My water heater isn't sitting level. How should I properly shim it?

If your water heater isn't level due to an uneven floor, use non-compressible, moisture-resistant shims like composite shims or treated wood blocks. Place them under the tank's feet or base to create a stable, level surface. For gas units, ensure you don't block the combustion air intake at the bottom. If the tank itself is unstable or rocking significantly, or if it's leaking, shimming won't fix structural damage or a leak, and you'll need a professional to inspect the unit for replacement.