Spray polyurethane foam (SPF) roofing is a popular choice because of its seamless, insulating, and waterproof qualities. However, when it comes to underlayment, SPF roofs differ from traditional roofing systems like asphalt shingles, which typically require a specific underlayment layer. SPF itself acts as a self-adhering, fully adhered roofing membrane that provides both insulation and waterproofing, so it generally does not need a separate underlayment. Instead, the SPF is applied directly to a clean, dry, and structurally sound substrate, such as metal, plywood, or concrete. Problems such as powdering, cracking, or shrinkage usually indicate substrate issues or improper installation, so carefully inspecting the surface before application is a key DIY step. You can gently probe for softness or moisture damage on the roof deck, but avoid any action that risks injury or further damage.
Even though SPF eliminates the traditional underlayment, it’s important to know that any warranties or building codes might require specific prep or primers before applying the foam. If you notice peeling, bubbling, or damaged foam, or if you are unsure about the condition of your roof substrate, it’s best to call in a roofing professional who specializes in SPF systems. When consulting a contractor, ask if they follow manufacturer specifications for substrate preparation and SPF thickness, and whether their work carries a manufacturer-backed warranty. These questions ensure that your SPF roof will perform well over time and meet warranty rules, protecting your investment from costly repairs later.