We have a new septic tank made of solid stone masonry. The masonry is held together by cement mortar and has been plastered with cement plaster.

A professional working on the job has ensured us that the tank is waterproof (i.e. it should not egress water into the soil.) However, posts online are telling me that neither masonry, mortar or plaster are waterproof.

So is this guy totally wrong? Do we need to apply a waterproofing system to the septic tank?

2 Answers 2


Probably not.

I have a precast cement tank. It is not otherwise waterproofed/coated, etc. It's perfectly standard to use them like that. They don't spray water everywhere - it simply seeps, rather slowly, through the walls.

Consider, if you will, what happens to water that you keep in the septic tank. It sits for a while, and then leaves by a pipe whereupon it's diffused in the soil in the drainage field.

Any seepage through the cement/mortar will be quite slow, and will also diffuse water into the soil. Sure, it will be the soil around the tank, but there's no significant difference.

If you feel the need of an official opinion, call the local health department (whoever does your septic permits/inspections) and ask for an official opinion.

  • One of the reasons I'm concerned about egress of fluid from the tank is because the tank is quite close to the building's footings.
    – AlfaZulu
    Oct 28, 2015 at 19:34
  • 2
    @AlfaZulu - You should be far far more concerned with water getting in along your foundation due to rain and other sources that are above ground. These other concerns are mitigated by proper landscape grading and installation of rain gutters to divert water away from the foundation.
    – Michael Karas
    Oct 28, 2015 at 19:42

The mortar, concrete, and plaster are not waterproof, but they don't have to be. The amount of water that will wick out will be minimal, and likely not even enough to be noticeable.

The septic tank is not pressurized, so there's nothing (other than gravity) trying to force the "water" through the walls of the tank. For all intents and purposes, the tank is water tight. And that's good enough for a septic system.

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