Below-grade waterproofing is described by which face of the wall it's applied to relative to the water. Positive-side waterproofing goes on the exterior face — the side actually in contact with the wet soil — and stops water before it ever enters the concrete. Negative-side waterproofing goes on the interior face, the dry side you can see standing inside the basement, and instead has to resist water that's already pushed its way through the wall.

Positive-side is the stronger approach for one simple reason of physics: the groundwater pressure pushing toward the building also pushes the exterior membrane tightly against the wall, reinforcing the seal rather than working against it. It's the standard specification for new construction, applied before backfilling. Its major drawback shows up on existing buildings — reaching the exterior face means excavating around the foundation, which is disruptive and expensive on a completed structure.

Negative-side waterproofing exists largely as a retrofit option for exactly that situation: when excavation isn't practical because the wall abuts a neighbouring building, a driveway, or a property boundary. Because it's fighting water that's already inside the wall, the material has to withstand ongoing hydrostatic pressure from behind and is generally limited to cementitious systems capable of bonding to a damp or wet substrate. It stops water reaching the occupied space, but — unlike positive-side systems — it doesn't protect the concrete and steel reinforcement inside the wall from ongoing water damage.

Integral waterproofing (admixtures mixed into the concrete) is a third strategy that sits outside this positive/negative framework entirely, since it protects the whole thickness of the wall rather than one face. On a serious basement or reservoir project, the strongest specifications combine integral concrete with a positive-side membrane and correctly detailed waterstops at every joint — belt, braces, and a backup plan.