Applying Tar Paper to Roof: A Practical Guide

Tar paper, also known as roofing felt, serves as a crucial underlayment that protects roof decks from moisture and wind-driven rain before the final shingles are installed. This guide provides a concise, actionable approach for homeowners and contractors in the United States to apply tar paper correctly. Proper installation improves roof durability, reduces leak potential, and supports longer-lasting roofing systems. The focus here is on common residential roofs, with practical tips for selecting the right weight, orientation, and sealing methods to ensure a consistent, weather-resistant barrier.

Preparation

Before laying tar paper, inspect the roof deck for damage, rot, or loose boards. Replace any compromised material to ensure a solid base. Clean the deck and remove debris, nails, and dust that could hinder adhesion or create voids beneath the underlayment. Establish a clean work sequence that minimizes wind exposure and keeps underlayment clean during installation. Gather all materials and tools in a ready-to-use setup to maintain steady progress and reduce the risk of rework.

Materials And Tools

The most common tar paper options are 15-pound (15 lb) and 30-pound (30 lb) roofing felt, with synthetic underlayment as an alternative in some homes. For steeper or exposed locations, consider heavy-weight felt or synthetic products for added durability. Key tools include a utility knife, hammer or pneumatic nailer, roofing nails with flat heads, a chalk line, roofer’s tape, a staple gun (if using staples), and a staple-free method for certain applications. In colder climates, be mindful of temperature-sensitive adhesive seals or fasteners that may fail in freezing conditions.

Step By Step Application

Begin at the eaves and work uphill toward the ridge. Start with a starter strip at the bottom edge, ensuring a straight, aligned line. Roll out the first sheet, align with the eave edge, and keep it taut to avoid wrinkles. Secure with galvanized roofing nails every 6 to 8 inches along the edge and every 12 to 18 inches along the field, depending on local codes and roof pitch. Continue rolling and securing successive sheets, overlapping the upper sheet over the lower one by about 2 inches to create a continuous water barrier.

When working on a ladder or scaffolding, maintain three points of contact and avoid overreaching. Cut the tar paper to fit around vents, chimneys, and skylights with precision, leaving appropriate overlaps for flashing. Conform to roof contours using a utility knife, trimming excess material after nailing to prevent fluttering at higher wind loads. Keep the roll edges aligned to preserve a clean, uniform surface and minimize the risk of gaps that could admit moisture.

Overlaps And Seams

Proper lap seams are critical to prevent water intrusion. Typically, horizontal laps should be 2 inches, and vertical laps should be 6 inches on the underlayment. Ensure the upper layer fully covers the lower seam, and avoid gaps or buckling at any seam. Seal seams with roofing cement or compatible sealant if recommended by the manufacturer, particularly around roof penetrations. For roof sections with higher exposure, consider overlapping more than standard to compensate for wind uplift.

In areas prone to ice dams or heavy precipitation, maintain generous overlaps and verify that flashing integrates seamlessly with the tar paper. Some builders choose synthetic underlayment in combination with tar paper for enhanced performance; if using mixed materials, follow the manufacturer’s compatibility guidelines to prevent separation or reduced effectiveness.

Edges, Flashing And Penetrations

At eaves and rakes, ensure the tar paper extends over the drip edge and underlayment layers properly to support water shedding. Around valleys, hips, and ridges, install additional underlayment or transition to flashing materials per local codes. Vents, pipes, and chimneys require precise cutouts with ample sealing. Use a combination of flashing and sealant to secure any gaps where the underlayment meets the metal or vinyl flashing. Maintaining a tight seal around penetrations is essential to long-term roof integrity.

Safety And Weather Considerations

Work only in dry conditions with temperatures above 40°F (4°C) when possible to ensure the underlayment adheres and nails hold securely. Use fall protection on pitched roofs and follow OSHA guidelines. Wear appropriate footwear with good traction and a hard hat if necessary. Protect tar paper from direct sunlight and wind when transporting, and avoid working on windy days that could cause sheets to flap and tear. If rain is imminent, postpone installation to prevent moisture entrapment beneath the underlayment.

Common Mistakes

• Not aligning starter strips or misplacing laps, which creates weak seams.
• Using nails too close to the edge or spacing too far apart, causing wind uplift risk.
• Cutting around penetrations without precise fitting, leading to leaks.
• Allowing wrinkles or air pockets that trap moisture and compromise performance.
• Choosing the wrong underlayment weight for the climate and roof pitch.

Maintenance And Inspection

After installation, periodically inspect the roof underlayment, especially after heavy storms or high winds. Look for lifted edges, tears, or loose fasteners and address issues promptly. In newer homes, underlayment should be part of a broader inspection of roofing components, including shingles, flashing, and drainage. If signs of moisture intrusion appear inside the attic, investigate promptly to determine whether the tar paper underlayment has maintained its barrier function. Routine maintenance extends the lifespan of the roof system and helps prevent costly repairs.

Additional Tips

• Follow local building codes and manufacturer instructions for underlayment weight and installation methods.
• Store tar paper flat and protected from moisture to prevent warping before installation.
• Use corrosion-resistant nails and fasteners to enhance longevity in humid climates.