Foam & flat roofing

Spray polyurethane foam is applied liquid, expands on contact, and cures into a single seamless membrane that follows the exact shape of the deck, no laps, no seams, no fastener penetrations for water to find. That's the part that makes it a genuinely good system for a flat or low-slope roof, where standing water has nowhere to run off toward.

The foam itself isn't left exposed. It gets topped with an elastomeric coating that handles UV exposure and reflects heat. The coating is the sacrificial layer. It's built to wear down and get renewed. The foam underneath, protected, is the part that can last indefinitely.

The coating chalks, then hairline-cracks, then goes. Once it's gone, the foam underneath is exposed to direct UV for the first time, and it starts breaking down on its own. Going brittle and friable instead of the dense, sealed surface it's supposed to be. Water finds pinholes, and because the whole membrane is monolithic, it can travel under the surface and pool between the foam and the deck completely out of sight from above.

By the time that shows up as a stain on your ceiling, it's usually already spread well past the spot where the water actually got in. At that point you're not recoating. You're grinding the failed foam off and starting the system over, which costs a great deal more than staying on the recoat schedule ever would have.

Very few Valley houses are fully flat-roofed. Foam usually shows up on additions, covered patios, and low-slope tie-ins bridging into a tile or shingle main roof. Sections that are hard to see from the street and easy to forget exist. Those same low areas are also where water from the pitched roof above tends to drain toward, so they take more punishment than the main roof while getting looked at less often.

When we inspect a house with a mixed roof, we check the flat sections on their own schedule, separately from the primary material, because "the roof looks fine" almost always means someone looked at the tile or shingle and never walked the flat part at all.