Final answer:
To make the joints around vent pipes watertight, the traditional and effective method is the use of lead flashing, which provides a durable seal due to its flexibility and conformity to the joint's shape.
Step-by-step explanation:
The question asked is related to how to ensure that joints around vent pipes on a roof are made watertight. To achieve a watertight seal around these critical points, where vent pipes penetrate the roof, the use of lead flashing is a common and effective practice. Lead flashing involves preparing the joint and installing a sheet of lead into the joint to prevent water ingress. The flexibility of the lead allows it to conform to the shapes of both the pipe and the roof, making a watertight seal. While silicone sealants, rubber gaskets, and cement mortar may also be used in certain roofing applications, the traditional and durable method involves the application of lead flashing which, when installed properly by an experienced roofer, provides a long-term solution against water leaks.