The Science Behind Below Grade Waterproofing and Water Control

Controlling and managing water below grade has been around since Ancient Roman times. And fortunately, we’ve come a long way in keeping harmful water away from buildings.

Watch Dr. Joe Lstirburek in this A Cup of Joe episode as he digs into below-grade waterproofing strategies: how did they start, where they went, and how we control hydrostatic pressure against buildings today.

A summary of below-grade waterproofing from Dr. Joe

A leak requires three things: Water, a hole, and force. You can’t stop water, and assuming materials won’t have holes at some point is a false premise. So, the most robust approach to below-grade water management is to remove the force, or hydrostatic pressure

Controlling water below grade today means controlling hydrostatic pressure, which is simply the weight of accumulated water pushing against a structure.

hydrostatic pressure example

To put into perspective how much force water can hold, just one inch of water is equivalent to 250 Pascals (Pa). Or, about the force of 70 mph winds. Again, that’s just one inch of water, so imagine the force upon a building that has several inches of water on foundation walls. 

So, the key is to drain the water away from the foundation. The Romans originally did this with free-draining materials like rocks and sand – which in retrospect isn’t the optimal choice considering those materials are also filled with holes. But the idea was there. 

If you’re able to provide drainage, hydrostatic pressure won’t build. This can be done by draining the water down to a subgrade drain or perimeter drain, which lowers the groundwater table adjacent to the foundation wall. So if there’s a crack or hole, there isn’t water present because it’s been drained away to control the hydrostatic pressure.

Recent innovations, thankfully, are a step up from rocks and sand. Instead, applying a layer on the outside of the foundation wall can effectively drain water away and relieve hydrostatic pressure. This layer or material can be a drainage board or dimple mat, for example, and should have larger dimples.

exterior moisture sources

You’ll also want to provide a capillary break. Fortunately, that same drainage board or large-dimpled mat can be used as one for your below-grade concrete walls.             

So, use materials that:

  • Span the cracks.
  • Control hydrostatic pressure.
  • Provide a capillary break.

It doesn’t get better than those three things when it comes to your foundation.

Now that you know the best way to control hydrostatic pressure, the next step is determining if your dimple mat is truly up to the task.

About Dr. Joe Lstiburek 

Dr. Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. Dr. Lstiburek holds a Bachelor of Applied Science in Mechanical Engineering, a Master of Engineering in Civil Engineering, and a Doctor of Philosophy (Ph.D.) in Building Science. Get the full scope of Dr. Lstiburek’s work, accolades, and contributions to the industry over at Building Science Corporation.