Flat roofs: An architect’s dream, a PMP’s nightmare


May 18, 2018

Don Putnam, an Austin, Texas-based construction consultant, performs a
roof moisture survey using a Tramex Dec Scanner.
Photo: Putnam Construction Consulting

Flat roofs have become the “new normal” on many modern building designs, both residential and commercial. They’re becoming common for hospitals, hotels, restaurants, office buildings and warehouses. Although they look stylish and are less expensive to construct, they eventually leak.

Pest management professionals (PMPs) all know moisture attracts many insects and permits breeding. Common pests include fungus gnats, fungus beetles, psocids and springtails. The presence of psocids and springtails sometimes attracts and enhances the breeding of erythraeid mites, which become a problem when they invade buildings. Unlike pitched roofs, which cause leaked water to run off, flat roofs cause water to pool in low areas and wet the underside of the roof structure. Leaking pitched roofs can cause indirect problems in the structure below, but only seldom do they permit the breeding of moisture-loving insects.

Flat roofs usually have a rubber membrane that keeps the water out. The problem is that the membranes crack and/or have weak seams that leak with age. The water seeps downward into the substrate. The substrate can be insulation, wood decking or combinations of materials (concrete often is directly below the organic substrates). Organic materials, such as wood and some types of insulation, support mold growth when wetted continuously. Flat roofs seldom have any provision to “breathe,” so once the substrates are wet, they never dry out. Patching such roofs does not solve the pest problem because the wet substrate beneath will continue to support pest problems.

First things first

When encountering such problems, you must conduct a thorough inspection to determine the nature and source of the problem. Begin by identifying the pests. If it is a moisture-loving pest, then you must track down the moisture source. This is not always easy. If the substrate of the roof lies atop concrete, you can’t measure the moisture in the substrates from below. The rubber membrane on top may not show you where moisture problems lie, either.

A moisture reader on a roller can quickly traverse a flat roof and detect areas of moisture. Sometimes you must bite the bullet and ask the customer to open up the roof so it can be inspected and the moisture level determined. That becomes a challenge in most cases.

If drop ceilings are present, you can remove random panels and look upward to find signs of insect activity. Look in the area of vent pipes that penetrate the roof (and concrete, if present). Generally speaking, insects breeding in the roof will enter around the vent pipes:

  • Fungus gnats will hover around the opening around the vent pipe.
  • Fungus beetles often can be found around ceiling lights.
  • Psocids will be difficult to see.

Sometimes you can’t locate the pests, but a temporary insect light trap (ILT) and/or sticky trap will. If you find pests above the drop ceiling, you need to have the roof opened for inspection. This is the most difficult part of dealing with the customer. The customer doesn’t want to spend the money when you can’t “guarantee” the roof is the problem. Have the roof opened around vent pipes, as you can often see the insects traveling to the vent pipe area. Then follow the moisture to the actual source.

If the roof isn’t leaking

Erythraeid mite infestations stem from roof moisture problems, but not necessarily because the roof is leaking. These mites are predacious on small insects such as springtails.

Many flat roofs are not perfectly flat, and rainwater collects in some areas. During rainy periods, the water in these little “ponds” never dries out. Springtail populations begin. If the moisture problem persists long enough, erythaeid mites appear. These mites can be found crawling on the roof, on the upper exterior walls of the building and around upper floor windows.

Erythraeid mite problems are often misdiagnosed as clover mite problems, because both mites are red. Don’t be fooled: If you have a “red mite” problem on the upper floors of a flat-roofed building, you likely have an erythraeid mite infestation.

Removing low spots on the roof usually is not practical. Rather, an appropriate insecticidal spray on the roof will eliminate both the springtail problem and the mite problem.

Beyond the roof

Not all moisture-loving insects are related to roof problems. It is not uncommon to find psocids, fungus gnats and fungus beetles in new construction, but not coming from the roof.

New construction usually has a lot of moisture in it. Framing wood (studs, etc.) often contain as much as 19 percent moisture, although it will continue to lose moisture as it ages in place. However, consider wood that is stacked at a building site for extended periods and is rained on repeatedly. It, too, will contain higher-than-usual moisture. Also consider a building that is framed, but the roof is not yet installed or only the plywood sheathing is in place — not shingles. Every shower or storm puts more moisture into the building.

After the first heating season, the wood typically dries out to its normal moisture level range of 8 percent to 12 percent. Higher levels can produce surface molds and mildews. These, in turn, can support some of the moisture-loving insects. If you encounter moisture-loving insects in a new building, don’t automatically assume the roof is the problem.

A practical solution

A recent inspection of a building that had been occupied for less than a year found fungus beetles in ceiling lights and wall sconces. Wall and ceiling moisture readings were in the range of 12 percent to 14 percent. This is not sufficient to support wood-decay fungi, but can support many molds and mildews.

The fungus beetles apparently were breeding in the walls and ceilings. When they tried to escape, they were caught in the light fixtures. Monitors in the attic failed to capture any of the pests — indicating the roof was not the problem. Further discussions indicated the structure had been “open” several months after the walls were erected. Heavy rain during that roofless period contributed to the moisture problem.

Pesticides are not the practical solution to this problem. The next heating season would dry out the walls sufficiently to eliminate the problem.

Under similar conditions, psocids are also a frequent problem. Again, pesticides are not practical. The next heating season will resolve the problem.

The bottom line on solving moisture-loving insect problems is to eliminate the moisture source. In the case of flat roofs, opening them up, removing the wet materials and closing the roof with appropriate materials and methods solves the problem. It is not a problem that can be solved using pesticides.

The real challenge for our industry is educating the customer and taking the appropriate steps to verify the problem source and eliminate the moisture.


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