Anatomy of a Roof Leak

by Katie Lee

9 ways water can be entering your building other than from a roof leak.

By Anthony Vross

The telltale signs are there. A puddle on the floor. Stains on a ceiling tile. An active drip. So how is the water getting in? It must be a roof leak, right?

Maybe. That’s certainly the first inclination in many cases.

But first a little homework might be in order. Before incriminating your roof and spending money unnecessarily, don’t too quickly discount the other potential sources of water entry into the building.

Consider this. I was visiting my sister-in-law’s house and water starts dripping from a light fixture. The assumption from the homeowners was it was coming from a roof leak. The problem is, we were on the first floor of a two-story house. And it was as sunny as it could be outside. Good thing they had an experienced roofer on-site at the time, right? I suspected it was a plumbing issue, not the roof. Sure enough, the young daughter came running down the steps with toilet paper stuck to her shoe and said the commode was overflowing upstairs.

The point is, it’s easy and common to suspect the roof at the first sign of water intrusion. But not uncommonly it’s coming from other issues — duct work, plumbing or HVAC to name a few.

Emergency calls like this happen all the time in the commercial space, but keep in mind the person calling is often not at the site himself. It’s a regional facilities manager based in another location who doesn’t see the issue in-person, but whose responsibility the water intrusion falls to. Now you’re paying for a technician to drive out there, get a ladder out, climb up on the roof, investigate the leak, and then potentially determine it’s not a roof leak after all.

water infographic july2016Countless numbers of roofs have been repaired — and even replaced — only to see the leak re-emerge because it wasn’t the roof leaking in the first place. The water was coming from another source. Further complicating matters, sometimes it’s a combination of sources — roof damage and plumbing issues, for instance — yet only one of them gets addressed before the roof is replaced.

Roof leaks typically don’t happen overnight. A roof failure is usually a slow process, unless there are extenuating circumstances. As an example of that, we were called out to a roof that was in relatively good shape; it had never had a problem. And all of a sudden it had 42 roof leaks. When this happens we immediately think, “Was there a hail event or another severe weather situation?” Was there an outside contractor doing work on the roof who might have damaged it; for example, someone shoveling snow? We went online to a service that tracks hail events, and sure enough, there was hail recently at that location. 

But outside of these one-time traumatic events, it’s highly unusual for a moderately worn, low-problem roof to all of a sudden experience a leak.

So if it’s not the roof, what is it? Here’s a checklist of other potential trouble areas:


Mechanical issues are among the more common sources if not coming from the roof.

“Most leaks attributed to HVAC units are from condensate,” says Ron Fagert, vice president of York Mahoning, a mechanical contractor in Youngstown, Ohio. “If the HVAC is old or not maintained well, the condensate can get plugged or the dripping pan could rust; both conditions would lead to an overflow situation and water gets into the duct work.

Fagert says three easy checks to rule out HVAC issues causing water issues in the building include:

1. If it’s air conditioning season, check the filters. If they’re plugged, the evaporator coil freezes up and gets ice on it, and when it melts it’s too much for the drain to handle, causing dripping.

2. Make sure the condensate trap is clean.

3. Check to ensure your panels are properly attached to the unit. Sometimes on a service call the panels are not put back on correctly, and when a heavy wind comes it can blow poorly installed panels right off, allowing water to get in.

Drain Pipes / Plumbing

Plugged drains or a malfunctioning sprinkler system can be a plumbing-related source of water. If you’re seeing water intrusion and you’re not on the top floor of the building, give a hard look for plumbing issues as the potential cause.


Water can get around the window glazing, particularly after heavy winds and hard rains.


Sometimes we detect water leaking from a door threshold.


This is a type of waterproofing issue where you have water that’s not moving around the building, and it’s backing up and getting in where the floor meets the foundation.


Cracks and joints in walls are other potential trouble spots for water.

Penetrations Through Walls

We’ve seen instances where conduit was poked through an exterior wall that created an opening for water; or new building signage was installed above the flashings and not caulked correctly, allowing a water leak into the building.

Gutters / Downspouts

Downspouts are typically not made to hold water, but rather to allow water to run through them. If it becomes clogged with debris or is not wide enough to handle the runoff — or if there aren’t enough downspouts on the building — they can create a situation where water gets in the building.


Sometimes it’s literally just spilled water on the floor, and someone walks by and sees it and reports it to maintenance as a roof leak.


You can be in a metal building without insulation and experience a huge differential in temperature between the outside and inside, which creates condensation that can drip off ceilings. 

Water on a roof runs. It could be leaking through the wall and hit the decking and run some more before dropping through the ceiling. It could be leaking from a plumbing pipe, hit a purlin and run that structure for a stretch before dripping into the building. It can truly be a challenge to find it.

Sometimes it’s a process of elimination to identify the source. It’s not always black and white. And it could be coming from multiple sources. You might close off two and not get the third, and it’s still leaking after the repair.

The goal when there’s a water source should be to act quickly and mitigate it. It’s not always clear as to its origin, so it’s handy to have some go-to resources who are responsive and trustworthy, namely a roofer, mechanical contractor and plumber.


— Anthony Vross is a co-owner of Simon Roofing, a national commercial roofing installer/manufacturer that’s among the largest roofing contractors in the United States.


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