Ponding After a Re-Cover With Raised Drains
A 30,000 sq ft one-story retail building has an existing roof sloping about 1/8 in. per foot to two roof drains, with minor historic ponding. The re-cover design adds 2.5 in. of polyiso directly over the existing perlite, then a fully adhered 60-mil EPDM, and reuses the two existing drains with taller clamping rings so the strainers sit about 1 in. above the new membrane surface. After the first heavy rain, two areas, each roughly 20 ft by 30 ft, hold about 1.5 in. of water. Sixty hours after the rain stops, those same two areas still hold about 1 in. of water. The owner's maintenance manager says the old roof 'always had a puddle there,' and the contractor says the water is 'within tolerance' because the drains are clear. The local plumbing code is enforced by the AHJ. The structural drawings show the joists were designed with no allowance for ponding, and there is no overflow drainage on the roof.
1. Apply the accepted definition of ponding and decide whether this roof ponds. What further evaluation does that answer require?
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Ponding is commonly defined as water remaining on the roof surface 48 hours after rain has stopped. At 60 hours with 1 in. still standing, this roof ponds. The contractor's 'within tolerance' and the owner's 'it always puddled' are not technical defenses, particularly on a new re-cover where the opportunity to correct slope is being spent.
The next step is to classify the cause, because the fix depends on it:
- Construction ponding: the roof was built without adequate slope or with a local low spot. Adding 2.5 in. of insulation over an irregular perlite substrate without re-establishing slope can create or relocate low points, and the 1/8 in. per foot existing slope was already marginal.
- Drain-related ponding: raising the clamping ring puts the strainer weir above the surrounding surface, and if the drain is not set in a sump, water has to stand to the weir depth before it can leave. On a low-slope area this alone creates a permanent puddle ring. Verify the weir height relative to the low point.
- Deflection ponding: progressive sagging of the structure under standing water.
Evaluate with a string line or laser level over the two areas and around each drain, plus a visual check of whether the low point coincides with a drain. Measure the actual weir height. If the low area is not at a drain, the drain detail is not the cause and the fix is slope or a new drain location.
2. Quantify the structural issue and explain why the missing overflow drainage changes the risk.
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Water is heavy and the structure was not designed for it.
Unit weight of water is about 62.4 lb per cubic foot, or 5.2 psf per inch of depth. At the observed 1.5 in. peak, each 20 ft by 30 ft area (600 sq ft) carries about 7.8 psf, or roughly 4,700 lb of water. At the 1 in. 60-hour level it is still about 5.2 psf over each area. Against a joist design with zero ponding allowance, that is unbudgeted load sitting in two specific bays, and it is not evenly distributed.
The missing overflow is the larger issue. Without overflow scuppers or overflow drains, the only limit on water depth is the top of the drain ring or the roof edge. If a strainer plugs with debris, water rises until it finds the roof edge, a door, or a joint, and the load can go far beyond 7.8 psf. Ponding also tends to be self-reinforcing: sustained load deflects the structure, the low spot deepens, and more water collects there.
Practical conclusion: this is not a cosmetic drainage complaint. Recommend a structural review of the two bays under the observed loading plus a blocked-drain scenario, and treat overflow drainage as required remediation, not an upgrade. Coordinate the overflow requirement with the AHJ, since local plumbing code governs.
3. The owner wants the cheapest option that keeps the membrane warranty intact. What do you recommend and why?
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Rank the options by whether they eliminate the water or merely tolerate it.
Re-slope the low areas with tapered insulation and a cricket, or install a new drain at the low point with a proper sump. This removes the water and is cheapest now, while the roof is open and the membrane is being finished. Re-sloping a localized 600 sq ft area is a small quantity of tapered product and labor.
Add overflow drainage. Overflow scuppers or overflow drains at a defined height are typically required by plumbing code and are almost always a condition of membrane warranty when ponding is acknowledged. Confirm the required height and location with the AHJ.
Fix the drain detail. Confirm whether the raised clamping ring created a weir above the low point; if so, lower the sump or set the drain in a sump so the weir is at the low point, not above it.
Accept in writing only as a last resort. If the owner refuses, get a structural review confirming capacity for the ponding plus blocked-drain depth, a written owner acknowledgment, and a maintenance plan for strainer cleaning. Do not let the contractor's 'within tolerance' line become the record without the 48-hour criterion and the load numbers on paper.
The recommendation is option 1 plus 2 and 3. Deferring this is the expensive path, because the next repair involves removing finished membrane rather than installing it.