Low and Dry: Basement Waterproofing

August 25, 2022 (Reston, Virginia)

Even in an empty basement, water can damage structural integrity and any systems located there. In a finished basement or one used for storage, finishes and property can be destroyed by water damage or mold from sustained humidity.

It takes a well-executed, multi-tiered approach, based on sound building science, to keep water out of below-ground basements. While the process is commonly called “waterproofing”, a house is never watertight like a boat. Even the best waterproofing isn’t infallible. Diligence and routine maintenance ensure the system can keep doing its job.

To remain stationary, modern houses sit on concrete footings that go down below the frostline. The ground rises and sinks as it freezes and thaws in a process called frost heaving. With footers below the frostline, the house remains stable, greatly reducing the chances of foundation cracks that could enable water penetration

As builders excavate around and inside the foundation, they install a drainage system around the exterior and interior home perimeters. This consists of perforated piping called drain tiles, which are surrounded by gravel or crushed stone to keep soil from clogging them. Water drains into the pipes which carry it away from the foundation, often assisted by a sump pump. Drain tiles are also installed under the slab, and because concrete is porous, a special membrane is installed as a water/vapor barrier. Builders also often treat exterior foundation walls with a spray-on product to seal the concrete against water seepage.

The waterproofing system is vital to resisting water entry, but the most common source of foundation leaks is improper grading.. If water can pool against a house, it may eventually get in. Anywhere the earth meets the house, the ground must slope away. Typically, the final grading of the lot includes swales around the house to direct water away and down. Even if there is a hill behind a house, the grading should create a slope immediately around the house down before the hill, and swales to divert water.

Over time, even the best engineered waterproofing systems can fail. The most common causes:

• Insufficient slope from the earth settling over time, improper landscaping, or new hardscape or structures that change the terrain and steer runoff toward the house.
• Gutter problems such as clogged or sagging gutters or downspouts that are too short or improperly positioned.
• Foundation damage, such as cracks from large trees or shrubs planted too close to the house.
• Clogged or crushed drain tiles.
• Water penetration around doors or windows. Regular inspections and caulking are important, and older windows may need replacement. Window wells and well covers need maintenance as well. Heavy snows, as they melt, are a common cause of leaking basement windows. Blowing or shoveling snow well away from the foundation can help.
• Sump pump failure. An old pump might be inoperable or underpowered for significant downpours. Check the pump at least annually. The storm conditions bringing heavy rain may also cause power failures, so a battery backup pump is a smart investment.