What do roof underlayment installation instructions say?
Short answer. Underlayment instructions come from the product's manufacturer and usually cover the minimum slope, which side faces up, the head and end lap widths marked on the sheet, the fastener type and spacing pattern, how long the sheet can be left exposed, and how it laps with drip edge, leak barrier and flashing. Following them matters because both code and warranties point back to them.
Where to find them
Instructions are printed on the roll wrapper or a label inside the core, and the full version is on the manufacturer's website, often linked from a code on the roll. Product data sheets and code evaluation reports add test results and limits.
What each section means
- Slope limits. The minimum pitch for standard installation and what changes on low slopes, usually double coverage or a self-adhered membrane.
- Orientation. Which side faces up. Synthetics have a walkable, slip-resistant side, and installing it upside down makes the roof dangerous to walk.
- Laps. Head lap and end lap widths, often shown by printed lines and marks on the sheet.
- Fasteners. Cap nails or cap staples, with a spacing pattern that is tighter at laps and edges. Bare staples are rarely allowed for synthetics.
- Exposure. How long the sheet can sit in the sun before shingles go on. Exceeding it can void the product warranty.
- Conditions. Do not install over wet or frosty decking, and observe temperature limits.
- Details. How to handle eaves, rakes, valleys, walls and penetrations alongside drip edge and leak barrier.
Why this is not optional
The residential code ties underlayment installation to the manufacturer's instructions in many cases, and shingle makers often require their own brand of underlayment for enhanced system warranties. An installation that ignores the instructions can fail inspection, lose warranty coverage or simply leak.
Compare product types in synthetic underlayment vs felt, and see how system warranties work in roofing warranties explained.