This guide walks you through installing 15-lb felt, 30-lb felt, or synthetic roof underlayment over a clean roof deck. Topics include starting at the eave with ice & water shield (extend 24" past interior wall line), running the first felt course, proper 2-4 inch horizontal overlap and 6-inch end laps, fastener spacing (cap nails 12" o.c. on the field, 6" at edges), how to handle hips, ridges, and valleys, and integrating with drip edge and step flashing. Pairs with our Underlayment Calculator.
Underlayment is the secondary water barrier under the shingles. It matters most in the hours before the roof is finished and in the years after a shingle lifts, which is why the detail at the eaves and the laps is worth getting right.
Order of work at the eave
Eave drip edge goes on the bare deck first.
Ice and water barrier goes over the eave drip edge, extending at least 24 inches inside the exterior wall line measured along the slope — two courses on a low pitch with a deep overhang.
Field underlayment starts above the ice barrier and laps over it.
Rake drip edge goes on top of the underlayment, the opposite way round from the eave.
Laying the field
Work from the bottom up so every course laps over the one below.
Horizontal laps as printed on the roll, usually 2 to 4 inches; end laps 6 inches.
Stagger the end joints between courses.
Fix with cap nails or plastic-cap staples where permitted — plain staples tear through.
Keep it flat. A wrinkle telegraphs through the finished shingles and shows for the life of the roof.
Valleys and penetrations
Run self-adhering barrier the full length of every valley before the field underlayment, centred on the valley line.
Field underlayment then laps over it from each side.
Cut and seal around every pipe, and lap the underlayment over the flange of anything that has one.
At a wall, run the underlayment up the wall a few inches behind where the step flashing will sit.
Common mistakes
Putting the eave drip edge over the underlayment instead of under it, so water drains behind the fascia.
Leaving felt exposed past its rating, where it wrinkles and cannot be flattened out again.
Running the ice barrier only to the wall line rather than 24 inches past it.
Skipping the end laps entirely, which are the easiest joints to forget.
Fixing with plain staples, which pull through under wind before the shingles go on.
Frequently asked questions
Does underlayment go over or under the drip edge?
Under at the rakes, over at the eaves. At the eave the drip edge goes on the deck first and the underlayment laps over it; at the rake the underlayment goes down first and the drip edge over it.
How far up should ice and water barrier go?
At least 24 inches inside the exterior wall line, measured along the slope. On a low-pitch roof with a deep overhang that often means two courses of 36-inch barrier rather than one.
How much should underlayment overlap?
Follow the lines printed on the roll — commonly 2 to 4 inches horizontally and 6 inches at end laps. The manufacturer figure is what the warranty follows.
Can I staple underlayment down?
Use cap nails or plastic-cap staples. Plain staples tear through, especially before the shingles go on, and most manufacturers require capped fasteners for the warranty to stand.