Trained Eye


Leaking Holding Tank


Hello Ari Marantz!

Literally dozens of cottage owners in Manitoba are suffering from ground water leaking into their fibreglass holding tanks, caused by heavy rainfall, which raises the groundwater table. The leakage occurs at the junction of the tank’s body to its extension collar. Trying to seal this from the outside, the collar has to be dug up to below its junction. Trying to seal this area with roofing tar and or silicon lasts only for 1-2 seasons.

Do you know of any material that would seal this area permanently?

Sincerely thanking you for your reply.


I agree that ground water leaking into holding tanks, especially at the collar, is a very common problem. For those that have only a holding tank and no connection to a low-pressure sewer or field, this defect can add substantial cost to the system. The need for pumping out the tank will be much more frequent, depending on the extent of the leak. To offer a suggestion for repairs, we will consider the cause of the water intrusion, first.

The primary reason for the leakage is the difficulty in properly sealing a large joint between the main body of the holding tank and the collar. This is not exclusive to fibreglass tanks and occurs with concrete tanks, as well. Any small gap or opening that occurs will allow water to seep into the tank. Sealing this from the exterior will be the only way to stop or minimize this occurrence.

The soil around a buried holding tank is quite heavy and often is composed of expansive clay. This clay soil will expand and contract with changes in moisture content and temperature. As the water level rises, due to heavy rains or snow melt, the soil will swell and put pressure on the tank. A Fibreglass tank is somewhat flexible and designed to withstand this soil pressure, but the movement may open joints between the tank and the collar. The water in the soil will force its way into the tank by hydrostatic pressure. This can also occur with concrete tanks as small hairline cracks or openings may occur after several years of use. Regardless of the method of repair, the tank will have to be excavated below the area where the collar enters the tank. This should be done with sufficient area exposed to allow for proper cleaning and repairs.

I agree that asphalt cement or silicone caulking will have limited success in stopping the leaks. A better alternative may be the use of a membrane that will not easily open when the tank flexes. Fibreglass components are constructed of layers of thin Fibreglass mesh embedded in strong resins. The mesh gives tremendous strength to the structure and allows a fair amount of flexibility without cracking or damage. If the area around the collar is sufficiently exposed and clean, the same method may be employed for repair as with original construction. Fibreglass repair kits are readily available at automotive shops and boat accessory dealers. With a little practice, a reasonably good patch could be installed around the entire collar junction area. Alternatively, there are various waterproofing membranes, commonly called “blue-skin” membranes, that may be an easier-to-install solution. These membranes are self-adhesive and may require a primer to ensure proper adherence.

The difficulty in both of these proposed solution is dependent on the current condition of the tank. Neither system will work unless the tank is completely clean in the area of the repair. This may require wire brushing or light sanding/grinding to remove all the existing dirt and soil. Washing and drying the area may be required before repairs. If the collar is sufficient misshapen from soil pressure, or the area uneven, a repair may not be possible. I would caution that I have never attempted such a repair, but have seen these types of repairs successfully done on other similar materials.

The final item to consider is the soil surrounding the holding tank, itself. If the backfilled material is heavy clay soil, then this is part of the problem. Once the collar of the tank is excavated and the area exposed and repaired, the clay should not simply be replaced. A smooth granular fill, such as sand or river wash stone, may be installed around the collar to improve drainage. This may also help prevent hydrostatic pressure from moisture trapped in the heavy clay gumbo. A thin layer of clay or topsoil may be built up on top of the granular fill to help with grading away from the tank. Under this topsoil, a landscape fabric should be installed to prevent the soil from washing into the granular fill, defeating the purpose.

It may be possible to sufficiently patch the collar of your tank to prevent excess leakage and prevent frequent pumping, but the benefits should be weighed against the work involved. If the fibreglass collar or tank is badly warped or deteriorated, as is common, then excavation and replacement may be the only practical option. It may be considerably less expensive to pay for several additional pump-outs in a season, than proper repair or replacement. If leakage is occurring from the contents of the tank to the surrounding soil, then environmental concerns should also be taken into consideration when exploring tank replacement.




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