How basements start leaking

A rainstorm or melting snow temporarily raises the groundwater level. When water accumulates around the foundation, hydrostatic pressure builds up and causes the basement to leak. Clay-rich soils do not drain well and hold rainwater right against the foundation walls. Water pushes its way inside through cracks or joints and the pores in concrete.

As houses settle, concrete develops stress cracks that leak water. Exterior waterproofing cracks and disintegrates or separates due to the "alkali attack." Water also corrodes imbedded steel, which expands and cracks the concrete.

Water penetrates into the pores in concrete, dissolves alkalis, and enlarges the pores. As concrete ages, it becomes more and more porous. Initially, the seeping water evaporates, leaving on the surface lime that reacts with carbon dioxide in the air and forms limestone. Efflorescence is a telltale sign of capillary water seepage.

Slabs get even less protection than basement walls. The plastic "vapor barrier" soon disintegrates due to the lime in concrete and over time, the layer of gravel ("drainage pad") silts up. The concrete slab pulls in groundwater against gravity by capillary suction.

How can water get through concrete?

Concrete is naturally porous - typically 10 to 18 percent air (extremes 2-60%). To make concrete "workable," a water/cement ratio of 0.45-0.50 is used but cement only needs 0.25 w/c ratio for hydration. As concrete cures, the surplus water escapes to its surface where it evaporates. This "bleeding water" leaves a network of tiny capillaries (pores).

We cannot see the pores. Their size ranges from 3 nm (millionths of a mm) to 0.1 mm (the diameter of a human hair). The median is about 1 micron (1,000 nm) but a water molecule is 3,000 times smaller (0.28 nm). By all logic, concrete should leak like a sieve!

So, what holds the water back? Not the concrete but the physics of water itself. Although not as viscous as molasses, it takes pressure to push water through dry microscopic pores. Surface tension holds the "blob" of water inside the dry pore until its surface gets wet. But add a little water pressure during occasional rainstorms or condensation on the interior and after a while, the pores get wet and the concrete starts leaking. Concrete is hygroscopic - it attracts water by sorption and once wet, the pores draw in groundwater by wicking action (capillary suction).

But nothing holds back gases - invisible water vapor and radon gas flow right through the pores!

Types of Basement

A "walk-out" or "daylight" basement is a modern architectural form where the house is situated on a slope and part of the basement is above ground. Occupants can walk out at that point without having to use the stairs. For example, if the ground slopes downwards towards the back of the house, the basement is at or above grade (ground level) at the back of the house. It is a modern design because of the added complexity of uneven foundations; where the basement is above grade, the foundation is deeper at that point and must still be below the frost line.

In a "look-out" basement, the basement walls extend sufficiently above ground level that some of the basement windows are above ground level. Where the site slopes gently and is insufficient for a walk-out basement, a look-out basement will result. Sometimes, a look-out basement is deliberately constructed even on a flat site. The advantage is that the basement windows are all above grade. The disadvantage is that the main floor entry is above grade as well, necessitating steps to get up to the main floor. The raised bungalow design solves this by lowering the entry half-way between the main floor and basement to make a dramatic, high-ceiling foyer. It is a very economical design because the basement is shallower, and excavation costs are minimized.

A “walk-up” basement is any basement that has an exterior entrance via a stairwell. Some designs cover the stairwell with angled “basement doors” or "bulkhead doors" to keep rain water from accumulating in the stairwell.

When initially built, the main floor joists are often exposed and the walls and floors concrete (with insulation, where appropriate). Unfinished basements allow for easy access to the main floor for renovation to the main floor. Finishing the basement can add significant floor space to a house (doubling it in the case of a bungalow) and is a major renovation project.

Cellars

A cellar is a type of basement, primarily used for the storage of food and drink (especially wine) for use throughout the year. A cellar is intended to remain at a constant cool (not freezing) temperature all year round. Cellars are more common in older houses than in modern houses, and were important shelters from air raids during World War II.

Except for Britain, cellars are popular in most western countries. In Britain, people tend to store food and drink in a garage, if at all. However, the majority of continental Europeans have cellars. In North America, cellars usually are found in rural or older homes.

Crawlspace

A crawlspace (as the name suggests) is a type of basement in which one cannot stand up — the height may be as little as a foot, and the surface is often soil. While this cannot be used as living space, it can be used as storage, often for infrequently used items. These can be placed directly on the dirt, but it is more desirable to finish with either plastic or some sort of wood or concrete flooring. These are rarely included in houses as standard due to health and safety issues. However, they are frequently added on by home owners installing a small door into the foundations.

Basement Waterproofing and Crawl Space Moisture Control

If you have a wet basement or crawl space moisture problem of any kind, call us and we'll be there with experience and professionalism that will surpass your expectations. After all, drying a wet basement and wet crawl space is our business.

As a full-time Basement Waterproofing Contractor, Basement Systems strives to develop and provide superior basement waterproofing products and performance to solve wet basement and dirt crawl space problems.