While the environment for union laborers has been improving lately, the work has been changing—and the field isn’t without its challenges.

During a conversation with Patch, State Rep. Marty Walsh said that one of the major challenges facing Boston’s building trades these days is keeping a reliable source of labor within the city.

Walsh, who also serves as the Secretary Treasurer of the Building & Construction Trades Council of the Metropolitan District, said that union workers have a tendency to leave the city as they age, earn a savings account and start a family. They remain in the trades, he said, they just drift outside of Boston’s borders.

That may be a positive development for the worker, but it creates a challenge for city projects that aim to meet a quota of sourcing half of their workers from within the city.

To fill that gap, the Boston Housing Authority and the Building Trades council have partnered for a program called “Building Pathways.” The seven-week course, according to material provided online, prepares Boston residents from certain economic strata for apprenticeship programs in the building and construction trades. Walsh said the program targets mainly residents of Dorchester, Mattapan and Roxbury, but its scope isn’t limited to those neighborhoods.

How the work is changing.

While Walsh and his council cope with shifting demographics, union workers, he said, are dealing with shifting work patterns.

Buildings today are constructed faster, Walsh said, and built with different techniques.

As an example, he pointed to the Vertex Pharmaceuticals building currently under construction in South Boston. The building’s designers clustered all of its infrastructure—air ducts, wiring, etc.—at its core. In the past, Walsh said, those ducts and pipes would be spread out, and the new design requires laborers to work in tighter spaces.

And they have to work faster. Ten years ago, Walsh said, construction on the 1 million square foot building would last three, maybe four years.

In today’s environment, Walsh said, union crews are expected to complete the project in just 24 months.