10/22/2026
Two materials can sit beneath your shingles and still serve different purposes.
Synthetic underlayment covers the roof deck and acts as a secondary water-shedding layer. It helps move water down the roof if moisture reaches beneath the shingles.
Ice & water barrier is installed in more vulnerable areas, including the eaves and valleys. It creates a watertight seal around the roof deck and fasteners where water is more likely to collect or move beneath the shingles.
These materials also need to connect correctly. The synthetic underlayment overlaps the ice & water barrier, helping create a continuous path for water to move down and off the roof.
Understanding the difference helps you know what materials are included in your roofing system and why both have a place beneath the finished shingles.