What is the difference between positive and negative hydrostatic pressure and what products can you use?

Guest Blog Written By: Robert Goff, Technical Services Manager

Understanding hydrostatic pressure is crucial for managing moisture in construction and waterproofing projects. This blog explains what hydrostatic pressure is and its impact on structures. Positive hydrostatic pressure refers to water exerting force from the outside of a structure, such as groundwater pushing against basement walls. In contrast, negative hydrostatic pressure occurs when water is drawn inward, typically through cracks or porous materials. Knowing the difference between positive and negative hydrostatic pressure helps in selecting the right waterproofing solutions to protect buildings from moisture damage and ensure long-term structural integrity.

Positive pressure refers to the side of the building or structure on the same side as the hydrostatic pressure. Think of the positive side as the side that, during construction, meets and needs to be waterproofed against, moisture going down.

For example, when building a swimming pool, the inside of the pool is the positive side, as it must be waterproofed against the water that the pool will hold. The inside walls of the pool receive positive hydrostatic pressure. For this type of application you can use our waterproofing products like HYDRO BAN^® Cementitious Waterproofing Membrane.

Conversely, let’s say you’re standing in your finished basement and you see water stains in your walls, this is negative pressure and it comes from water accumulation in the soil around the walls. Moisture in the soil can also make its through the slab and create problems for certain finishes like wood floors or impervious tiles. In these cases, products like VAPOR BAN™ Primer ER can be used to reduce the amount of moisture coming through the slab and make the moisture level tolerable for the chosen floor finish.

In cases where moisture may be an issue it is best to used tests designed for this purpose like calcium chloride tests or moisture probes that can determine the relative humidity of the slab. This is necessary when products used in the assembly or the finished floor may be negatively impacted when exposed to excessive moisture.

Have questions? Contact LATICRETE Technical Services.