What role does building paper play behind stucco walls in Ottawa?

Building paper behind stucco is one of those hidden details that most Ottawa homeowners never think about — until something gooes wrong. It's actually your most important line of defence against moisture damage, and understanding what it does helps you make smart decisions about stucco maintenance and renovation.

Building Paper Is Your True Waterproofing Layer

Here's something that surprises many homeowners: stucco is not waterproof. It's designed to shed most water, but it's a porous, cement-based material that absorbs moisture — especially during Ottawa's prolonged fall rains and spring thaw cycles. The building paper (technically called the weather-resistive barrier or WRB) behind the stucco is what actually keeps water out of your wall framing and sheathing.

The Ontario Building Code requires at minimum two layers of Grade D building paper (also known as 60-minute paper) behind stucco cladding. The "60-minute" rating means it can resist water penetration for 60 minutes under lab conditions. Two layers are required because:

• The metal lath (wire mesh) that holds the stucco is fastened through the paper with staples or nails, creating thousands of tiny holes. The second layer catches water that gets through those punctures.
• During stucco application, the wet cement mix is pressed into the lath and can saturate the first paper layer. The second layer provides backup protection during this vulnerable curing period.

What happens when building paper fails in Ottawa:

• Water reaches the wood sheathing (usually OSB or plywood) and can't dry out because the stucco on the outside slows evaporation
• In winter, trapped moisture freezes inside the wall cavity, then thaws and soaks the insulation
• Over months and years, this leads to sheathing rot, mould growth, rusted fasteners, and potentially structural damage
• Repair costs at this stage are significant: $150–$300 per square foot to remove stucco, replace sheathing and insulation, install new WRB, and re-stucco — easily $20,000–$50,000 for a badly affected wall

Modern alternatives to traditional building paper: Many Ottawa contractors now use synthetic housewrap products instead of asphalt-saturated building paper. These include:

• Tyvek StuccoWrap — Has a textured surface that creates micro-drainage channels between the stucco and the wrap. Very popular in the Ottawa market.
• Delta-Dry or MTI Sure Cavity — Drainage mat products that create a physical air gap behind the stucco. More expensive but provide true rain screen drainage.
• Fluid-applied WRBs (like Henry BlueSkin VP160) — Roller or spray-applied membranes that create a seamless, self-healing barrier. Increasingly specified on high-end Ottawa custom homes.

One important note for older Ottawa homes: houses built before the 1990s may have only a single layer of building paper, or in some cases, no paper at all behind the stucco. If you're doing any stucco repair or replacement on an older home, always check the WRB condition. If it's degraded or missing, that section of wall should be properly rebuilt with modern materials — it's not a place to cut corners, especially in our climate.

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

• Justyn Rook Contracting
• RenoMotion Inc.
• Steven Labelle - Your Complete Home Renovator
• BFI Renovations
• The Next Reno

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