Quick Answer
Underlayment is the water-resistant layer between the roof deck and the metal panels. It acts as a backup barrier against wind-driven rain, helps manage condensation, and protects the deck during installation. In San Antonio's heat, synthetic or high-temperature underlayment is usually a better choice under metal than traditional felt, which can break down when the roof gets very hot.
What Underlayment Does
Metal panels shed most water, but wind-driven rain, ice from hail piled in valleys, and condensation can get underneath. Underlayment is the second line of defense.
It also separates the metal from the wood deck and protects the deck from weather while the roof is being installed.
Why Heat Matters
Metal roofs in direct San Antonio sun can get very hot on the underside. Traditional asphalt felt can soften, dry out, or deteriorate under that heat over the years.
Synthetic underlayments are lighter, stronger, and more resistant to tearing. High-temperature underlayments are designed specifically for metal and other roofs that run hot.
Common Underlayment Options
Your roofer should explain which product is being used and why.
- Asphalt felt: traditional and inexpensive, less heat-tolerant
- Synthetic underlayment: lightweight, tear-resistant, common under metal
- High-temperature underlayment: designed for hot roof surfaces
- Self-adhered membrane: used in valleys, around penetrations, and at low slopes
Special Areas That Need Extra Protection
Some parts of a roof need more than standard underlayment. Valleys, where two roof slopes meet, carry a large volume of water. Areas around chimneys, skylights, and wall intersections are also prone to leaks.
Self-adhered membranes are often used in these areas because they seal around fasteners and provide a stronger barrier. Low-slope sections may need full membrane coverage depending on the manufacturer's requirements. A careful roofer plans these details before the panels go on.
Ask What's Under Your Roof
Underlayment is one of the easiest places for a contractor to cut costs, because you never see it once the roof is finished. Make sure your estimate lists the product by name.
Sanchez Metal Roofing uses underlayment suited to metal roofing and San Antonio's climate, and we list it on every estimate.
Frequently Asked Questions
Is underlayment required under a metal roof?
Yes. Building codes and manufacturers generally require underlayment, and it is important for long-term performance.
Is synthetic underlayment better than felt?
For metal roofs in hot climates, synthetic or high-temperature underlayment is usually the better choice.
Does underlayment reduce noise?
It helps a little. Solid decking and attic insulation do most of the work in keeping rain noise down.
Contact Sanchez Metal Roofing
Phone: (210) 708-3308. Email: sanchezmetalroofing123@gmail.com. Address: 520 Fredericksburg Rd, San Antonio, TX 78201. Hours: Monday through Saturday 8:00 AM to 5:00 PM. Free roof inspections across San Antonio. Call (210) 708-3308.