Most of what has been posted is from fairly high in the chain of command, so I’ll give a “Grunt in the Trenches” perspective. As others have said, a good General Contractor makes a job go smoothly, and heads off problems before they become problems onsite. However, no Contractor can anticipate EVERY potential problem, so they do crop up. One common problem that was mentioned, is that sometimes trade-specific drawings are often done by different people, who don’t always refer to each others’ drawings. That’s when you get electrical, plumbing, and ductwork (my part) running through the same space, and you get conflict on the jobsite. A couple of examples come to mind: Once, I worked on a small one-story bungalow that was being upgraded with a second floor and a finished attic living space. The furnace was in the basement, with a supply riser (vertical main ductwork) running up through a “chase” (enclosed passageway) to the attic, where the ductwork branched out to supply the upper rooms. (the lower rooms were served by ductwork in the basement ceiling) The attic ductwork design was pretty much the same as it had been in the original attic, and so that was straightforward, but the Architect, in designing the second floor Master Suite, had not allowed for the chase, but had instead located the Master vanity in that space. This meant that if we all had built “per design”, the furnace would have blown air up into the riser, which would have abruptly ended at ceiling height on the first floor. The top end of the riser, in the attic, would have simply been a metal box with duct branching off of it, but no actual air being supplied TO it. The result would have been that the first floor would have gotten great airflow for heating and cooling, but the entire duct system for the second floor and attic would have been COMPLETELY non-functional. At this point, we had to approach the Super. and show him the drawings, he had to take it to the General Contractor, who in turn had to show the drawings to the Architect who had originally done the drawings. Of course the fault wasn’t the Architects, (How could it have been? He was only the guy who designed and drew the second floor without a chase; it’s not HIS fault that what he drew made the HVAC system physically impossible! 
) The Architect had to change the plans, leaving to the Contractor or even the Super the job of explaining to the homeowner why he couldn’t have the fancy his/hers vanity he had been promised.
On another job, a commercial building, we were running ductwork in the area above what was to be a dropped ceiling. The first problem we ran into, was that we had been scheduled AFTER the guys who put in the ceiling grid. (the white metal framework you see holding up the white ceiling tiles in many commercial and public buildings) This meant that we couldn’t use scaffolding or regular construction lifts, because everything we brought up had to fit through 2’x4’ openings. We had to use a “catpole” lift, which is large enough to hold only one man, and which has to be lowered to the floor everytime you want to move it to the next opening. Had we gotten there BEFORE the ceiling guys, we could have used a large, rolling motor-driven lift which would have allowed us to bring up two or three guys and large runs of duct, and roll around as needed to reach each hanger or connection. This meant more “manhours” on our part, which got the Contractor on our case, and we (or rather, our boss) had to show that we were forced into the longer manhours by poor scheduling. The electricians and plumbers had to do pretty much the same thing. Another problem we, the electricians, and plumbers, ran into, was that we’d find that out various fixtures didn’t seem to end up in the right places. We checked and double-checked our measurements, and we couldn’t find where we’d gone wrong. We all had to adapt, and do what was necessary to put the fixtures where they needed ot be, which meant that sometimes, we got in each others’ way. However, since we were all experiencing the same problems, we quickly learned to work together to try to avoid causing each other problems. Again, more manhours for all of us. Finally once we had all gotten all our ductwork, wiring, pipe, and fixtures in place we found out what the problem was: The actual building was two feet longer than the drawings, and when the carpenters built the room walls, they started from one end of the building, only finding that things were off when they got to the other end, while we Ductmen, Plumbers, and Electricians started at the end where power and water came into the building. This mix-up was likely the fault of the Surveyors, but on a concrete ‘tilt-up’ building, once it’s up, it’s up, and it’s what we have to work with. This is why, as someone mentioned earlier, there’s a difference between “as drawn” and “as built”. Between the person who draws the plan, and the person who swings the hammer, there are many layers of potential error, and so each level has to be able to adapt to the unexpected, and communication both between levels and between trades can make a HUGE difference in how smoothly a job goes. Hats off to everybody in the chain of command, from Architect to Carpenter’s Bi…um, helper!
