Underlayment: The Unsung Hero of Roofing

When it comes to roofing, the shingles often steal the spotlight, but beneath those durable layers lies a silent protector: the underlayment. Roofing underlayment is a critical component of any roofing system, providing a secondary layer of defense against moisture and ensuring the longevity of your roof. This blog post will cover the application of underlayment on low slopes and new construction, shedding light on why this often-overlooked element is so vital.

What is Roofing Underlayment?

• Shingle underlayment, also known as tar paper and roofing felt, is a roll product applied over the roof deck before the shingles are installed.
• Underlayments consist of a felt or mat (fiberglass or polyester) that may be impregnated or coated with asphalt, stabilized asphalt coating, polymer-modified asphalt, non-asphaltic polymeric coating, or a combination of these materials.
• It acts as a water-resistant or waterproof barrier, protecting the roof deck from rain, snow, and ice.

Why is Underlayment Important?

• Secondary Water Protection: Underlayment provides a crucial barrier against water intrusion, especially in cases of wind-driven rain or ice dams.
• Deck Protection During Installation: It keeps the roof deck dry until shingles can be applied, preventing issues that can arise from installing shingles on wet decks. Shingles should be applied as soon as possible after the application of the underlayment.
• Fire Resistance: Underlayment contributes to the overall fire classification of the roof assembly.
• Reduces Picture Framing: It helps minimize the visible outline of deck panels caused by irregularities in roof decking thicknesses.
• Protects Shingles: Underlayment shields shingles from resins that may come from wood-based sheathing.

Underlayment Types

• Saturated Felt Underlayments: This type of asphalt roofing product consists of a dry felt impregnated with an asphalt saturant.
• Synthetic Underlayments: Many synthetic underlayments are available that may or may not be self-adhering.
• Self-Adhering Shingle Underlayment: These are polymer-modified bituminous sheet membranes used in critical roof areas. They resist water penetration caused by the backup of water from ice dams, melting snow, or wind.

Underlayment Application on New Construction

1. Deck Preparation: Ensure the roof deck is smooth, firm, and dry. Plywood with a minimum thickness of 1/2″ (12 mm) is recommended for best roof performance.
2. Drip Edge Installation:
   • On eaves, install drip edge below the underlayment so the water will shed efficiently off the roof if it gets under the shingles.
   • On rakes, install drip edge on top of the underlayment to stop wind-driven rain from getting underneath.
3. Underlayment Installation:
   • Apply underlayment courses parallel to the eaves, lapping each course at least 2″ over the underlying one.
   • Secure the felt with only enough nails to hold it in place.
   • If two or more pieces are needed to continue the course, lap ends at least 4″.
   • End laps in a succeeding course should be located 6 ft. or more from the end laps of the preceding course.
   • Lap felt a minimum of 6″ from both sides over hips, ridges, and any valley underlayments.
   • Where the roof meets a vertical surface, continue the underlayment at least 4″ up the surface.

Underlayment Application on Low Slopes

• Low-slope roofs can have slopes as minor as 1⁄8 inch per 12 inches.

1. Special Considerations: On slopes below 4/12, down to 2/12, the risk of leaks is great, caused by phenomena like wind-driven rain and capillary action. Never apply shingles on slopes lower than 2 inches per 12 inches.
2. Two-Layer Application: Cover the deck with two layers of asphalt-saturated felt.
3. Starter Strip: Fasten a 19-inch-wide strip of underlayment along the eaves so that it overhangs the drip edge by 1⁄4 to 3⁄8 inch.
4. Full Sheet: Place a full 36-inch-wide sheet over the starter strip with the long edge placed along the eaves and completely overlapping the starter strip.
5. Overlapping: Position all succeeding courses to overlap the preceding course by 19 inches.
6. Cementing: In areas where icing along the eave is anticipated, asphalt roof cement or lap cement may be used.

Ice Dam Protection

• In extreme ice dam areas, a good option is to install waterproof underlayments under the drip edge and down the fascia at the eave.

Valley Underlayment

1. Run a 36″ wide centered strip of underlayment up each valley.
2. Secure it 1″ from the felt edge with just enough nails to hold in place.
3. If two or more strips of underlayment are needed to cover the entire valley length, overlap the upper piece 12″ over the lower and bond it with asphalt plastic cement.

Best Practices

• Follow Manufacturer’s Instructions: Always adhere to the underlayment manufacturer’s specific installation guidelines.
• Check Local Codes: Ensure compliance with local building codes for underlayment requirements.
• Proper Ventilation: Ensure adequate attic ventilation, especially when using self-adhering underlayment.

While it may not be the most glamorous part of a roofing project, underlayment is a crucial investment in the long-term performance and protection of your roof. By understanding the different types of underlayment and following proper application techniques for new construction and low slopes, you can ensure that your roof stands strong against the elements for years to come.