Storage Tank Roofs - Issues with Metal and Corrugated Metal Roofs

Since Colorado’s storage tank rule went into effect in 2016 we have published many articles on different aspects of finished water storage tanks. Today we would like to focus on another important potential avenue of contamination, corrugated and metal storage tank roofs. 

In recent inspections for systems that have tanks with sheet metal and corrugated metal roofs, the water quality control division (division) has seen an increase in issues with seals between the roof of the tank, and the tank walls, vents, and access hatches. Corrugated tank roofs are of particular concern due to the gaps created by their corrugated ridges. Below are some of our most important lessons learned about metal and corrugated tank roofs.

Corrugated Tank Roofs Can be Difficult to Seal

The voids created between the ridges of a corrugated roof and the walls of a tank can be difficult to effectively seal and may be a desirable shelter for rodents and other small animals. It is also much more challenging to correctly seal roof penetrations for things like tank vents in a corrugated roof. If your system has a preexisting tank with a corrugated roof the division recommends more frequent periodic inspections of these tanks with a particular focus on the seals around the corrugated materials. 

Figure 1 - Improper seal between two corrugated panels

Figure 2 & 3 - Gaps left between corrugated roof and tank walls that can allow rodents and bugs to enter the tank, creating a public health risk.

Figure 4 - Light is visible from the interior of a tank around a vent in a corrugated roof showing that it is improperly sealed.

Figure 5 - Improper seal around a tank hatch.

Expanding Foam is not an Effective Method to Seal Gaps in Tank Openings

Figure 6 - Foam used to seal the interior of a tank.

It is not uncommon to see expanding foam used to seal gaps in corrugated tanks because it can easily fill large gaps. The use of this foam can lead to potential public health risks and sanitary defects. Foam used in exterior gaps can degrade rapidly due to weather and may be used by small animals as nesting material. Foam that is used as a sealant on the interior of tanks may deteriorate and flake off into the water posing a contamination risk itself over time. Condensation collecting on the foam can leach contaminants and then drip into the drinking water.

Metal Tank Roofs Can be Challenging to Inspect

We have seen tanks with metal roofs where it isn’t possible to examine the interface between the walls of the tanks and the metal roof without an interior inspection. A dislocation or break in the gasket material or a warpage or dislocation of the metal panels could allow the entrance of contaminants such as plants, insects, birds, and small animals into the tank. A brief walk around a tank is not effective in finding potential openings. An extendable mirror can be helpful for looking at tough to see locations around all storage tanks.

Drinking Water Design Criteria for Tanks

The division has outlined the requirements for tank roofs in the design criteria. This section of the criteria is currency undergoing a stakeholder update process. If you are interested in receiving updates on this process or getting involved you can find more information on our Drinking Water Design Criteria webpage. 

New or modified storage tanks being submitted to the Department for approval must meet the design criteria. Also, the Department intends to update the design criteria to clarify that we will not approve tanks moving forward with corrugated metal roofing. However, during a sanitary survey an inspector will not issue a significant deficiency solely based on your existing tank not meeting the current design requirements unless there is a condition identified during the sanitary survey that is a potential threat to public health (examples include unprotected openings to tanks, missing/damaged screens, unsealed/un-gasketed hatches, improperly sealed tank roofs, etc.).

Figure 7 - Excerpt from the current Design Criteria for Potable Water Systems (2022 Edition)

Identification of Sanitary Defects and Risks to Public Health

If a supplier finds an animal or other concerning debris in an improperly sealed storage tank please contact the division’s emergency line at 1-877-518-5608 as soon as possible but within 24 hours of discovery. The drinking water acute team will consult with you on next steps.

If suppliers identify a sanitary defect, it must be documented upon discovery. Also, a corrective action schedule in accordance with the Storage Tank Plan must be developed to fix it as soon as possible. Take photographs before and after making repairs. By doing so, suppliers can avoid a violation for not correctly implementing the storage tank rule.

Sanitary defects that are discovered during sanitary surveys may be cited as significant deficiencies or possibly even as a Tier 2 (treatment technique) violation of the storage tank rule for permitting a known sanitary defect. The risk of violations is especially present for defects that should be ‘obvious’ to anyone performing a periodic tank inspection.

Additionally, if suppliers do not follow the corrective action schedule that they developed, they will also be in violation of the storage tank rule. Avoid violations of the storage tank rule by proactively identifying sanitary defects and then documenting that the supplier is following a corrective action schedule to fix them.

➽ Chelsea Cotton, P.E., Lead Drinking Water Engineer