after dealing with substandard work in my bathroom remodel. Don't pay anymore money, be VERY direct, specific and upfront about how you expect this to get resolved.
They will either do the right thing and fix it, or they will walk.
If you have already paid, you feel kinda stuck if they walk. If you haven't paid you have leverage. A lot of this stuff is dependent on the structure of your contract and how far you are already in financially. It's a strange spot for sure, but you need to do your best job to convince this person they need to fix it.
Using terms like 'implied warranty' and 'professional workmanlike manner' should give some hints about certain codes/laws the project must follow...if it gets to that in the conversation.
good luck
EDIT: might also want to make them aware how serious of an issue this is by a certified letter. State your dissatisfaction, use photos, be as detailed as you need to be. Talk is cheap, but starting this process with something in writing (which might actually be part of your contract anyway) might be a better strategy. Doesn't need to come from a lawyer, but will let your builder know you are serious.
When my remodel quality started to show signs of poor workmanship, I brought it to their attention. They demanded to be paid the next draw on the schedule before proceeding with repairs and I refused. They terminated their contract and then put a mechanics lien on my home - for the FULL amount of the project (after leaving it in disarray and in complete). Fast Forward to just last week, the statute of limitations in which to file suit on the lien expired...so they never pursued. I would like to take them to small claims, to recover some monies I already paid for some of the work, but I just don't know if that is worth it. Trying to talk to an attorney is becoming difficult.