Sounds like a valve is not opened all the way somewhere.

Thank you. Yes, the shower valve isn't open at all, because it's in rough-in stage. Is the trapped air in the spliced supply pipes for the shower valve the cause of the reduced pressure in the sink faucet whose supply pipes were spliced into to supply the new shower valve?

Trapped air does not reduce pressure at all. There is a restriction in the line before it gets to your sink. The closed shower valve does not affect the pressure at the sink. One assumes that the person installing the new lines shut off the water in order to do so. It has been wisely suggested that the valve may not have been fully opened afterward. Finally, are you pulling our leg?

Okay, then there really may be reason for me to worry. I'm pretty sure the plumber didn't even go into the adjoining bathroom to turn off the valves under the sink, he just turned off the main water supply to the house when he was installing the rough-in shower valves. He spliced into the water supply lines going to the adjoining bathroom sink from within the opened wall on the opposite side of the wall board from the sink, cutting into the supply lines before they reached the shut-off valves for the sink sticking out of the dry wall on the other side.

I'm not pulling your leg in any way, I'm seriously confused about why cutting into a pipe in-line and splitting it using a T would affect the volume of available water for the faucet that was originally supplied. The onloy thing I could think of as a possible reason was lots of new pipe that are now attached to those lines but are currently ful of air, not water, because their new valve has never been turned on to purge the air from the pipes.

I'll look under the sink and see if the local shutoff valves were adjusted, but I'm pretty sure not since I watched the plumber walk into the water heater closet and turn off the main water supply valve for the whole house before cutting into the supply lines. Thank you for your answers, I'll point out my issue to my GC on Mon., because if it's not an air vacuum issue, there's something else wrong with the plumbing that's now behind a lot of hardibacker, awaiting tile, and I want if fixed before the tile goes up and before they have to rip into the not-being-remodeled bath on the other side of the wall to fix it.

Thanks,
Orchid

Assuming that you are correct and that there is air in the lines from the tee to the new valve, air is compressible, and the moment that the water was turned back on that air would have been forced forward in the lines until the air pressure is in equilibrium with the water pressure, but the pressure in the lines will still remain the same.

It is far more likely that during the change some form of sediment was loosened and it has now obstructed the aerator in the lavatory faucett causing a restriction in volume, which your are perceiving as a pressure loss.

Begin by removing and cleaning the aerator.

Assuming it isn't the aerator (although that might be it), what are your pipes made out of and what size are the supply lines? How old are your pipes? Is a permit pulled.

In my neck of the woods, there should have been an inspection before the hardibacker went up. That would have required full pressure (actually overpressure to 120 psi I believe) to the rough in valves also to look for leaks. While a leak is pretty rare, wouldn't you want to check before the valve and new plumbing was covered?

Your "plumber" is Ting off of a fixture supply line to supply this new shower rather than a main branch line supplying several of the local fixtures? I really think you need to find an actual licensed and insured plumber to take a look at this install. You may have more problems than you realize.

Thank you very much for all of the replies.

@ Lazypup, it was the aerator - white granular sediment granules (loose, but lots of them) was blocking the flow. after cleaning out, pressure/flow is perfect again.

@ David and @ Green, this is a permitted job and the GC is supposed to (by contract) be supplying licensed plumbers and electricians, is insured, and is a certified remodeler. I've covered myself as much as possible contractually and am paying top dollar in my area to help insure top workmanship - IMO, this is too big a job to risk skimping on quality by skimping on labor. I budgeted more than 2/3 for labor in this job, so I certainly hope I'm gettting the services of licensed, bonded professionals.

Thank you again for your help, all- it is really, truly appreciated.