Residential Water Leak Troubleshooting & Repair Solution Generator

You are a licensed master plumber with 15+ years of residential service experience, expert in Texas plumbing code and leak detection techniques.My Business:
Austin Plumbing Pros, licensed in Austin, Texas.Client Details:  Property type: Single-family home  
Year built: 1998  
Plumbing system type: Copper with PEX repipe in 2015  
Reported leak issue: "water stain on living room ceiling under upstairs master bathroom, hot water bill has doubled in the last 2 months, no visible drips in the bathroom"  
Recent service history: "water heater replaced in 2022, no other recent repairs"

Your Task:
Generate a complete, step-by-step leak troubleshooting and repair workflow. The workflow must include:Pre-diagnostic safety & prep steps
(water shutoff protocol, PPE requirements, client prep).
4-6 most likely root causes of the leak, listed in order of probability, with specific details for the property type and system age.
For each root cause:
A sequential list of non-invasive testing steps first, then invasive testing, with tools required and clear pass/fail criteria.
For each confirmed root cause:
2 repair options:  Temporary emergency fix  
Permanent code-compliant repair
Include parts list and labor estimate for each.

Water damage mitigation steps to prevent mold and further damage.
Preventive maintenance tips to avoid future leaks.

Requirements:
All content must be compliant with local plumbing code (Texas/Austin), practical for field use, and tailored to the specific leak issue: water stain on living room ceiling under upstairs master bathroom, doubled hot water bill over 2 months, no visible drips in the bathroom.

Residential Water Leak Troubleshooting Workflow1. Pre-Diagnostic Safety & Prep StepsConfirm location of main water shutoff valve and hot water shutoff; review with client.
PPE Required: Nitrile gloves, safety glasses, moisture meter, leak detector.
Client Prep: Clear access to bathroom plumbing, ceiling access panel, and water heater.

2. Most Likely Root Causes (In Order)2.1 Leaking Shower Pan / Shower Drain Assembly
(55% probability for bathroom ceiling leak)Non-Invasive Testing Steps:Perform flood test on shower pan: plug drain, fill shower with 1" water, mark level, monitor for 30 minutes.
Use moisture meter to check ceiling directly under shower drain vs. supply lines.
Run shower water for 10 minutes, monitor moisture meter readings for changes.

Invasive Testing:
Remove shower drain cover, inspect drain gasket and pan liner for cracks.Permanent Repair:
Remove and replace shower pan liner and drain assembly, re-waterproof shower base.
Labor Estimate: 4 hours, $480–$650.
Parts: PVC drain assembly, shower pan liner, waterproofing membrane.