Basement waterproofing

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Basement waterproofing refers to techniques or materials used to reduce water penetrating the basement of a structure. Waterproofing involves the application of materials and/or sealants and installation of drains and possibly a sump pump to collect, remove and/or redirect water away from the basement.


Waterproofing is often required for structures built at or below ground level. Waterproofing and drainage considerations are especially important in cases where ground water is likely to build up in the soil and raise the water table. A higher water table causes hydrostatic pressure to be exerted underneath basement floors and against basement walls. Hydrostatic pressure forces water in through cracks in foundation walls, and through openings caused by expansion and contraction of the footing-foundation wall joint, or up through floor cracks. Hydrostatic pressure can cause major structural damage to foundation walls, and is likely to contribute to mold, decay, and other moisture-related problems.


Several measures exist to prevent water penetrating a basement foundation; they do differ greatly in theory and design. The methods include:

  • placing interior wall and floor sealers
  • providing interior water drainage
  • including exterior drainage combined with waterproofing coatings
  • having sump pumps
  • increasing ventilation and air flow

Interior sealants[edit]

In poured concrete foundations, cracks and pipe penetrations are the most common entry points for seepage. These openings can be sealed from the interior. Epoxies, which are strong adhesives, or urethanes are pressure injected into the openings, penetrating the foundation through to the exterior, thereby cutting off the path of the seepage.

In masonry foundations, interior sealers will not provide permanent protection from water infiltration where hydrostatic pressure is present. Interior sealers are good for preventing high atmospheric humidity inside the basement from absorbing into the porous masonry and causing spalling. Spalling is a condition where constant high humidity or moisture breaks down masonry surfaces, causing deterioration and shedding of the concrete surfaces.

When the foundation is constructed of concrete blocks, an interior drain tile system, described below, can capture the water entering the block system and drain the water into a sump. Typically, weep holes are drilled into the lowest course of block, allowing the drainage to occur.

Interior water drainage[edit]

Although interior water drainage is not technically waterproofing, it is a widely accepted technique in mitigating basement water and is generally referred to as a basement waterproofing solution. Many interior drainage systems are patented and recognized by BOCA(Building Officials and Code Administrators) as being effective in controlling basement water.

A channel is created by removing a narrow section of concrete around the perimeter of the basement alongside the foundation footers. Then a French drain, PVC pipe, or a patented drainage system is installed in the newly made trench. The installed drain is covered with new cement.[1]

The drainage system collects any water entering the basement and drains it to an internally placed sump pump system, which will then pump the water out of the basement. Wall conduits such as dimple boards or other membranes are fastened to the foundation wall and extend over the new drainage to guide any moisture down into the system.

Foundation sump pumps can be installed through do-it-yourself kits, plumber installations, or by a professional waterproofing contractor and generally come in plastic and cast-iron models.

In summary:

  • Water enters the structure via the basement wall/floor joint, through cracks in the foundation walls and/or holes created by faulty or decaying masonry/brick.
  • A perimeter trench drain, such as a French drain, collects the water before it enters into the basement.
  • Wall vapor barriers/retarders and drip moldings are used and incorporated into the sub-slab perimeter drain to collect water coming from wall cracks and other foundation wall defects, such as pipe protrusions.
  • The drain directs the water to a sump pump.
  • The sump pump directs the water out of the structure.

The sump pump is placed inside a sealed sump pit which prevents the high humidity inside the sump from entering the basement environment. The sealed pit also helps prevent against radon gas entering the habitable space. Interior drainage systems can include a battery backup for the sump pump in case of power failure.

Interior waterproofing[edit]

Interior basement waterproofing using coatings is effective where condensation is the main source of wetness. It is also effective if the problem is minor dampness. Usually, major leaks are unable to be solved by internal coatings. In such cases, if outside waterproofing has been ruled out, sheet or tile coverings which conceal drainage structures to carry the water out and drain it from the structure may be employed.

Internal drainage systems also include breaking a trench in the concrete floor along the perimeter walls. Weeping tiles are then installed in the trench and filled with 3/4" gravel. A waterproof dimple board is then installed on top extending up the foundation walls. The new weeping tiles are connected to a sump pump or drainage and new concrete is then poured over the system.

Exterior waterproofing[edit]

Exterior waterproofing prevents water from entering foundation walls therefore preventing the wicking and molding of building materials. Waterproofing a structure from the exterior is the only method the IBC (International Building Code) recognizes as adequate to prevent structural damage caused by water intrusion. Prior to the 1980s much of the original exterior waterproofing was damp-proofing using a degradable asphalt-based covering.

Waterproofing an existing basement begins with excavating to the bottom sides of the footings. Once excavated, the walls are then power washed and allowed to dry. The dry walls are sealed with a waterproofing membrane[2] and new drainage (weeping tiles) are placed at the side of the footing.

Polymer-based compounds[edit]

Over the past ten years, polymer-based waterproofing products have been developed. Polymer-based products last for the lifetime of the building, and are not affected by soil pH. Polymer-based waterproofing materials have the advantage of a low enough viscosity that they can be sprayed directly onto a wall, are very fast curing, and are semi-flexible, allowing for some movement of the substrate.

Causes of water seepage and leaks[edit]

Water seepage in basement and crawl spaces usually occurs over long periods of time, and can be caused by numerous factors. Often, water damage may begin as early as the construction process but may be worsened by compounding issues.

  • Concrete is one of the most commonly used materials in home construction. When pockets of air are not removed, or the mixture is not allowed to cure properly the concrete can crack, which allows water to gradually force its way through the wall.
  • Foundations (footings) are horizontal pads that define the perimeter of foundation walls. When footings are too narrow or are not laid deep enough, they are susceptible to movement caused by soil erosion.
  • Gutters and downspouts are used to catch rain water as it falls, and discharge it away from houses and buildings or into underground storm drains. When gutters are clogged or downspouts are broken rainwater is absorbed by the soil near the foundation, increasing hydrostatic pressure.
  • Subsidence is the downward shifting of soil under or near a structure's foundation caused by water saturation, the removal of groundwater or as an aftereffect of mining.
  • Weeping tile is a porous plastic drain pipe installed around the perimeter of the house. The main purpose of external weeping tile is preventing water from getting into the basement. Internal weeping tile systems redirect water to the sump pump pit. When drain pipes are clogged or damaged excess water puts pressure on internal walls and basement floor.

Sump pumps[edit]

There are various types of sump pump systems used for basement waterproofing. The majority of sump pump systems are submersible. A sump well is installed in the ground and a submersible pump is placed in the bottom of the pit. Once the well fills up with water, a floating mechanism on the pump allows the pump to turn on and the water is discharged.

Warning signs of water damage[edit]

Signs that water is seeping into a basement or crawlspace often take years to develop and may not be easily visible. Over time, multiple signs of damage will become evident and could lead to structural failure.

  • Cracked walls: Cracks may be horizontal, vertical, diagonal or stair-stepped. Severe pressure or structural damage is evident by widening cracks.
  • Buckling walls: Usually caused by hydrostatic pressure. Walls appear to be bowed inward.
  • Peeling Paint: Water seeping through walls may lead to bubbling or peeling paint along basement walls.[3]
  • Efflorescence: White, powdery residue found on basement walls near the floor.
  • Mold: Fungi that usually grow in damp, dark areas and can cause respiratory problems after prolonged exposure.


  1. ^ Kearny, Nancy. "Tackling that leaky basement". Retrieved 2 November 2015. 
  2. ^ Carter, Tim. "How to redirect water around a damp garage". The Washington Post. Retrieved 2 November 2015. 
  3. ^ Chodorov, Jill. "Basement flooding may put a damper on your home sale". The Washington Post. Retrieved 2 November 2015.