hiring "under the table" labor

[Tim Robinson]

i am new to real estate investing, I actually have not even done a deal yet. This is something that I have been pondering for some time now.  I often see people do jobs and get paid "under the table". To be honest, I myself, when I was in my 20's did jobs like this. Put on a new roof, maybe a vinyl siding job, a flooring job, or some landscaping.  I feel like our society accepts a lot of under the table jobs like its no big deal. I mean you pay the neighbor kid 30 bucks to mow your yard. No big deal, right? Lets say you got a permit to put on a new roof, instead of doing it yourself, you pay a couple coworkers 400 bucks each to help you out. No big deal, right? My question is this... Is it a big deal? Is there a line drawn between legal and illegal? what sort of trouble could this land you in?  And just to be clear, its not like I would pay some yahoo to put on a roof, but I do work in an industry where I literally know dozens of guys that are knowledgeable on how to do work like this.  And, I guess another question would be, what is the legal way for one person to hire another person to do labor?

Thanks