When roof longevity is discussed, attention usually goes to shingles, tiles, or metal panels. However, there's a hidden but vital component that often roof underlayment.

Underlayments serve as the roof’s secondary barrier, protecting the deck from water, moisture, wind, and other environmental stressors. Yet one issue rarely considered is shrinkage. When underlayment contracts, it can create wrinkles, gaps, and vulnerabilities that shorten the roof’s service life, sometimes drastically.

What Causes Roof Underlayment to Shrink? It typically occurs due to:

• Prolonged UV exposure before the roof covering is installed
• Dramatic Temperature fluctuations 
• Moisture absorption and drying cycles
• Use of lower-grade materials or installation errors.

This shrinkage can result in the underlayment pulling away from critical areas, such as valleys, eaves, seams and around penetrations, where it is precisely most needed.

Why Is Underlayment Shrinkage a Big Deal?

Shrinkage isn’t a minor defect. Its impact cascades through the entire system, for example:

1. Loss of Waterproofing Integrity. Gaps around seams or fasteners can let in, leading to

               • Leaks in the attic and ceiling

               • Mold and mildew development

               • Rotted sheathing or framing.

2. Compromised Wind Uplift Resistance. Many modern underlayments contribute to the roof system’s ability to withstand uplift forces from high winds. If the underlayment shrinks and loses adhesion or fastener hold, it weakens the entire system’s structural integrity, especially in hurricane or storm-prone areas.

3. Delamination and Wrinkling Beneath Shingles. Shrinkage can cause the underlayment to wrinkle or buckle. This creates an uneven surface that can telegraph through asphalt shingles, causing them to lift or wear prematurely. Worse yet, wrinkled underlayment may prevent shingles from sealing properly, leaving them vulnerable to blow-off.

4. Shortened Roof Life. All of the above add up to a single painful truth: if your underlayment fails, your roof will likely fail long before its intended service life. Even if you invest in premium shingles or tiles, a shrinking underlayment undermines that investment from day one.

How to Prevent Underlayment Shrinkage

Fortunately, underlayment shrinkage is preventable, but only if roofing contractors have the right materials and practice the correct installation techniques.  

• Choose high-quality, low-shrink underlayments. Modern synthetic options typically outperform traditional felt in terms of shrink resistance, dimensional stability, and overall durability. 
• Limit exposure times. Minimize the time underlayments are left uncovered to limit UV and moisture effects.
• Follow manufacturer guidelines. Each underlayment has specific requirements for installation, fastening, and overlap. Cutting corners can lead to shrinkage and other failures. 
• Proper ventilation and moisture control. Minimizing moisture buildup in the attic and roof deck will reduce the chance of underlayment degradation over time.

A roof is only as strong as its weakest layer. While underlayment may be out of sight, it should never be out of mind — especially when it comes to shrinkage. Using the right products and installation techniques today can save homeowners thousands of dollars in premature roof repairs or replacements down the line.

If you're a roofing contractor, make sure your team understands the importance of underlayment selection and proper handling. And expect homeowners not to hesitate and ask what type of underlayment you're using and why.  

Because in roofing, it’s what’s underneath that truly counts.