Multi-Unit Source Attribution: Supply vs Roof vs Grade Decision Tree

Why this matters

In multi-unit buildings, the difference between a supply-line failure, a roof breach, and grade-driven hydrostatic intrusion decides who owns the loss, what category you assign, and which carrier or association subrogates against whom. Misattribution costs you twice: a wrongly-categorized loss invites coverage denial, and a wrongly-attributed source invites a subrogation reversal months later when an engineer reads your moisture map. The water travels far from its origin in stacked construction, so the wet drywall in unit 2B may originate three floors up or from the foundation footing. Your job on day one is to build a defensible attribution from physical evidence: moisture gradient direction, chloride and conductivity readings, and the architecture of the wet path. Get this wrong and you have dried the symptom while the source keeps running.

Symptom presentation

Three patterns dominate. A supply or domestic-water failure presents as warm or ambient clean water, often under pressure, with a wet path that radiates from a chase, riser, or in-wall line and frequently spans multiple floors vertically through a common plumbing stack. A roof or building-envelope breach presents as top-down staining tied to rain events, with the highest moisture content at the ceiling plane of the topmost affected unit and a tapering gradient downward. Grade-driven intrusion presents at or below the lowest occupied level: wet base plates, efflorescence on foundation walls, rising-damp gradients in slab-on-grade, and moisture that correlates with storm or snowmelt rather than fixture use.

Quick checks

Run these before you commit to a source.

• Map moisture content vertically. Pin and pinless readings at the ceiling, mid-wall, and base plate of each affected unit reveal gradient direction. Highest reading at the top points to roof or upper-floor origin; highest at the base points to grade or a lower-floor line.
• Check water temperature at the wettest extraction point. Pressurized hot or warm clean water at a riser indicates domestic supply.
• Correlate timing. Pull the resident report and the local rain log. Intrusion that tracks rainfall is envelope or grade; intrusion that tracks occupancy or fixture use is supply or drain.
• Test conductivity and chlorides on standing water. Municipal supply carries a chlorine and chloride signature; rainwater is low-conductivity; grade water often carries soil minerals and elevated conductivity from ground contact.
• Inspect the common stack. In stacked construction, open the chase access in the lowest wet unit and look upward for active drip or staining.

Isolation tree

Start at the wettest documented point and work toward the source.

1. Is moisture highest at the ceiling of the top affected unit? Yes: suspect roof or upper-envelope. Confirm by inspecting roof penetrations, flashing, and the deck above. No: continue.
2. Is the wet path vertical through a plumbing chase across multiple floors with warm or pressurized clean water? Yes: suspect domestic supply. Isolate by shutting the building or riser valve and re-reading pressure or active flow after 30 minutes. No: continue.
3. Is moisture concentrated at base plates, slab edges, or below-grade walls with efflorescence and storm correlation? Yes: suspect grade or hydrostatic. Confirm with an exterior grade and downspout inspection plus an interior calcium-chloride or in-situ RH probe on the slab. No: continue.
4. Is there a drain or waste component (gray or black water, odor, biological load)? Yes: reclassify category before proceeding; drain failures are at least Category 2 and often Category 3 regardless of source level.
5. None confirmed? Treat as multi-source until disproven and document each candidate path separately.

Confirming diagnosis

A defensible attribution rests on three converging lines of evidence. First, the moisture gradient must point toward the claimed origin: water moves down and out, so the source is uphill of the highest reading along a continuous wet path. Second, a physical mechanism must be located and, where possible, demonstrated; a shut riser that stops active flow confirms supply, a flashing gap that drips under hose test confirms roof. Third, the water chemistry must fit; chlorides for municipal supply, low conductivity for rain, mineral load for grade. When the building has a shared wall or stack across ownership lines, document each unit's readings on a dated diagram and photograph the access openings so the attribution survives an adjuster or engineer review.

Remediation

Attribution drives the work order. For supply, the trade plumber stops the source before drying begins; you cannot dry against an active leak. For roof, coordinate a temporary dry-in (tarp or membrane) so the chamber is not re-wet by the next rain, then dry top-down. For grade, address the exterior cause (regrade, downspout, sump) within the host trade's scope and treat any standing intrusion as at minimum Category 2 given soil contact, escalating PPE and antimicrobial selection accordingly. In every case, dry each affected unit as its own documented chamber with its own GPP and moisture-content log, because shared-wall losses are billed and subrogated per ownership boundary.

References

• ANSI/IICRC S500-2021, Standard for Professional Water Damage Restoration, Section 10 (Inspection, Preliminary Determination and Pre-Restoration Evaluation) and Section 12.2 (Category of Water).
• ANSI/IICRC S500-2021, Section 13.2, Principles of Drying and creation of drying chambers.
• EPA, Moisture Control Guidance for Building Design, Construction and Maintenance (EPA 402-F-13053) on envelope and grade moisture pathways.
• ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes, for slab-source confirmation.