Roofing felt, or tar paper, provides a protective underlayment that helps shed moisture and add a moisture barrier beneath shingles. This guide explains how to lay roofing felt step by step, with practical tips to ensure proper overlap, secure fastening, and a weather-tight roof surface. Clear instructions, safety reminders, and common mistakes are highlighted to help DIY homeowners and professionals achieve durable results.
Tools And Materials
Before starting, assemble the necessary tools and materials to ensure efficient workflow. A well-prepared kit reduces delays and improves seam integrity. The following items are commonly used for installing roofing felt:
- Roofing felt rolls (typical 15-lb or 30-lb weight)
- Measuring tape and straightedge
- Sharp utility knife or roofing knife
- Chalk line for straight, accurate lines
- Hammer or nail gun with roofing nails
- Roofing nails or staples (galvanized or stainless steel)
- Sealant tape or bituminous seam tape
- Roofing tar or mastic for edge sealing
- Carpenter’s pencil or marker
- Safety gear (gloves, eye protection, non-slip footwear)
- Ladder stabilizer and harness, if needed
- Utility knife replacement blades
Preparing The Roof Surface
Proper preparation ensures the felt adheres and lasts. Inspect the roof deck for damage, replace rotten boards, and remove debris. Clean the surface and ensure it is dry before installing felt. Any moisture trapped behind the felt can cause mold or deck rot over time. If using self-adhesive or laminated felt, make sure the roof deck is free from dirt and dust to promote adhesion. For best results, install on a dry, calm day with temperatures above freezing to avoid premature tearing of the material.
Cutting And Measuring
Accurate cutting minimizes waste and ensures clean seams. Measure the length from eave to ridge and cut the felt with an extra 6–8 inches for the overhang at the bottom edge. Trim around protrusions, such as chimneys, vents, and skylights, by scoring and carefully bending the felt to fit. Always measure twice and cut once to prevent unnecessary waste. Use a chalk line to mark straight starting lines across the roof, aligning with the eaves for consistent rows.
Laying The Felt
Begin at the eaves, working upward toward the ridge. Unroll the first strip with the underside facing the deck and align it with the chalk line. Secure the felt with fasteners along the top edge, spacing nails about 12 inches apart. For long roofs, use full-length sheets when possible to minimize seams. Ensure the felt lies flat with no wrinkles or air pockets, as these can trap moisture and cause bubbling later on. Overlap each subsequent row by 2 to 4 inches, depending on the product’s instructions, to create a water-shedding layer.
Seams And Overlaps
Overlap direction should face upward to promote water runoff. When two sheets meet, the upper sheet should overlap the lower sheet by the recommended amount. For self-adhesive felt, peel backing gradually as you press the sheet into place to improve adhesion. For non-adhesive felt, use seam tape or a compatible sealant along the overlaps to prevent water intrusion. Keep the overlaps consistent across the entire surface to avoid weak spots.
Fastening And Securing
Secure the felt along the entire length of each row. Common methods include roofing nails or staples with caps to prevent tearing. Space fasteners every 8 to 12 inches along the center of each strip, particularly near the edges and along eaves where wind uplift is a concern. For exposed nails, seal the heads with a compatible sealant or tar to prevent water ingress. If working in windy conditions, temporarily staple or weigh down the edges to keep the sheet in place until the next step.
Edge Sealing And Finishing
Edge sealing is essential to prevent moisture infiltration at eaves and rakes. Apply a continuous bead of sealant along the edges where the felt meets the decking or flashing. For hard-to-seal joints or transitions around vents and chimneys, use a flexible bituminous sealant or specialized flashing tape. After sealing, inspect the entire surface for any loose corners or lifts and re-secure them promptly. Some installations may require a second layer or additional flashing at critical points to ensure a weather-tight finish.
Common Mistakes To Avoid
- Installing felt on wet or damp decks
- Underestimating the required overlap or misaligning seams
- Using damaged or curled sheets that don’t lay flat
- Skipping edge sealing or improper fastening near eaves and ridges
- Failing to allow for proper ventilation in some roof assemblies
Maintenance And Inspection
After installation, periodically inspect the roof for lifted edges, tears, or sealant failures, especially after storms or high winds. Re-secure loose sheets and replace damaged sections promptly. Maintain the underlayment by ensuring the overlying shingles are installed correctly to maximize protection. If replacing roofing materials, consider whether a new felt layer is warranted based on the roof’s age and condition.