Roofing Underlayment Methods: Match the Installation to Your Roof Shape

The installation method changes with the roof, not just the material. A shallow roof needs double coverage. A steep roof needs back nailing and different handling. Valleys, ridges, and eaves each get their own treatment. Start with the slope band your roof falls into, then follow the matching method.

Which underlayment method does your roof slope need?

Match the slope first, then the method. These bands come from manufacturer install guides and the military roofing design manual that codifies the same thresholds.

How do you lay underlayment on a standard-slope roof?

Work horizontally from the eave toward the ridge so each upper course laps over the lower one. Unroll with the printed side up, keep the sheet flat without wrinkles, and nail it down as you go.

With a synthetic membrane, overlap horizontal laps 4 inches and end laps 12 inches, and stagger end laps at least 6 feet from the end laps in the course below. Fasten with corrosion-resistant cap nails that have at least 1-inch-diameter heads. At the ridge, fold the membrane over both sides with at least 6 inches of overlap.

Course count is simple arithmetic once the lap is fixed. A 36-inch-wide roll with 4-inch laps covers 32 inches of fresh deck per course. A 12-foot eave-to-ridge run needs ceil(144 / 32) = 5 courses. Verify this before ordering: an extra course at the ridge costs less than a stretch of deck left uncovered.

How does the method change on a low-slope roof?

Water moves slowly below 4:12, so a single layer is not enough. Install double coverage: lay a starter course along the eaves, then full courses that each overlap the previous one by roughly half their width.

One manufacturer method for synthetic starts with a full 25.5-inch starter strip along the eave, followed by 48-inch sheets laid so each covers 22.5 inches of new deck (25.5 inches of overlap). End laps overlap 12 inches and stay at least 36 inches away from end laps in neighboring courses. For a 16-foot run: ceil((192 - 25.5) / 22.5) = 8 courses after the starter, so 9 runs total. Double-check the last course still lands on the deck with full overlap.

What changes on a steep roof?

Gravity becomes the main problem. On steep slopes, back nail self-adhering membranes so the sheets do not creep before the roofing goes on. Unroll downhill-side down, smooth from the center outward, and never walk the membrane in a way that wrinkles it.

Steep work also changes the tools: a soft push broom or a lighter roller is enough on steep slopes, where a heavy weighted roller is the low-slope choice. Synthetic membranes give better footing than smooth felt, which is one reason they are common on steep residential roofs. Keep the exposure time within the product's UV rating; most synthetics are rated for weeks or months, not a full season.

How do you handle valleys, hips, and ridges?

Valleys carry the most water per square foot on the roof, so they get the strongest membrane: a self-adhering waterproof layer goes down in the valley before the field courses arrive. Overlap the valley membrane at least 6 inches or run it straight through the valley, and keep every fastener at least 12 inches away from the valley center so nails do not create leak paths where water concentrates.

Hips and ridges get a 12-inch overlap where the courses from each side meet. If a ridge vent goes in, follow the vent manufacturer's instructions for how the underlayment terminates around it.

How is underlayment handled under a metal roof?

Metal moves with temperature, so the underlayment under it must not bond to the panels in a way that tears or squeaks. Use a separation layer or a synthetic underlayment rated for metal roofing, and follow the panel manufacturer's instructions exactly: some allow direct-to-deck installation with an approved product, others require a slip sheet between membrane and panel.

Self-adhering membrane is common under metal in ice-prone and wind-prone areas because it seals around fasteners. Roll or broom it fully, pay attention to side and end laps, and never install it on a wet, dusty, or frozen deck.

How do you treat eaves, ice-dam regions, and penetrations?

In regions where ice forms at the eaves, add an ice barrier before the field courses: self-adhering membrane running from the lowest roof edge to at least 24 inches inside the exterior wall line. Measure from the wall line, not the drip edge; on a wide overhang the barrier runs much farther than 24 inches from the edge.

Set the drip edge under the underlayment at the eaves and over the underlayment at the rake edges. This puts the metal where it sheds water instead of trapping it. Around vents, stacks, and skylights, integrate the flashing boot with the underlayment in the same shingle fashion: underlayment below the flange on the downhill side, over it on the uphill side, sealed per the flashing instructions.

Which method mistakes cause the most leaks?

Reversed laps cause the most damage for the least effort: a course lapped uphill directs water under the membrane instead of over it. Check every course before the next one covers it.

Staples are the second classic failure. Some synthetic manufacturers forbid staples entirely because the membrane tears around them in wind; use the specified cap nails. Installing on a wet or dusty deck is the third: self-adhering membrane loses adhesion and mechanical courses trap moisture. Finally, misaligned courses compound: if one course goes crooked, start that run over instead of following it uphill into wrinkles and fishmouths.

Safety and professional scope

Roof work carries fall risk, and steep-slope work is not a beginner task. Use fall protection, stage materials so nothing slides, and stop when the deck is wet, icy, or wind-blown. Local code editions and manufacturer instructions control the final method; when they conflict, the stricter requirement wins.