Tri-Flex® mechanically-attached roofing underlayment is a high performance water-shedding underlayment, designed to overcome the shortcomings of traditional roofing felt. It is stronger than #30 felt and withstands high wind conditions; it is lighter than #30 felt and covers more area. The product is mechani¬cally attached to the roof and can be used alone or in conjunction with self-adhered underlayments.

GCP roofing underlayment provides a stable base for the application of mechanically-attached roof coverings and serves as a secondary water shed to help prevent leaks if water penetrates the primary roof covering. The product may also be used to temporarily “dry in” the structure for a period of 6 months before the roof construction is finished or if the primary roof coverings become damaged.

GCP underlayment is suitable for application as part of all major roof systems.



Below are some of the major benefits of GCP roofing underlayment compared to conventional #30 roofing felt:

  • It’s lighter and covers more area — A roll of underlayment is around half the weight of #30 felt, and has nearly 5 times the coverage.
  • It’s stronger — Tri-Flex® roofing underlayment is 20 times stronger than felt and won’t tear away from nails even in high winds.
  • It’s more durable — GCP underlayment can be left exposed for up to 6 months. It doesn’t dry out or rot at high temperatures. It doesn’t crack or become brittle at low temperatures.
  • It’s slip resistant — GCP roofing underlayment is formulated for excellent skid-resistance even in wet conditions.
  • Suitable for use under all major roof coverings — Tri-Flex® mechanically-attached underlayment can be used under shingles, tiles, slate, metal and cedar shakes. Refer to subsequent sections for more details.
  • Facilitates faster and easier installation — The lighter rolls make the one-person application fast and easy. It also means fewer trips up the ladder and lower labor costs
  • Technical support — Tri-Flex® underlayment is backed by a team of local technical support personnel that help ensure every application goes smoothly.

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