Underlayment is the layer nobody sees and everybody depends on: the water-resistant membrane between the roof deck and the shingles that backstops wind-driven rain, ice dams, and the inevitable day a shingle blows off. Proper roof underlayment installation is not complicated, but it is unforgiving of sequence errors — laps run backward, ice barrier missing at the eaves, fasteners underdriven — because every mistake is buried under shingles the same afternoon and only reports back as a ceiling stain years later. I have decked and papered plenty of roofs alongside roofing crews, and the process below is the standard asphalt-shingle sequence for US homes: material choice, prep, ice and water shield, field courses, and the detail work at valleys, penetrations, and edges that actually decides whether a roof leaks.
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Choose your underlayment: felt vs. synthetic vs. peel-and-stick
| Type | Cost per square (100 sq ft) | Exposure time | Notes |
|---|---|---|---|
| #15 asphalt felt | $5–$10 | Days | Code-minimum classic; tears, wrinkles when wet |
| #30 asphalt felt | $10–$15 | Days–1 wk | Thicker, flatter; good under heavy shingles, tile, cedar |
| Synthetic (polypropylene) | $15–$30 | 30–180 days (per brand) | Lighter, stronger, walkable, printed lap lines; today’s default |
| Self-adhered ice & water shield | $50–$100 | Varies | Required at eaves in cold climates, valleys, penetrations |
My standing recommendation: synthetic in the field, self-adhered membrane at eaves, valleys, and penetrations. A typical 20-square roof needs roughly $400 to $700 in underlayment materials — around 2 percent of the reroof cost — so this is not the layer to economize on. Whatever you choose, confirm it is compatible with your shingle manufacturer’s warranty and your local code; high-wind and high-heat regions (Florida, desert Southwest) often have specific requirements, and unvented or low-slope assemblies change the rules.
Slope decides the rules
Standard asphalt-shingle underlayment applies on roofs 4:12 and steeper: single layer, 2 inch horizontal laps, 4 inch (some codes 6 inch) end laps. Between 2:12 and 4:12, shingles are still permitted but underlayment doubles: 19 inch overlaps creating two-ply coverage, or a full self-adhered membrane. Below 2:12, shingles are off the table entirely — that is membrane roofing territory. Check your slope before buying anything, because a low-slope porch tying into the main roof is the most common place this rule gets missed, and the leak shows up at the transition.
Step 1: Prep the deck
Underlayment telegraphs and punctures over bad decking. Strip the roof to bare sheathing, then: renail or screw any loose or squeaky panels, replace delaminated or rotten OSB/plywood (probe around old leaks, chimneys, and eaves), pull or drive flush every old nail, and sweep the deck clean. Gaps between panels should be under 1/4 inch. Install drip edge along the eaves now — underlayment goes over the eave drip edge, while at the rakes, drip edge goes over the underlayment. That eave/rake distinction is a code detail (and a shed-water logic detail) that even experienced crews occasionally flip.
Step 2: Ice and water shield where it counts
In any climate with ice dams (roughly the northern half of the country and mountain regions), code requires a self-adhered ice barrier from the eave edge up the slope to at least 24 inches inside the exterior wall line — which on typical overhangs and pitches means two courses of 36 inch membrane. Peel-and-stick basics: roll it out and cut to length, tack the top edge, peel the release film progressively while pressing from the center outward, and roll or hand-press firmly, especially at laps (minimum 3.5 inch side laps, 6 inch end laps). Cold mornings hurt adhesion; most membranes want 40 to 45°F and rising. Even outside ice-dam country, I run self-adhered membrane in every valley (36 inches wide, centered), around chimneys and skylights, and along roof-wall intersections. Those four locations produce most roof leaks, and $150 of membrane there outperforms any amount of felt.
Step 3: Run the field courses
- Start at the eave, over the ice barrier, lapping the membrane by at least 2 inches (never butting to it). Roll the underlayment horizontally, parallel to the eave, drum-tight and square. Synthetics have printed lines that make straight courses and correct laps nearly automatic.
- Work up-slope, shingle-style. Every upper course laps over the one below by the printed lap line or 2 inches minimum (19 inches on low slope for double coverage). Water must always cross laps downhill onto the layer below — that single principle is the entire trade of dry-in. End (vertical) laps: 4 to 6 inches, staggered between courses so they never stack.
- Fasten per the label. Felt takes 3/8 inch head roofing nails or, better, plastic cap nails; synthetics require cap nails or cap staples — smooth staples void most synthetic warranties and tear out in wind. Typical pattern: one fastener every 8 to 12 inches along laps and two staggered rows in the field at 12 to 24 inches, tightened up substantially (often to 6 inch spacing) in high-wind zones or when the roof will sit exposed before shingles. Drive heads flush; an overdriven cap cuts the sheet, an underdriven one holds the shingle off the deck.
- Hips and ridges: wrap courses over the hip/ridge from each side, lapping 6 inches, so the peak is double-covered.
A two-person crew can dry-in a simple 20-square gable roof in half a day with synthetic. Do not let paper wrinkle: wrinkles telegraph through shingles as visible ripples, and every wrinkle is a place where the shingle bridges instead of bearing.
Step 4: Penetrations, valleys, and walls
Field paper is easy; the details are the roof. At plumbing vents, cut the underlayment tight around the pipe, then flash with the boot during shingling — many crews add a self-adhered patch lapped shingle-style around the pipe first, which I endorse. At sidewalls and chimneys, run the underlayment up the vertical surface 3 to 6 inches behind where the step flashing and housewrap or siding will lap it. In valleys, the field courses from each plane lap across the pre-installed valley membrane by at least 6 inches; never leave a valley relying on field laps alone. Skylights get the manufacturer’s flashing sequence, with underlayment and membrane integrated exactly per their diagram — improvised skylight flashing is a leak with a delay timer.
DIY or crew work?
Dry-in is the most DIY-accessible phase of roofing on a walkable single-story gable: the materials are cheap, the technique is laps and cap nails, and mistakes are still visible and fixable before shingles bury them. Where I draw the amateur line is pitch and complexity — anything above a comfortable 6:12 walk, cut-up roofs with multiple valleys and dormers, and any two-story eave work belong to people with staging and harness habits. A useful middle path if you are reroofing yourself: have a helper on the ground cutting and passing, dry-in one plane at a time so the house is never open overnight, and photograph every valley, penetration, and lap before shingling. Those photos settle warranty arguments and make the next storm-damage insurance claim ten minutes instead of ten letters.
Exposure limits and weather sense
Underlayment is a temporary weather barrier, not a roof. #15 felt buckles after one dew-heavy night and should be shingled within days; synthetics carry printed exposure ratings from 30 to 180 days, which cover real UV degradation limits, not an invitation to leave it all summer. If rain is coming mid-project, fasten laps tighter, seal any suspect areas, and understand that cap-nailed sheet goods resist ordinary rain but not ponding or driven storms. And basic safety, since this is the part of the job that hospitalizes people: harness and anchor above 6:12 or on anything wet, synthetics are slippery with dew even when rated walkable, and felt underfoot on a 45-degree morning deserves the same respect as ice.
Final judgment from the tear-off side of the business: when we strip a failed roof, the diagnosis is almost always written in the underlayment — backwards laps at a porch tie-in, no membrane in a valley, staples through synthetic, ice barrier stopping short of the wall line. None of those errors cost more than an hour to prevent. Buy synthetic, membrane the eaves and valleys, lap everything downhill, cap-nail on pattern, and get shingles on before the exposure clock runs out. Roof underlayment installation done to that standard disappears from your life for twenty-five years, which is exactly what the layer under the shingles is supposed to do.
