Which type of pipe is not permitted outside a building unless protected from frost?

The type of pipe that is not permitted outside a building unless protected from frost is the soil pipe. Soil pipes are designed to carry wastewater and sewage from bathrooms and kitchens to the sewer system. Because they transport materials that can freeze, it is crucial to protect them from cold temperatures to prevent blockages and potential damage. Frost can lead to solid waste freezing within the pipe, obstructing flow and creating significant maintenance challenges.

The other pipe types do not have the same critical vulnerabilities related to freezing when placed outside. Water pipes, for example, can also be susceptible to freezing; however, soil pipes are specifically noted for their need for protection outside due to their function and the nature of the waste they carry. Vent pipes, designed to equalize pressure in the plumbing system, and exhaust pipes, typically related to furnaces or similar appliances, do not retain liquids and thus are less affected by freezing conditions than soil pipes.