Underlayment (roof deck protection)

Definition: Underlayment is a crucial component of the roofing system, serving as a protective layer installed directly over the roof deck (sheathing) and beneath the roofing materials such as shingles, metal, or tiles. It acts as an additional barrier against water infiltration, helping to prevent leaks and moisture damage, and it can also provide extra insulation.

Purpose and Functionality

• Waterproofing: One of the main functions of membrane is to provide an extra layer of waterproofing. In the event that the primary roofing material (like shingles or metal panels) is damaged or water manages to seep through, the underlayment helps protect the roof deck by keeping moisture away. This is especially important in regions that experience heavy rainfall, snow, or ice dams.
• Protection Against Wind-Driven Rain: In high-wind situations, rainwater can be driven under the edges of the roofing material. Roof sheathing layer offers additional protection by preventing water from penetrating into the roof structure, which can cause leaks or rotting over time.
• Insulation and Temperature Control: Some types of this layer also add a layer of insulation to the roof, helping to regulate indoor temperature. By acting as a thermal barrier, underlayment can prevent heat from escaping during winter and reduce heat gain during the summer, improving the home’s energy efficiency.

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Types of Roofing Underlayment

• Asphalt-Saturated Felt: Also known as felt paper or tar paper, this traditional type of underlayment is made from organic or fiberglass materials saturated with asphalt. It comes in different weights, such as 15-pound or 30-pound felt, with the heavier variety offering greater durability and water resistance. Although it’s less expensive, it may not last as long as more modern materials.
• Synthetic: This type is typically made from polypropylene or polyethylene and is rapidly becoming the preferred choice for modern roofing systems. Synthetic underlayment is lighter, more durable, and resistant to tearing compared to felt. It also provides better UV resistance, which is important if the layer is exposed to sunlight for an extended period during installation.
• Rubberized Asphalt: Known for its superior waterproofing properties, rubberized asphalt underlayment features a self-adhesive backing that creates a watertight seal. This type of underlayment is particularly effective in areas prone to ice dams or heavy rain. It’s commonly used in vulnerable areas of the roof, such as valleys, eaves, and around penetrations like chimneys and skylights.

Benefits of Underlayment

• Extended Roof Lifespan: By providing an additional layer of protection, underlayment helps extend the overall lifespan of the roofing system. It reduces the risk of water damage, which can weaken the roof structure and lead to costly repairs.
• Fire Resistance: Certain types of roof deck protection, especially those made from synthetic materials, offer added fire resistance, helping to improve the overall safety of the building.
• Cost-Effectiveness: While underlayment adds to the initial cost of a roofing project, it helps prevent long-term damage from water infiltration and other environmental factors, making it a cost-effective investment.

Read more: Metal Roof Cost Ontario in 2025 

Disadvantages:

• Installation Cost and Time: Adding membrane to a roofing project increases both the time required for installation and the overall cost of materials and labor. However, these costs are generally outweighed by the long-term protection provided.
• Weather Sensitivity: Certain types of underlayment, such as felt paper, are more vulnerable to weather conditions like strong winds or excessive moisture during installation, making them less durable in extreme environments.