Roofing discussions and local services.
Discussion options
Intermittent Ponding Near Commercial Roof Drain: Clog, Slope, or Settled Deck?
A low-slope commercial roof develops ponding near one internal drain, but only after heavier rain. The water is approximately 1 to 2 inches deep and appears to remain for roughly 24 to 48 hours, while lighter rainfall drains normally. The pond is several feet from the drain rather than directly over the strainer.
The roof is about 15 years old, but the membrane type has not yet been confirmed. The drain was cleaned during routine maintenance several months ago. I do not know whether the cleaning included the drain leader below the strainer, and there is no obvious overflow route visible from the roof edge.
What observations would help distinguish a partially restricted strainer or leader from inadequate slope, a settled roof deck, or an undersized drain? Is it reasonable to have the strainer and leader professionally cleaned first, then monitor the drainage time, or should a roof survey be done before any cleaning?
I would avoid probing the membrane or adding weight near the pond. Would checking the drain basket, documenting the pond’s deepest point and dimensions, and observing whether water reaches the drain during rainfall provide useful information? Also, should the absence of a visible overflow scupper be treated as an urgent issue, even if the roof has not yet overtopped?
Join the conversation
Share your experience or ask a follow-up question.
3 replies
Cleaning and documenting the drainage behavior are sensible first steps, but a clear drain still may not be adequate for the roof area it serves. Have a commercial roofing professional verify the drain’s capacity against its contributing roof area and the rainfall intensity required by local code. A 1- to 2-inch pond after heavy rain could reflect restricted flow, but it could also mean the drain is undersized for those storms.
The 24- to 48-hour duration is useful evidence, especially if water is still reaching the drain during rainfall. Ask the contractor to document:
- drain and leader condition, including the connection below the strainer
- roof area draining to that outlet
- drain size and any restrictions in the leader
- low points and the direction of the actual slope
- whether the membrane or deck has settled around the pond
I’d avoid treating the missing visible overflow as proof of an emergency, since some systems use internal overflow provisions that aren’t obvious from the roof edge. But the professional should confirm whether a secondary drainage path exists and where it discharges. If none exists, that deserves prompt attention, particularly if a blocked or undersized primary drain could let water rise to roof edges, penetrations, or occupied spaces.
The 24- to 48-hour drainage time is useful for budgeting as well as diagnosis, but I’d ask for separate prices rather than one bundled “roof repair” allowance:
- strainer and leader cleaning, including access and any camera inspection
- diagnostic work, such as level measurements, moisture mapping, or an infrared survey where appropriate
- corrective work, priced separately for membrane detailing, tapered insulation, drain modifications, or deck repair
That separation matters because cleaning may improve the pond without eliminating a low spot, while a survey may show that the membrane is sound and the issue is limited to slope. Conversely, if moisture or deck settlement is found, the repair cost can be substantially different from a routine maintenance visit. Ask the contractor to state what each price includes and whether opening the roof is excluded unless authorized.
For similar cases, have the contractor record the pond’s maximum depth and footprint after comparable storms, along with whether water is visibly moving through the drain during and after rainfall.
If cleaning produces a clear reduction in depth, area, or the 24- to 48-hour drainage time, a restriction becomes more likely. If those measurements stay nearly unchanged while the drain flows freely, a persistent low spot or settled deck becomes more likely. Repeating the measurements can narrow the question before opening the roof.