A mounting system for a roof-mounted structure comprises: a roof having a surface; a shaft having a first end opposing a second end, and including a roof fastening portion at the first end and secured within the roof, an anchor portion for securing the roof-mounted structure at the second end, a drive part and a radially projecting shoulder surface oriented toward the roof fastening portion; and a washer positioned on the surface of the roof and disposed on the shaft such that the shoulder surface urges the washer into watertight sealing contact with the surface of the roof.