Once the decking is confirmed stable, underlayment is rolled across the surface to establish a critical secondary moisture barrier. Most modern installations favor high-temperature synthetic underlayments that resist tearing and handle wide temperature ranges—particularly important underneath darker metal roofs. In regions prone to ice damming or heavy downpours, a self-adhering membrane (sometimes called ice-and-water shield) may be layered along eaves or valleys, creating a watertight seal that contours around fasteners. Installers overlap underlayment courses per the manufacturer’s guidelines—commonly by 4 to 6 inches—and secure them using plastic-capped nails or pneumatic cap staplers. This strategy reduces the chance of tears that can compromise moisture defense. Care is taken around roof penetrations or corners: butyl tape or additional adhesive strips may help seal transitions
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Professional crews track the environment during underlayment placement, avoiding windy conditions that can cause wrinkles or misalignment in the sheets. Horizontal chalk lines often guide consistent overlap, ensuring each row tracks evenly from eave to ridge. Some contractors also incorporate venting accessories or breathable underlayment in particular scenarios where moisture might accumulate under the metal panels. In high-heat climates, specialized synthetic membranes tolerate surface temperatures that can exceed 200°F under a metal roof, preventing bonding or melting. If standing seam panels are planned, slip-resistant or high-temp underlayment is critical to allow panels to expand and contract freely, rather than stick. Certain premium membranes include a protective top film that reduces scuffing during panel installation—helpful for preserving the metal’s underside finish.

Chimneys, skylights, pipes, and other roof penetrations demand tailored metal flashing or specialized boots. Aluminum or steel flashing aprons direct water around rectangular objects like chimneys or dormers, preventing leaks where different planes meet. Vent pipes often use rubberized pipe boots sized to the pipe diameter, secured with ring-shank screws and sealed with adhesives that adhere to metal surfaces. Crews carefully integrate these boots under the uphill course of metal while overlapping on the downhill side, forcing water to flow outward onto the roof surface rather than beneath it. Skylights receive step flashing pieces or kits recommended by the skylight manufacturer, combining with the metal roofing layout to guide runoff around the opening.
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Installers commonly rely on butyl tape or high-grade polyurethane sealants tested for metal roofing applications, forming a tenacious bond that endures temperature swings without cracking. Wider expansions, such as along chimneys, can require an upper saddle or cricket formed from matching metal, diverting water around the protrusion. Soldered flashing is occasionally used in premium materials like copper, ensuring a permanent seal. For complicated transitions, on-site fabrication with a portable brake or press helps create custom flashing profiles that precisely nest with panel ribs. Proper alignment keeps edges well-lapped, and the top side of the flashing tucks under an upslope panel, stopping water infiltration. Any fasteners in critical areas are sealed meticulously, and installer checks confirm no gap or misalignment can allow future seepage.