Concrete, plastic, and fiberglass are the three common septic tank materials, and each trades off differently: concrete is heavy and durable but can corrode from the inside over decades, plastic is light and corrosion-resistant but can float in a high water table if not anchored, and fiberglass sits between the two on weight and cost.
Concrete is the traditional choice and is widely used because its weight keeps it in the ground without special anchoring, and it has a long track record. Its main long-term weakness is that hydrogen sulfide gas inside the tank can slowly corrode the concrete above the waterline over many years, which is one reason older concrete tanks eventually need structural evaluation.
Plastic (polyethylene) tanks are lighter, easier to transport and set, and immune to the corrosion that affects concrete. Their lighter weight is also their main risk: an empty or partially empty plastic tank in a high water table or saturated soil can float or shift unless it is properly anchored during installation.
Fiberglass tanks share plastic’s corrosion resistance with somewhat more rigidity, and are often chosen where truck access limits weight but a stiffer structure is wanted. Cost and local availability tend to be the deciding factor between plastic and fiberglass in practice, more than a performance gap between them.
Whichever material, the installed capacity and compartment configuration matter more day to day than the material itself — material affects longevity and installation constraints, not treatment performance.
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Concrete tanks have the longest track record and their weight keeps them stable in the ground, but they can corrode from sewer gas above the waterline over decades. Plastic and fiberglass resist that corrosion but must be properly anchored against floating.
A lightweight tank, usually plastic, can float or shift in a high water table or saturated soil if it isn't anchored during installation. This is a known risk that installers plan for, not a common failure when done correctly.
No. Material affects durability, weight, and installation requirements, not the settling process, which depends on tank capacity and compartment design.
Guide last reviewed 2026-08-31.