What is the difference between roof underlayment and ice and water shield?
Short answer. Standard underlayment, felt or synthetic, is a water-resistant sheet that is nailed down and covers the whole deck as a backup layer. Ice and water shield is a thicker self-adhered membrane that bonds to the deck and seals around nails, making it waterproof, so it is used in the high-risk spots like valleys, eaves, walls and penetrations, with regular underlayment everywhere else.
Side by side
- Attachment: underlayment is mechanically fastened with cap nails or staples. Ice and water shield is adhered across its whole back.
- Water protection: underlayment is water-resistant and sheds water. Ice and water shield is waterproof and seals around nail shanks.
- Vapor: many synthetics and felts allow some vapor through; self-adhered membranes essentially block it.
- Cost: underlayment is much cheaper per square.
- Exposure: both have limits on how long they can sit uncovered, set by the product.
- Removal: underlayment comes off with the shingles; membrane bonds to the deck and often damages it at tear-off.
- Standards: felt is made to ASTM standards such as D226 or D4869, synthetics are usually backed by code evaluation reports, and self-adhered membranes are tested to ASTM D1970.
How they work together
On a typical Georgia shingle roof, membrane goes down first in valleys, around chimneys, skylights and pipes, along roof-to-wall joints and on low-slope sections, sometimes at the eaves too. Underlayment covers the rest, lapping over the membrane's top edges so water flows from one to the other without entering a seam.
When membrane covers more
Low-slope shingle roofs, metal roofs with high temperature membrane, and sealed roof deck programs can call for full membrane coverage. That adds cost and makes attic ventilation more important.
The underlayment choices are compared in synthetic underlayment vs felt, and membrane placement in ice and water shield in Georgia.