Supply Line vs Drain Line vs Roof Source Decision Tree

Why this matters

Source determination drives category, category drives scope, scope drives every line on the estimate. A roof loss misread as a supply-line break gets dried in place when it should be tarped, opened, and dehumidified through the attic plane. A drain-line backup misread as a clean supply break gets dried without antimicrobial and re-occupied while pathogen exposure is still rising. Per IICRC S500 Section 10, source classification is the first action on site, not a paperwork step done later. Get the first 20 minutes right and the next 5 days run themselves.

Symptom presentation on intake

Three signals from the homeowner narrow source before you arrive:

• "Water was coming down the wall" with no rain in the last 24 hours: supply-line, water-heater, or appliance-fill. Pressure-fed.
• "It only happens when we flush, run dishwasher, or take a shower": drain-line. DWV failure, branch or main.
• "It started after the storm" or "always after heavy rain": building envelope. Roof, flashing, window, foundation.

Ask three questions on the call: time of onset relative to last rainfall, whether shutting the main stopped flow within 10 minutes, and what was running when it started. A "no" to the main-shutoff stop is roof or drain by elimination.

Quick checks at the door

Before opening walls, walk the path. Confirm what the homeowner reported and stage the moisture map.

• Read the water meter with all fixtures off. A spinning low-flow indicator with no fixture demand is a supply leak somewhere downstream of the meter.
• Smell. Clean-water supply is odorless. Sulfur or sewer odor at the affected area is a Category 3 drain source until proven otherwise.
• Look up. A ceiling stain with a defined edge directly under a bathroom is suspicious for supply or drain from above. A stain that widens upward toward an exterior wall or eave is envelope.
• Temperature touch. Hot supply leaks present warm above the affected ceiling for hours. Cold supply and drain present at ambient. Roof and envelope present cold during winter intrusion.

Isolation tree

Step 1: shut the home's main supply valve. Wait 10 minutes. Re-read the meter.

• Meter stops AND active flow at the affected area stops: supply-line branch. Go to Step 2A.
• Meter stops but flow continues at the affected area: water already in the cavity is finishing its path. Likely supply but possibly a tank-style appliance (water heater, dishwasher reservoir) that holds residual. Recheck in 30 minutes.
• Meter does not stop: meter is sticking, irrigation is bleeding, or there is no supply leak. Move to Step 2B.

Step 2A (supply confirmed):

• Isolate by fixture stops. Close the dishwasher, washer, ice-maker, and toilet stops. Re-read meter. The stop that kills the flow names the appliance. If meter still spins, the leak is in fixed copper, PEX, or a manifold.
• Hot vs cold: touch the visible copper runs at the panel or manifold. The warm leg under load points to hot-side; the cold leg confirms cold-side.

Step 2B (no supply involvement):

• Run a single drain fixture. Stand at the affected area. If intrusion resumes during or 60-180 seconds after running that fixture, the drain or vent branch serving that fixture is the source. Test one fixture at a time; do not run multiple in parallel.
• If no fixture drain reproduces, isolate by location. Roof, window, foundation, slab. See Step 3.

Step 3 (envelope and slab):

• Check the last 72-hour weather. No rain and no fixture reproduction: slab or service-entry leak. Thermal-image the slab perimeter for cold lines.
• Recent rain with intrusion at exterior walls only: window, flashing, or siding. Inspect from outside; spray-test top-down per ASTM E1105 protocol if accessible.
• Intrusion at ceiling under a roof plane with active or recent rain: roof source. Inspect attic plane for the rundown path - water rarely drops straight through. Trace uphill from the stain.

Confirming diagnosis

Three confirmation tests before scope is locked:

• Moisture mapping with a calibrated pin-meter or capacitance meter across the affected area and 18 to 24 inches beyond visible damage. Document highest reading, location, and material per S500 documentation requirements.
• Thermal imaging across the suspect plane during a temperature differential of at least 10 degrees F. Cold blue against warm field on a winter loss is intrusion; do not confuse with a stud cavity drying out from prior loss.
• Pressure test on supply (60-80 psi static hold for 15 minutes) or vacuum-air test on DWV if the leak is presumed pipe-internal and not visible.

Category assignment is then driven by S500 Section 10.5:

• Source water from a sanitary supply pipe: Category 1.
• Roof, window, foundation rainwater intrusion through clean substrate: Category 1 if not contacting contaminated surfaces and remediated within 24-72 hours.
• Dishwasher discharge, washing machine drain, aquarium: Category 2.
• Toilet flange leaks with urine, sewer backups, ground surface intrusion, all DWV mains: Category 3.

Remediation by source

• Supply line: stop the source at the fixture stop or main, document, dry per S500 Class 1-4, and hand off plumbing repair to a licensed plumber. Antimicrobial is optional on Category 1 within the 24-hour window.
• Drain line (Category 3): containment per S500 Section 12, source-removal of saturated porous materials (drywall, insulation, carpet pad, particleboard cabinet boxes), HEPA air filtration, EPA-registered antimicrobial, full PPE per OSHA 29 CFR 1910.134. Do not attempt dry-in-place on porous materials below the flood line.
• Roof/envelope: emergency tarp or temporary patch first, then dry. Do not start dehumidification until the source is closed - you will run equipment indefinitely against an open intake. Document with photos before tarp and after.

References

• IICRC S500 Standard for Professional Water Damage Restoration, Sections 10.5, 10.5.5, 12 (2021 edition)
• ANSI/IICRC S500-2021, Reference Guide Chapters on source identification and category determination
• OSHA 29 CFR 1910.134, Respiratory Protection (Category 3 PPE requirements)
• ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows
• EPA Mold Remediation in Schools and Commercial Buildings (EPA 402-K-01-001), source-control principles