Can I re-roof my garage with dimensional shingles over existing 3-tab in New Brunswick?

Yes, you can typically install dimensional (architectural) shingles over existing 3-tab shingles in New Brunswick, but only if you currently have just one layer of shingles and the roof deck is structurally sound. This approach can save $2,000-$4,000 in tear-off and disposal costs on a typical two-car garage.

Structural Considerations for NB Snow Loads

The critical factor in New Brunswick is whether your garage roof structure can handle the additional weight. Dimensional shingles weigh approximately 240-300 pounds per square (100 square feet), while 3-tab shingles weigh about 200-240 pounds per square. Adding a second layer increases the total roof load by roughly 2.5-3 pounds per square foot.

In most of New Brunswick, garage roofs must be designed for ground snow loads of 2.4-4.8 kPa (50-100 pounds per square foot), with northern communities like Bathurst and Edmundston seeing the higher loads. If your garage was built to current code standards with proper rafter or truss spacing (typically 16 or 24 inches on center), the structure should handle the additional shingle weight without issue. However, older garages built before modern snow load requirements may have undersized framing that cannot safely carry the extra load.

When Re-Roofing Over Existing Shingles Works

This approach is most successful when your existing 3-tab shingles are relatively flat and well-adhered. The new dimensional shingles will follow the contours of what's underneath, so any curled, buckled, or loose tabs will telegraph through the new roof. Walk your garage roof (safely) and check for loose or missing shingles, significant granule loss, or areas where the roof deck is visible.

The existing roof must also be properly ventilated. In New Brunswick's climate, inadequate attic ventilation leads to ice dam formation and premature shingle deterioration. If your garage has soffit vents and ridge vents or gable vents providing adequate airflow, the new shingles will perform well. Poor ventilation will cause problems regardless of whether you tear off or overlay.

NB Building Code and Permit Requirements

Most New Brunswick municipalities allow overlay installations but limit total roof covering to two layers maximum. If your garage already has two layers of shingles, you must tear off to the deck before installing new material. Some municipalities require a building permit for re-roofing work, while others exempt simple shingle replacement. Check with your local building department before starting.

The overlay method requires longer roofing nails to penetrate through both shingle layers and into the roof deck. Standard 1-inch nails used for single-layer installation are insufficient. You'll need 1.5-inch nails minimum, and many contractors prefer 1.75-inch nails for better holding power in NB's wind conditions.

Installation Details for Maritime Climate

Pay special attention to the starter course and ice dam protection. Install a full starter strip along the eaves, and ensure your ice and water shield membrane extends at least 36 inches up from the eave edge (NB code minimum). Many experienced contractors in New Brunswick extend this protection 48-72 inches given our ice dam potential, especially on attached garages where heat loss from the house can create freeze-thaw cycles.

The dimensional shingles should overhang the existing drip edge by 0.5-0.75 inches to ensure proper water drainage. In coastal New Brunswick communities, use shingles rated for high wind speeds (110+ mph) and consider upgrading to 6-nail installation pattern instead of the standard 4-nail pattern for enhanced wind resistance.

Cost Comparison and Timing

Overlay installation typically costs $4-$7 per square foot in New Brunswick, compared to $6-$10 per square foot for complete tear-off and replacement. On a typical 600-square-foot garage roof, this represents savings of $1,200-$1,800. However, the overlay method reduces the roof's total lifespan by 3-5 years compared to installation over a clean deck.

Plan your re-roofing for late spring through early fall when temperatures consistently stay above 10°C. Dimensional shingles seal properly in warm weather, and working conditions are safer for contractors on sloped garage roofs.

When to Hire a Professional

Garage re-roofing involves working at height with proper safety equipment and requires knowledge of NB's specific ice dam and wind resistance requirements. Professional installation includes proper nail placement, starter course details, and integration with existing flashing and ventilation systems. Most contractors provide 5-10 year workmanship warranties on overlay installations.

Need help finding a roofing contractor experienced with garage re-roofing? Browse contractors through the New Brunswick Construction Network directory for professionals familiar with NB climate requirements and overlay installation techniques.