How Thick Is Roofing Felt: Guide to Types, Uses, and Thicknesses

Roofing felt thickness varies by material, weight, and intended use, affecting durability, waterproofing, and installation. This article explains common felt weights, how thickness is measured, differences between asphalt-saturated and synthetic underlayments, and guidance for selecting the right product for a roofing project. Understanding felt thickness helps homeowners and contractors make better choices for protection and cost.

Felt Type	Common Weight	Approx. Thickness
15‑Pound Asphalt Felt	~15 lb per 100 sq ft	~0.04–0.06 inches
30‑Pound Asphalt Felt	~30 lb per 100 sq ft	~0.08–0.12 inches
Roofing Underlayment (Synthetic)	Varies, often lighter by weight	~0.02–0.06 inches

What Roofing Felt Is And Why Thickness Matters

Roofing felt, also called tar paper, is an underlayment placed between roof decking and the outer roof covering to provide temporary protection and secondary moisture resistance. Thickness influences durability, tear resistance, and water-shedding capability, so knowing felt thickness helps predict performance during installation and over the roof’s life.

Common Types Of Roofing Felt

There are two main categories: traditional asphalt‑saturated felt and modern synthetic underlayments. Asphalt felts come in nominal weights like 15‑ and 30‑pound; synthetics use polymers and fiberglass or polypropylene backing. Each type has different thickness profiles, weight-to-thickness relationships, and performance traits.

Standard Felt Weights And Their Approximate Thicknesses

Roofing felt is commonly specified by weight per 100 square feet rather than strict thickness. Typical options include 15‑pound and 30‑pound asphalt felts and various synthetic products. Weight correlates to thickness but is not an exact measure—manufacturing and material composition affect final thickness.

• 15‑Pound Felt: Usually about 0.04–0.06 inches thick, lighter, easier to handle, often used for re-roofing and single-layer applications.
• 30‑Pound Felt: Typically around 0.08–0.12 inches thick, heavier and more durable, preferred for steep-slope roofs and when underlayment longevity is needed.
• Synthetic Underlayment: Thickness ranges ~0.02–0.06 inches depending on design; offers high tensile strength and lighter weight compared with asphalt felt of similar performance.

How Thickness Is Measured And Why It Varies

Manufacturers report felt by weight (lb/100 sq ft) because asphalt-saturated products can have variable densities. Thickness is measured in inches or mils, but absorption and compression during storage can alter values. Temperature, manufacturing tolerances, and reinforcement layers (e.g., fiberglass) cause thickness variation between brands and batches.

Performance Differences Related To Thickness

Thicker felts generally resist tearing and punctures better and provide thicker moisture barriers. However, thickness alone doesn’t guarantee superior performance. Material formulation, reinforcement, and surface treatment (coatings or silicone) often influence longevity and water resistance as much as raw thickness.

Installation Considerations For Different Thicknesses

Heavier, thicker felts like 30‑pound are more durable but heavier to carry and lay, which can slow installation and require more fasteners in windy conditions. Lighter materials and many synthetics are easier to handle and can cover roof areas faster. Installers should follow manufacturer fastening patterns and overlap recommendations because improper installation negates thickness benefits.

Comparing Asphalt Felt And Synthetic Underlayment

Asphalt felts are bulkier for a given weight and provide a traditional moisture barrier. Synthetics can be thinner yet stronger, offering superior tear resistance and UV exposure tolerance during construction. Synthetics often outperform asphalt felt when measured by performance per thickness, but cost, code acceptance, and compatibility with the roofing material influence selection.

Code, Warranty, And Manufacturer Guidance

Local building codes and roofing manufacturer warranties sometimes mandate specific underlayment types or weights. For example, some shingle warranties specify 30‑pound felt or a designated synthetic product. Always verify code and warranty requirements before choosing felt thickness or switching to an alternative underlayment.

How To Choose The Right Felt Thickness For A Project

Selection depends on roof slope, exposure to weather, contractor preference, and budget. For steep-slope roofs, 30‑pound felt is commonly recommended for its added protection. For temporary protection during short-term work, 15‑pound may suffice. Synthetics are ideal when weight, tear resistance, and UV exposure during construction are high priorities.

Cost Implications Of Thickness Choices

Thicker felt typically costs more per roll and increases labor time due to heavier handling. Synthetics may be pricier per square but can save time and reduce the need for multiple layers. Budget decisions should weigh material cost against potential savings from longer life, fewer repairs, and warranty compliance.

Measuring Felt Thickness In The Field

Field measurement can use calipers to measure across an edge, but compression can skew readings. Manufacturers provide technical data sheets with nominal thicknesses and tolerances. Rely on manufacturer specs and test samples under controlled conditions for accurate thickness comparisons.

Practical Tips For Installers And Homeowners

Store rolls flat and protected from heat to avoid deformation. During installation, ensure proper overlaps, fastener spacing, and that seams are sealed where required. Even thicker felts fail when improperly installed, so following best practices is essential for maximum protection.

When To Replace Underlayment And How Thickness Affects Longevity

Underlayment should be inspected when re-roofing or if leaks are detected. Thicker, higher-quality underlayments typically provide longer service before replacement. However, underlayment age, water exposure, and roof covering integrity are key factors determining replacement timing.

Environmental And Safety Considerations

Asphalt felts contain petroleum products and produce fumes when heated; proper ventilation and PPE are important during installation. Some synthetic underlayments are recyclable and lighter, reducing transportation emissions. Choosing materials with lower environmental impact may affect felt thickness and cost but can improve job-site safety and sustainability.

Common Questions About Felt Thickness

• Does thicker felt mean better waterproofing? Thicker felt generally improves resistance to puncture and water penetration, but material quality and installation matter more than thickness alone.
• Can synthetic underlayment replace 30‑pound felt? Many synthetics meet or exceed the performance of 30‑pound felt and are accepted by manufacturers and codes, but confirmation from the shingle manufacturer and local code is necessary.
• How to check if underlayment meets warranty requirements? Review the roofing product warranty and the underlayment manufacturer’s technical data sheet for compatibility and required specifications.

Resources For Further Reference

Consult product technical data sheets, local building codes, and shingle manufacturer installation guides for authoritative specifications. Organizations such as the Asphalt Roofing Manufacturers Association and major underlayment manufacturers publish detailed comparisons and performance charts. These sources provide exact thickness, tensile strength, and permeability values needed for precise selection.