What is used on a rooftop to ensure a watertight connection for a package unit?

A roof curb is specifically designed for mounting rooftop HVAC equipment, such as package units, ensuring a watertight connection to the roof surface. The curb elevates the unit above the roof, which allows for proper drainage of rain and other moisture, preventing water from pooling around the base of the unit.

Additionally, the design of a roof curb often includes a flange that is built to fit snugly against the roofing material. This feature not only helps in achieving a secure fit but also facilitates the proper installation of waterproofing materials, further enhancing the barrier against leaks.

The other choices generally do not provide the same level of watertight integrity. A roof jack is typically used for venting purposes rather than for securing units and managing rainwater. A flat section of the roof, while functional for support, does not inherently provide the watertight quality necessary to protect the internal components of HVAC systems. Lastly, silicon treated 4 X 6 timbers might be usable in certain construction contexts but do not serve the specialized purpose of mounting HVAC equipment as effectively as a roof curb.