Positive-side vs negative-side waterproofing

01Two faces of one wall

For a basement wall, the positive side is the soil face; for a tank, the inside face; for a roof slab, the top. The negative side is the opposite face: the inside of a basement, the outside of a tank.

02Positive-side waterproofing

Applied where the water comes from, the waterproofing keeps water out of the concrete altogether — protecting the reinforcement too — and water pressure presses the membrane onto the structure instead of off it. It needs access to that face — before a basement is backfilled, inside a tank, on top of a roof — and protection afterwards. Most sheet and liquid membranes are designed for this side.

03Negative-side waterproofing

Applied on the dry face, the system has to resist water that has already passed through the concrete and pushes from behind. Bonded systems made for it — some rigid cementitious coatings and crystalline treatments — work by bonding into the concrete surface or growing within it; most membranes would blister and peel off. It keeps the space usable, but the concrete itself stays wet, so the protection of the steel depends on the concrete quality and the cover.

04When each one is used

05In between: blind-side and in-concrete options

Two cases sit between the faces: blind-side (pre-applied) membranes, placed on the positive side before the concrete is cast against them; and integral protection — crystalline admixtures and water-resisting concrete — where the concrete body itself resists water.

06What negative-side treatment cannot do

• It does not stop water reaching the steel from the soil side.
• It cannot bridge moving cracks or joints; those are injected or given a joint system.
• It needs a sound substrate: weak or contaminated concrete is repaired first.
• Active leaks are stopped before coating.