Detecting a Leak Before it Ruins Your Roof

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Leaks rarely show up directly under the hole. Water can travel surprising distances, following the path of least resistance — across the deck flutes, around insulation joints, along structural members — before you even realize there is a leak.

That’s why “find the wet spot, patch above it” often turns into repeated service calls.

Leak detection isn’t guesswork — it’s a process. Below is a practical guide for narrowing down where the leak is coming from, what to document, and when it’s time to bring in a professional to detect the leak.

First: Safety and access (don’t skip this)

Before anyone goes up on the roof:

  • Use proper access (secured ladder or roof hatch) and follow your facility’s fall-protection rules.
  • Avoid wet or icy membranes — low-slope roofs can be extremely slick.
  • Watch for skylights and fragile deck areas (especially near old leaks).
  • Limit foot traffic if your roof is under warranty; careless traffic can damage seams and flashing.

If safe access isn’t possible, stop here and schedule a roof inspection. It’s not worth an injury.

Step 1: Confirm it’s a roof leak (not HVAC, plumbing, or condensation)

Many “roof leaks” are actually:

  • Condensate line issues (clogged drain, cracked line, overflowing pan)
  • Plumbing leaks (especially near restrooms, break rooms, or mechanical chases)
  • Sprinkler piping (slow pinhole leaks can mimic roof leaks)
  • Condensation from high interior humidity or poor insulation/vapor control

Quick checks:

  • Does the leak happen only during rain or also during dry weather?
  • Does it happen only during HVAC operation?
  • Is the staining near a mechanical unit, ductwork, or plumbing wall?

If the leak appears after long HVAC run times (even without rain), suspect condensation or condensate management before cutting into the roof.

Step 2: Record the leak “pattern” (this helps you diagnose faster)

Create a one-page log for each leak event:

  • Date/time observed
  • Rain conditions (heavy downpour, wind direction, duration)
  • Temperature (freeze/thaw matters)
  • Exact interior location(s) and photos
  • Whether the leak is dripping, staining, or wet insulation odor only
  • Any recent roof traffic or contractor work (HVAC, electrical, signage, solar, etc.)

Two important clues:

  1. Wind-driven rain often points to flashing details, parapet walls, or edge metal.
  2. Slow leaks after long rains often point to membrane seams, field damage, or saturated insulation.

Step 3: Map the interior leak location to the roof (accurately)

Don’t guess. Measure.

  • Pick two fixed references inside (ex: north wall and east wall).
  • Measure the leak point from each reference.
  • Transfer those measurements to the roof using the same building lines.

If you can safely access above-ceiling space, look for:

  • Damp deck underside
  • Rusted fasteners or deck staining
  • Water trails on steel members
  • Wet insulation or vapor barrier damage

Remember: the source can be upslope or sideways from where you see water.

Step 4: Look for the most common low-slope leak sources

Once you’ve mapped the likely roof “search zone,” inspect in this order:

1) Penetrations and flashings (the usual suspects)

Check around:

  • Pipes, conduits, and supports
  • Roof drains and scuppers
  • HVAC curbs and pitch pockets
  • Skylights and smoke hatches
  • Satellite mounts, sign anchors, guardrail posts

What to look for:

  • Cracked or split flashing
  • Loose termination bars
  • Aged sealant that’s separated from metal or membrane
  • Open corners or fishmouths
  • Pitch pockets that are shrunken, cracked, or missing material

2) Seams (especially on larger roofs)

Low-slope membranes rely heavily on seam integrity.

Signs of seam issues:

  • Gaps at overlaps
  • Wrinkling at the lap edge
  • Old patches lifting at corners
  • Debris lines where water ponds and works on a seam edge

3) Drains and ponding areas

Standing water isn’t automatically a leak, but it increases risk.

Check:

  • Drain bowls (clogging, separation, cracked membrane around the bowl)
  • Sump areas (depression around drain)
  • Overflow scuppers (are they working?)
  • Ponding zones for seam stress and surface deterioration

4) Roof edges and parapet walls

Wind-driven rain and capillary action can move water in ways that surprise people.

Inspect:

  • Coping joints and end laps
  • Edge metal terminations
  • Counterflashing (if present)
  • Membrane tie-in at vertical transitions

5) Field damage (punctures you’ll miss at a glance)

Look for:

  • Tool drops, screws, or metal shavings
  • Cuts from foot traffic
  • Animal damage
  • Damage near service paths to HVAC units

A small puncture can cause a big interior leak once water finds a pathway.

Step 5: Use the right leak-testing method for the situation

When visual inspection doesn’t reveal the source, testing helps you stop guessing.

Controlled hose testing (targeted, not random)

This can work when the leak is active or repeatable.

Best practice:

  • Test one small roof section at a time (15–30 minutes per area).
  • Keep a spotter inside on the phone.
  • Start lowest and work upslope to avoid flooding your test zone.

Avoid “spray everywhere” testing — you can create false positives.

Infrared (IR) scanning

IR can help locate moisture trapped in insulation by showing temperature differences, typically after a sunny day or at dusk. It’s helpful for identifying wet areas, but it’s not always definitive — confirm with core cuts when needed.

Moisture survey / capacitance meters

These tools can quickly find suspect wet insulation areas across a roof field. They are especially useful when leaks have been happening for a while.

Electronic leak detection (ELD)

ELD is one of the most precise options for single-ply or coated systems when conditions are right. It helps pinpoint breaches in the waterproofing layer without relying on visible symptoms.

In short: visual inspection tells you what’s obvious; testing tells you what’s hidden.

Step 6: Don’t confuse “wet area” with “leak source”

A key truth on commercial low-slope roofs:

  • The wettest insulation might be the lowest point, not the breach.
  • Water can enter high, travel, and collect somewhere else.
  • A patch in the wrong spot may temporarily “seem” to work — until the next storm proves it didn’t.

That’s why documentation and mapping matter. You’re building evidence, not guessing.

Step 7: Know when to call a professional leak investigation

Bring in an expert when:

  • The leak repeats after prior repairs
  • There are multiple leak points
  • You suspect wet insulation across a large area
  • The roof is under warranty (repairs may need to follow specific procedures)
  • You need a formal report for insurance, ownership, or capital planning

A good inspection typically includes interior review, roof walk, photo documentation, and a clear recommendation — repair, targeted restoration, or replacement planning.

What you should do today if you have an active leak

  1. Protect interior assets (move inventory/equipment, cover with plastic, use bins).
  2. Document everything (photos, time stamps, rainfall/wind notes).
  3. Map the leak location and mark it on a simple roof plan.
  4. Limit roof traffic until the inspection — foot traffic can worsen damage.
  5. Schedule a professional evaluation if the source isn’t obvious.

Need help isolating a low-slope roof leak?

If you’re tired of chasing the same leak, a structured inspection and (when needed) leak testing can save real money by preventing saturated insulation, deck deterioration, interior damage, and emergency calls.

Safety, Service, & Quality — every day since 1981.
Louisville: (502) 459-2216
Hebron: (859) 943-0012
Service: (888) 818-4842

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