Ice and water needs to go down first before synthetic underlayment. This roof is going to get a GAF golden pledge warranty, so we used GAF synthetic and GAF ice and water. Ice and water is a peel and stick product that goes in valleys and seals around nail heads. Plastic cap nails are used to fasten synthetic felt. Substrate needs to be prepared- this includes replacing rotten boards and no rotten rafters- both things that were done on this job. In summary, roofing underlayment is critical because it serves as a secondary barrier, preserving the life of the overall roofing system.
This roof is finished with GAF Timberline HD shingles, and this owner can expect their golden pledge warranty mailed to them via GAF a week after completion.