Water repellent on a new Ottawa deck: how long does it last?

A water-repellent preservative on a new deck lasts about 1 to 2 years, according to US data from the USDA Forest Products Laboratory. The same source rates semitransparent stain at 2 to 3 years on new wood and 3 to 6 years on weathered or saw-textured boards.

The Forest Products Laboratory recommends penetrating finishes for decks rather than film-forming ones. Paints and solid stains are rated low suitability for decking because film-forming finishes are very difficult to maintain on a walking surface. Penetrating finishes have to be refreshed every few years. The goal the FPL describes is keeping water beading on the surface rather than soaking in: that is what slows checking and splitting, which the FPL says is the most common reason wooden decks are replaced.

For older decks that were built before 2004, Health Canada notes they may have been made with CCA-treated lumber, and for those decks Health Canada suggests applying a sealant every year using an oil- or water-based stain. Paints and other film-forming finishes are not recommended for CCA decks either.

A couple of practical points worth keeping in mind:

• The 1 to 2 year and 2 to 3 year figures are US data from the Forest Products Laboratory, and the source says expected life depends on climate and exposure, so treat those intervals as a starting point rather than a guarantee.
• Follow the product label for the specific finish you choose. The label will tell you the application conditions and the recoat interval the manufacturer stands behind.

If your deck is attached to the house and its walking surface is more than 600 mm (24 in.) above grade, or if it is an elevated deck providing principal access to the building, the City of Ottawa requires a building permit for the structure itself. We found no City rule about refinishing an existing deck, and if you are planning any structural repairs or replacement of framing at the same time, it is worth confirming with the City's Building Code Services whether a permit is needed for that work before you start.