Answer:
The answer is increase.
Step-by-step explanation:
What unions do is basically restrict the number of workers in an industry. They act as a labor cartel in order to increase the unionized workers´ wages. In this specific case, if roofers unionize, eventually there will be less roofers working in the economy. That means if there are less roofers, the ones that remain doing the job will get paid better. On the other hand it also means that there will be a larger supply of workers for other jobs.