How do I flash a garage roof where it meets an existing house wall in NB?

Proper flashing at the garage roof-to-house wall connection is critical in New Brunswick's wet climate and requires step flashing integrated with the house's existing water-resistive barrier, plus ice and water shield protection extending well beyond the typical eave coverage due to NB's severe ice dam potential.

The roof-to-wall connection on an attached garage is one of the most vulnerable leak points in any structure, and in New Brunswick's climate of heavy snow, ice dams, and driving rain, this detail must be executed perfectly. Water intrusion at this junction causes structural damage, insulation failure, and interior damage that can cost tens of thousands to repair.

Step flashing is the foundation of this connection. Each piece of step flashing should be at least 5 inches wide and 7 inches long, made from galvanized steel or aluminum. The flashing pieces are woven between each course of shingles as they're installed, with each piece extending at least 4 inches up the wall and 3 inches onto the roof deck. The key is that each piece of step flashing overlaps the piece below it by at least 2 inches, creating a shingled effect that directs water down and away from the wall.

In New Brunswick, ice and water shield membrane must extend at least 36 inches up from the eave edge by code, but at roof-to-wall connections, experienced NB roofers extend it much further. The ice dam potential where a garage roof meets a heated house wall is extreme — warm air from the house melts snow on the upper roof, which refreezes at the garage roof line. Install ice and water shield from the eave edge to at least 24 inches beyond where the garage roof meets the house wall, and consider extending it all the way to the ridge if the garage roof is short.

The house wall preparation is equally important. If the existing house has vinyl or aluminum siding, the step flashing must be installed behind the siding, which means removing siding courses in the area where the garage roof meets the wall. Each piece of step flashing should extend behind the house's water-resistive barrier (house wrap or building paper) by at least 2 inches. If the house has brick or stone veneer, the step flashing must be integrated with the existing through-wall flashing system, which may require cutting reglets (horizontal slots) in the masonry and sealing them with appropriate sealants.

Counter-flashing completes the system. After the step flashing is installed and the roof shingles are in place, install counter-flashing over the step flashing. This is typically a continuous piece of aluminum or galvanized steel that covers the top edge of the step flashing and extends down over the shingles. The counter-flashing should be fastened to the wall only — never through the roof surface — and sealed at the top with a high-quality polyurethane or silicone sealant rated for exterior use in temperature extremes.

Snow drift considerations are critical in NB. Where the garage roof meets the house wall, snow will drift and accumulate to depths 2-3 times the normal ground snow load. This creates additional water volume during spring melt and increases the ice dam risk. The flashing system must handle not just normal rain, but the substantial water flow from heavy snow melt. Consider installing a wider valley or adding a snow retention system on the house roof above the garage to prevent snow slides that can damage the flashing.

Hire a Professional For This Work

Roof-to-wall flashing requires coordination between roofing and siding trades, knowledge of the existing house's water management systems, and experience with NB's specific ice dam challenges. Improper flashing installation is one of the most expensive mistakes in garage construction — water damage from a failed roof-to-wall connection can cost $15,000-$30,000 to repair, including structural repairs, insulation replacement, and interior finishing. Professional roofers experienced with attached garage construction understand the critical details and have the tools to integrate new flashing with existing house systems without compromising either structure's weather protection.

Need help finding a professional garage builder experienced with proper flashing details? New Brunswick Garages can match you with contractors who understand these critical weatherproofing requirements.