Home › Blog › Rooftop unit condensate leaks — stopping slow water damage on a commercial roof
How a facility team can spot a rooftop unit condensate leak from the hatch and the ceiling, what the mechanical code requires, and who fixes what.
A rooftop unit condensate leak rarely looks like a roof problem at first. The ceiling tile under the unit stains in August, after a week without rain. The roofer finds nothing wrong with the membrane. Then the stain comes back the next summer. A cooling coil produces water every hour it runs, and on a DMV summer day that is most of the day. If the drain line is cracked, blocked or disconnected, the water goes onto the membrane, around the curb or into the building.
Late October is the right time to deal with it. The cooling season has just ended, the heating changeover brings the HVAC contractor back to the roof, and you can still remember which tiles stained in July. This post covers what your facility team can check without going near an edge or touching the roof. It also covers what the mechanical code expects of the equipment, and where the job passes to the HVAC contractor and the roofer.
Every air conditioning or heat pump coil pulls moisture out of the air, and the drain pan under it collects that water. The condensate line then has to carry it somewhere approved. When it does not, the damage is slow and in one place.
Forensic engineers at Rimkus describe the mechanism. When a unit discharges condensate onto the roof surface instead of into a drain, the same spot stays wet for long periods. That moisture can work into the membrane and the materials beneath it, and condensate is mildly acidic, which may add to the wear (Rimkus). Rimkus also lists condensate drainage first among the ways rooftop HVAC units damage roofs, alongside vibration, foot traffic and flashing at the curb (Rimkus). Their recommendation is to route condensate to a proper drain, which removes the source before it becomes a problem.
The practical problem for a facility team is diagnosis. A condensate leak and a roof leak can look the same from the ceiling. One is the HVAC contractor's job and the other is the roofer's, and they will each tell you it is the other's until someone shows the pattern.
DC, Maryland and Virginia all build on the International Mechanical Code, with local amendments and their own adopted editions. Section 307 of the 2021 IMC sets the condensate rules, quoted here from the Maryland adoption on UpCodes (IMC 2021 §307). Check the edition and amendments your jurisdiction actually enforces with the local code official.
| IMC 2021 section | What it requires | What it means for you |
|---|---|---|
| §307.2.1 Condensate disposal | Condensate is conveyed from the drain pan to an approved place of disposal, sloped at least 1/8 in. per foot (1 percent); it must not discharge so as to cause a nuisance | A line that sags, back-falls or ends loose on the membrane is a question for the HVAC contractor |
| §307.2.3 Auxiliary and secondary drains | Where overflow could damage building components, each coil needs one of four protections: an auxiliary pan with its own drain, an overflow drain from the unit's pan, or a water-level detection device that shuts the unit off | Most rooftop units over occupied space should have one of these. Ask which one yours has |
| §307.2.3 Items 1 and 2 | The auxiliary or overflow drain discharges to a "conspicuous point of disposal to alert occupants" if the primary drain stops | Water from a secondary drain is a warning that the primary drain is blocked |
| §307.2.3 Items 3 and 4 | Detection devices conforming to UL 508 shut the equipment off before the pan overflows or when the primary drain is blocked | A unit that keeps shutting itself off in humid weather may be telling you its drain is blocked |
| §307.2.4 Traps | Condensate drains are trapped as the equipment manufacturer requires | Trap design is the HVAC contractor's call, not yours |
The code does not say where the "approved place of disposal" is on your roof. That is the local authority's decision, and practice varies between jurisdictions. Ask your mechanical contractor what was approved for your building, and get it in writing.
All of this is done from inside the building, from the roof hatch landing with the hatch open and nobody stepping out, or from a window that overlooks the roof. Nobody walks the roof for this. Anyone who does go onto a low-slope roof is covered by OSHA's rules in 29 CFR 1910.28(b)(13): fall protection within 6 feet of an edge, and protection, a designated area or an enforced work rule farther back. Skylights count as holes. The full table is in our fall roof maintenance checklist. Don't open the hatch in wet, frosty, snowy or windy weather.
Take the roof plan or a site aerial and mark every rooftop unit. On the floor plan below, mark every ceiling stain, replaced tile and drip report from the last year, with dates. A stain directly under or beside a unit's curb is the first clue.
This is the most useful thing a facility team can do, and it costs nothing. A roof leak follows rain. A condensate leak follows cooling load: hot, humid days and long run hours, often with no rain at all. If your drip reports cluster in dry spells in July and August, tell both contractors before they arrive.
From the hatch landing or a window, with a phone camera zoomed in, look for:
Photograph each one with the date and add it to your roof photo log.
Ask your building automation operator or HVAC contractor for any unit that has tripped on a condensate or float switch. Repeated trips in humid weather point to a drain that is blocking up.
The access log from your contractor roof access policy matters here. Stains that start soon after a service visit or a unit replacement suggest a line was disconnected or rerouted.
Most condensate problems need both trades. The table shows the usual split.
| Finding | First call | Second call |
|---|---|---|
| Line disconnected, cracked, sagging or ending on the membrane | HVAC contractor: reconnect, support and route to the approved disposal point | Roofer: inspect the wetted area and the curb flashing |
| Unit tripping on its float switch, or water from a secondary drain | HVAC contractor: clean the pan, trap and line | Roofer only if water reached the roof or the interior |
| Ceiling stain under a unit, no rain correlation | HVAC contractor: confirm the drain path is working | Roofer: check curb flashing and penetrations, and look for wet insulation |
| Discoloured, soft or blistered membrane at a unit | Roofer: assess and repair with a method compatible with your system and warranty | HVAC contractor: remove the water source first |
| A new line needs a roof penetration or support | Roofer, with manufacturer approval if the roof is warranted | HVAC contractor installs the line |
The order matters. A membrane repair under a unit that is still dripping on it is money spent twice. Get the water source fixed, then have the roof looked at. If water has been reaching the insulation for a season or more, a moisture survey may be worth the cost before anyone talks about patches or coatings.
Our roof repair and maintenance programme pages describe how we handle this kind of work, including notes for the HVAC contractor on what to fix first.
On a top floor under rooftop equipment, the usual suspects are condensate from a cooling coil, a plumbing line, or condensation inside the building. Log the dates against the weather and the HVAC run hours, and have an HVAC contractor check the condensate drain before a roofer opens the membrane.
The 2021 IMC requires condensate to go to an approved place of disposal, and it must not cause a nuisance. Where that is on your roof is the local code official's decision. A line dripping onto the same patch of membrane keeps it wet, and forensic engineers link that to membrane wear. Route it to a drain.
That depends on your service contracts and your roof warranty. Warranties commonly exclude damage caused by other trades. Read the exclusions in your own warranty and HVAC agreement, and keep dated photos and the access log.
The manufacturer's maintenance instructions and your HVAC service contract set the interval. Ask for the pan, trap and line to be checked at the spring start-up and again at the autumn changeover, and for the visit notes in writing.
Treat it as HVAC work for a licensed contractor. It means opening the unit, which puts staff close to electrical parts, and often working on the roof itself. Your team's job is to notice the problem and record it.
If you have a stain you can't explain under a rooftop unit, a written condition report will show what the roof needs and what the HVAC contractor needs to fix first. Book a free 30-minute consultation at https://dmvcommercialroofing.com/#book or call 202-798-7663.
Header image: AI-generated illustration (Magnific).
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