Also known as plumbers crack. A very common problem for guys in your line of work. Doesn't really matter what kind of underwear you have on it's still likely to happen. I am well acquainted with this issue since I have several friends and family members who are in the skilled trades. plumbing, electrician, carpet/tile installers ect...
A good belt might help for some but, the only fool proof way to prevent this issue is to wear a pair of suspenders.

Most simply ignore it as part of the job.