Steep-Slope Installation: Underlayment, Shingle Layout & Metal Panels

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Steep-Slope Installation: Underlayment, Shingle Layout & Metal Panels. Don't let the plain title fool you. Here's the part that actually matters on the job: Steep-slope roofing is won on sequence and placement — every layer lapped so water runs over the next one down, shingles nailed on the line in a straight offset, and metal panels fixed at one point so they can move. Learn it well and your roofs look straight from the street and survive the first windstorm.

The intro course covered the code minimums. This lesson is how a crew lays the roof: from the deck to the ridge, in order, with the layout and nailing decisions that separate a clean roof from a callback.

The deck and the lap sequence at the eave

After tear-off, damaged sheathing is repaired. Then the eave is built in strict order. Drip edge goes on the eave first, directly on the deck. The ice barrier (where required) and the underlayment lap over the drip edge at the eave so water on the underlayment runs onto the metal and off. At the rake the order flips: underlayment first, then drip edge over it, so wind-driven water cannot get under the underlayment. Underlayment courses lap at least 2 inches horizontally and 4 inches at end laps and are fastened with cap nails. On slopes from 2:12 up to 4:12 the underlayment is doubled, each course lapping the one below by about half its width.

Layout and the starter course

Measure from the eave at both ends and snap a line for the starter course so it runs parallel to the eave even when the fascia does not; then snap a vertical line square to it for the offset pattern. The starter strip goes on with its adhesive at the eave, overhanging the drip edge by about 1/4 to 3/4 inch, and most manufacturer details run a starter up the rakes too. Each course is offset by the amount the manufacturer specifies, commonly around 6 inches for laminated shingles, and the pattern is held with snapped lines every few courses rather than by eye. Racking laminated shingles straight up the roof is not recommended because it leaves nails out of the common bond line.

Nailing decides the wind rating

Every shingle has a nail line, and the wind class on the wrapper assumes the nails are on it: flush, straight, never cutting the mat, and at least 1 inch from the edges. Four nails per shingle is standard; six nails are required in high-wind areas and by many enhanced warranties. A high-nailed shingle, above the line and missing the shingle below, is the most common cause of a blow-off that passed the walk-through. The nail must penetrate 3/4 inch into the deck or through it. In cold weather the sealant strip may not activate for weeks, so rakes and eaves get hand-sealed with dabs of asphalt roof cement, never a continuous bead that traps water.

Valleys, hips, ridges, and ventilation

An open valley uses W-profile metal at least 24 inches wide over a 36-inch valley underlayment, with shingle corners clipped so water does not catch. A closed-cut valley runs the first plane through and cuts the second back about 2 inches from the centerline, with no nails within 6 inches of the center. A ridge vent only works with open soffit intake; the IRC net free vent area of 1/150 of the attic floor, or 1/300 with balanced intake and exhaust, is the target, and a ridge vent over blocked soffits pulls conditioned air out of the house instead of attic air.

Standing-seam panels and thermal movement

Metal moves. A 100-foot steel panel grows and shrinks roughly three-quarters of an inch across a 100-degree swing, and aluminum moves about twice as much. A standing-seam panel is therefore fixed at one point, usually the eave or the ridge, and carried on floating clips everywhere else. Pinning both ends tears the clips or buckles the panel. Panels go over a high-temperature self-adhering underlayment on a flat deck, because every deck irregularity telegraphs as oil canning, which is not a defect but is reduced with heavier gauge, striations, and careful substrate work. Layout keeps panels square to the eave on a measured module so the last panel is not a sliver; eave and ridge get foam closures, and the seam gets sealant where the slope is low. Exposed-fastener panels use gasketed screws driven straight to compress the washer without crushing it, on the flat or the crown as the manufacturer specifies.

Tile and slate in brief

Tile and slate rely on the underlayment for much of the waterproofing. Tile goes on battens that let water drain beneath it, with clips or foam adhesive in high-wind regions, following the Tile Roofing Industry Alliance installation manual. Slate hangs on copper or stainless nails driven to leave the slate hanging, not pinned, with a headlap of about 3 to 4 inches depending on slope.

Going Deeper (Intermediate)

Steep-slope quality shows up at transitions: where a roof meets a wall and where slopes change. Those are where underlayment runs up the wall before the siding goes on, where step flashing starts, and where a kickout is set, and they depend on sequence with other trades. A roofer who arrives after the siding is complete cannot build a correct sidewall flashing. Ask for the roof before the siding, or the wall gets opened.

Advanced / Pro-Level

Pros run production without losing the layout: one person snapping lines ahead of the nailers, nailer pressure rechecked when the compressor cycles or the deck changes, bundles staged over bearing points and never left loose on a steep slope overnight. They know the manufacturer's enhanced warranty typically requires a complete system from that manufacturer — starter, underlayment, ice barrier, hip and ridge, ventilation — installed by a certified crew, and they photograph the nailing before the next course covers it.

Practice Challenge

A new crew lays laminated shingles on a 6:12 roof in January. The shingles get four nails about 1 inch above the nail line to speed production, the courses are run by eye, and the rake edges are left to self-seal. A 60 mph gust comes through in March. What happens, and why? (Answer: high nailing left each shingle holding only its own course and missed the one below, the rakes never sealed in the cold, and the unsnapped layout left the offsets uneven; the wind lifts the rake edges and strips courses, and the manufacturer declines the claim because the nails were off the line and below its high-wind count.)

In Practice

A crew that builds the eave in order, snaps lines, nails on the line, and fixes the metal panel at one point hands over a roof that looks straight and stays on. A crew that eyeballs the layout and pins both ends of the panel gets a wavy roof and loses shingles off the rake in March.

Common Mistakes to Avoid

• Drip edge over the underlayment at the eave, or under it at the rake
• High-nailed or angled shingle nails, or four nails where the manufacturer requires six
• Courses run by eye without snapped lines, or laminated shingles racked straight up
• Standing-seam panels pinned at both ends, or installed over an uneven deck
• Ridge vent installed without open soffit intake

Takeaway: Steep-slope roofing is won on sequence and placement — every layer lapped so water runs over the next one down, shingles nailed on the line in a straight offset, and metal panels fixed at one point so they can move.

Educational content only — not roof design, engineering, legal, or safety advice, and not a substitute for hands-on training under a licensed roofing contractor or the judgment of a licensed design professional. Follow the IRC, IBC, and plumbing code as adopted and amended in your jurisdiction, OSHA 29 CFR 1926, the manufacturer's installation instructions and warranty requirements, and your company's procedures.