How to Install Asphalt Shingles on a Gambrel Roof

Gambrel roofs, characterized by their distinctive dual slopes on each side, offer aesthetic appeal and extra attic space but present unique challenges for shingle installation. The steeper lower slope and gentler upper slope require careful attention to transitions and securing methods to ensure longevity and weather resistance. This guide provides a thorough walkthrough for DIY enthusiasts or professionals tackling asphalt shingle application on such structures.

Understanding Gambrel Roof Anatomy

A gambrel roof features two pitches per side: a steep lower section for maximum headroom and a shallower upper portion leading to the ridge. This design, common in barns and colonial homes, demands shingles that flex well at the break point where slopes change. Asphalt shingles work well here due to their flexibility when warm, but proper underlayment and fastening are critical on steep inclines exceeding 21:12 pitch.

Key considerations include wind uplift resistance, water shedding at transitions, and thermal expansion. Always verify local building codes, as steep roofs may require enhanced nailing schedules or adhesives.

Essential Tools and Materials

Gather these items before starting to streamline the process:

• Asphalt shingles (3-tab or architectural for better durability)
• Galvanized roofing nails (1.25-inch for standard, longer for steep slopes)
• Nail gun or hammer
• Utility knife and shingle cutter
• Roofing cement or plastic cement (ASTM D4586 compliant for steep pitches)
• Chalk line, measuring tape, and ladder/scaffolding
• Safety harness, ropes, and roof jacks
• Underlayment (felt or synthetic)
• Ridge cap shingles

For steep gambrel sections, prioritize scaffold or jacks to maintain footing.

Prioritizing Safety on Steep Slopes

Safety is paramount on gambrel roofs due to the steep lower pitches. Install a safety harness anchored to a secure point, and use roof jacks spaced 2 feet apart with 2×10 planks for walkways. Work in pairs, with one person feeding materials. Choose dry, warm days above 50°F to enhance shingle pliability and reduce slip risks.

Accurate Roof Measurements and Material Estimation

Measure each roof plane separately: length and width of lower and upper slopes on all sides. Calculate square footage per section (length x width), sum totals, and add 5-10% for waste and overlaps. Divide by 100 for ‘squares’ needed—one square covers 100 sq ft.

Example Calculation: Lower slope (20×30 ft) = 600 sq ft; Upper slope (20×20 ft) x4 sides = 1600 sq ft. Total: 2200 sq ft +10% = 2420 sq ft /100 = 24.2 squares (buy 25).

Preparing the Roof Deck

Ensure plywood or OSB sheathing is clean, dry, and securely fastened (8d nails at 6-inch edges, 12-inch field). Install drip edge along eaves and rakes. Lay underlayment starting at eaves: full sheet over eaves, then 6-inch overlaps ascending. At transitions, double underlayment or use ice-and-water shield for leak prevention.

Installing Starter Strips

Begin at the eave edge of the lowest slope. Cut 3-tab shingles to remove one tab, creating starters. Nail with 4 nails per strip, 1 inch above sealant strip, staggering joints 4 inches from shingle courses above. This offsets water paths and seals edges.

Laying the First Course

Start at the bottom corner of the lower slope. Position full shingle over starter, flush to edge. Secure with 4-6 galvanized nails: two above each tab, 1 inch above sealant line, 1 inch from ends. Drive nails straight to avoid shingle damage. Complete row end-to-end, snapping chalk line for straightness.

Building Subsequent Courses

Offset second row by half shingle (cut first shingle lengthwise). Continue alternating offsets for staggered joints, preventing leak lines. Trim end shingles to fit rake edges with utility knife. Maintain 1.5-inch exposure per course unless manufacturer specifies otherwise.

• Row 1: Full shingles
• Row 2: Half shingle start
• Row 3: Full start
• Repeat pattern

Navigating the Slope Transition

The critical juncture: as courses reach the break, extend shingles 6-12 inches over onto upper slope. Nail upper portion only (5-8 nails), allowing lower tabs to lie flat. Apply roofing cement under overhanging tabs, press firmly (hold 30-60 seconds per instructions). Avoid nailing tabs to prevent exposure.

For pitches over 21:12, use six quarter-sized cement dollops per shingle: two under each tab, one near edges. Press firmly; excess cement causes blistering.

On steep transitions, bend shingles smoothly over the angle, securing with adhesive rather than nails on exposed lower faces.

Shingling the Upper Slope

Continue courses on gentler upper pitch, bending new shingles over transition edge. Glue undersides, nail above sealant strip. If needed, install a drip edge or colored flashing under first upper course for seamless coverage.

Completing Multiple Sides

Work one face at a time, starting lowest slopes. At valleys (if present), weave shingles or use valley metal. Ensure hips align with ridge direction. Symmetry across sides maintains visual balance.

Ridge Capping and Final Touches

At the peak, fold ridge cap shingles over both sides, nailing inner edge (two nails, 5.5 inches up). Cement exposed sealant strip. Trim for clean fit at ends. Inspect all nails, re-cement loose areas, and sweep debris.

Advanced Tips for Durability

– On high-wind areas, use 6 nails per shingle.
– Steep roofs (>60 degrees): Extra cement mandatory.
– Architectural shingles: Enhanced adhesives for better seal.
– Annual inspections prevent transition failures.

Common Mistakes to Avoid

FAQs

What if my gambrel roof is very steep?

Use scaffold, harness, and extra cement per ASTM standards.

Can I use metal flashing at transitions?

Yes, colored flashing or drip rail under upper courses hides nails effectively.

How many nails for steep gambrel roofs?

Atlas recommends six per shingle on slopes over 21:12.

Best shingle type for gambrel?

Architectural for flexibility and impact resistance.

Time estimate for DIY?

2-4 days for average garage, depending on helpers and weather.