ages and weathers.

The same principle applies to any adjacent fixed structure — garage walls, steps, fence posts set in concrete, or existing concrete slabs. Wherever your new patio meets any fixed element, install an isolation joint. The material cost is minimal (filler strip is $2-$5 per linear foot), and it is infinitely easier to install during the pour than to retrofit afterward.

If you are hiring a contractor for your patio, confirm that expansion joints are included in the scope before signing. It should be standard practice — if a contractor proposes pouring concrete directly against your foundation with no joint, that is a red flag.