Which Side of Roofing Felt Goes Down for Best Results

Choosing the correct orientation for roofing felt is essential for a durable, weather-resistant roof. The question, “which side of roofing felt goes down,” is common for DIYers and professionals alike. This guide explains the standard orientation, factors that influence the decision, and practical installation tips to help ensure a proper seal, wind resistance, and long-term performance.

Understanding Roofing Felt Orientation

Roofing felt, also known as tar paper or underlayment, is installed over the roof deck before shingles or other roofing materials. Most asphalt-based felts have a fibrous side with a rough texture and a smoother, sometimes shiny side. In traditional installation, the smoother side faces outward toward the shingles, while the rough side rests against the deck or previous layer. This orientation helps shed water toward the drip edge and ensures better seal with the roofing material. Always consult manufacturer instructions, as some products may differ slightly.

The general rule of thumb is that the smooth or glossy side goes down toward the deck or toward the previous layer interactions, and the rough side faces up toward the future roofing material. This arrangement can improve tear resistance, minimize wrinkles, and facilitate a secure nailing pattern when the shingles are applied. However, certain products and climates may call for alternative installations, so manufacturer guidance should be followed when available.

Standard Asphalt Felt Versus Synthetic Underlayment

Traditional asphalt felt typically has a distinct texture on each side. The common installation method is to place the smooth side facing up, toward the upcoming shingles, and the rough side facing down toward the roof deck. This configuration helps the nails grip the felt and reduces the risk of lifting in high winds. Synthetic underlayments, which are newer materials, may vary in texture and installation direction, so checking the packaging is essential.

Synthetic underlayments often feature a slip-resistant surface and may be designed to be installed with either side up depending on the product line. Some synthetics provide a moisture barrier on both sides, while others have a dedicated “top” side designed to interact with fasteners and roofing materials. If in doubt, prioritize the manufacturer’s recommended orientation and consider a compatibility check with the roofing material you plan to install.

Practical Installation Steps

When installing roofing felt, precise steps help ensure consistent performance and a long-lasting roof. The following sequence reflects common best practices:

• Prepare the roof deck: Ensure the deck is clean, dry, and free of nails or debris to prevent punctures and wrinkles.
• Start at the eave: Roll out the first strip from the bottom edge, ensuring the felt lies flat with no folds.
• Position orientation: Place the smooth side up, facing toward the future shingles, with the rough side contacting the deck where applicable.
• Nail pattern: Use a 6-inch staggered pattern along the edges and 8-12 inches in the field, depending on climate and product specification.
• Sealing and gaps: Overlap seams by 19-24 inches for thick felt or as recommended by the manufacturer, and seal seams with roofing nails or tape if required by the product.
• Continue up the slope: Overlap each subsequent strip, maintaining consistent orientation and minimizing wrinkles to avoid water pooling.

In steeper roofs or areas with high wind exposure, additional measures such as specialized fasteners, edge clips, or longer overlaps may be advised. Always verify the exact instructions on the packaging or with the installer guide for your specific felt product.

Common Mistakes To Avoid

Avoid these pitfalls that can compromise performance:

• Installing with the wrong side up: This can reduce tear resistance and affect adhesion of subsequent roofing layers.
• Skipping overlaps: Insufficient overlap leads to water infiltration at seams during rain or wind-driven moisture.
• Wrinkles and air pockets: Wrinkles trap moisture and can compromise shingle adherence.
• Inadequate nail pattern: Too few fasteners or incorrect spacing increases risk of blow-offs in storms.

Roof Type and Climate Considerations

The orientation of roofing felt can vary with roof type and climate. In regions with heavy rainfall or snow, a proper seam overlap and securement are critical to withstand weather events. For cold climates, some manufacturers recommend a tighter overlap and additional sealing methods to prevent ice dam development. In roof decks with radiant heat or attic ventilation concerns, the felt’s ability to shed moisture quickly remains important, reinforcing the need to follow the product’s guidelines for orientation and seam treatment.

Tips For Verifying Correct Orientation

To ensure correct installation, homeowners and installers can perform quick checks:

• Check packaging: The label usually indicates the correct side up or down, especially for synthetic underlayments.
• Ask the installer: If uncertainty remains, consult the installer or manufacturer support for the exact product line.
• Inspect seams during application: Ensure seams are properly overlapped and that the edging away from eaves is smooth and tight.
• Assess compatibility with shingles: Some shingles require a specific underlayment surface texture for best adhesion and wind resistance.

Tools and Materials You Might Need

The following tools help achieve a clean, durable installation:

• Roofing nails or cap nails, per product specification
• Hammer or nail gun with appropriate nail length
• Utility knife or scissors for trimming
• Measuring tape and chalk line for straight courses
• Roofing adhesive or seam tape if required by the product

Choosing the right underlayment and orientation can influence the ease of shingle installation, wind resistance, and long-term roof performance. Following the manufacturer’s instructions ensures compatibility with the chosen roofing materials and local building codes.