Mold vs. Mildew: What Is the Difference? How to find it, What to do About it

One of the most common causes of water damage in NYC buildings — and one of the most commonly misdiagnosed, because the visible damage often appears far below the actual leak point. Water travels along structural members before dropping, which means a stain on a sixth-floor ceiling tile may originate from a roof penetration two floors up.

Roof leaks become most noticeable after heavy rain, wind-driven rain, and snow melt. The NYC Department of Buildings identifies roof maintenance as a critical property owner responsibility — roof defects that allow water intrusion are a compliance issue as well as a facility management one.

Among the leading causes of indoor water damage — and the most difficult to catch early, because supply lines, drain lines, and HVAC condensate lines run inside walls and above ceilings where nobody sees them. A slow leak behind a wall can run for weeks before producing a visible stain, and by then the drywall, insulation, and framing it’s been wetting have likely already developed mold.

Common sources include supply and drain lines, toilets, water heaters, sinks, mechanical equipment, and HVAC condensate lines. According to the Insurance Information Institute, accidental discharge or overflow from plumbing systems is one of the most common property insurance claims in the United States.

Particularly common throughout New York City — a function of aging stormwater infrastructure, intense rainfall events, dense development that has reduced permeable ground surface, and groundwater pressure in low-lying areas. The flooding events that have affected Brooklyn, Queens, and lower Manhattan in recent years are not aberrations; they’re a pattern that facility managers in these areas need to plan for.

Common causes: heavy rainfall overwhelming drainage capacity, foundation cracks, poor site drainage, groundwater pressure, and sewer backups. NYC DEP’s stormwater resources provide guidance on flooding management and infrastructure for NYC properties.

A consistent source of water damage in commercial buildings — and a particularly troublesome one because HVAC systems are typically located above drop ceilings or inside mechanical rooms where problems go undetected for extended periods. By the time a damp ceiling tile or mold patch appears below an air handling unit, the unit itself may have been leaking for months.

HVAC-related mold is also a compliance consideration for commercial facilities: contaminated air handling systems can distribute spores building-wide before the source is identified. The EPA’s indoor air quality guidance notes that HVAC systems contribute to indoor moisture problems when condensation is not properly managed.

Water doesn’t always enter through the roof. In many NYC buildings — particularly pre-war commercial buildings and high-rise towers with curtain wall systems — moisture enters through windows, expansion joints, failed sealants, and deteriorated masonry, then travels inside the building envelope before appearing as interior damage.

Especially common in older buildings and landmarked properties where facade components have deteriorated. NYC’s Facade Inspection Safety Program (FISP) requires periodic exterior facade inspections for buildings over six stories — facade defects identified in FISP reports frequently include conditions that allow water intrusion.

Burst pipes release large volumes of water in a short time — and they typically happen when no one is in the building. A pipe that bursts on a Friday night or over a holiday weekend can saturate drywall, ceiling assemblies, flooring, and building contents for 48 hours or more before anyone responds.

According to NIOSH and the CDC, mold can begin developing on wet materials within 24–48 hours — which means a burst pipe not discovered and addressed within that window is likely to produce a mold remediation scope, not just a water damage cleanup. Common causes: freezing temperatures in inadequately heated spaces, corrosion in aging infrastructure, excess water pressure, and mechanical failure.

Among the most serious water damage events because the water involved may carry bacteria, viruses, and other contaminants — which means the cleanup protocol is more demanding than standard water damage. Affected materials often require removal rather than drying, and the space needs to be treated for contamination, not just dried.

For commercial facilities, a sewer backup event is also an OSHA compliance consideration — employee exposure to contaminated water falls within the General Duty Clause obligation to provide safe working conditions. NYC DEP advises property owners to address sewer backup incidents immediately and follow proper cleanup procedures.