Many parts of the Houston area have always been in a floodplain. But in the summer of 2001, Houstonians became very aware of the potential problems of living in a floodplain. Although it wasn ’t a hurricane, Tropical Storm Allison caused more physical damage to the Houston area than any hurricane on record. Most of this damage was from the flooding of homes, businesses, and the medical center. After a two-year study, new flood maps were developed and most cities began enforcing stricter rules when building in the floodplain. An understanding of these rules can help you and your builder plan for a foundation that will be attractive, structurally sound, and cost effective.

To qualify for the lowest flood insurance rates, the Federal Emergency Management Agency (FEMA) requires that the finished floor be a minimum of 12-inches above the Base Flood Elevation (BFE). Most of the cities in our area use the 12 inch rule, however some are more restrictive. Harris County, for example, requires that homes built outside the municipalities have finished floors 18 inches above BFE. FEMA rules also prohibit bringing fill into the floodplain. This means that you cannot add fill on your lot to raise the slab height to meet the 12-inch rule. Your only option is to build your new home on some type of raised foundation.

There are several options available for building on a raised foundation. The proper name for this is a crawl space foundation. However, in our area they are usually referred to as “pier and beam foundations” and were used for most of Houston’s early homes. Wood beams span brick or concrete blocks and provide support for the floor joists and wooden subfloor. Today, we usually install piers under the blocks for better structural support. The building code requires 18 inches of clear “crawl space” under a raised foundation. Adding the depth of your floor joists, typically 12 inches, the minimum height of a raised foundation is about 30 inches. Depending on the Base Flood Elevation, however, the required height can be much higher.

 If the house is covered with brick or stucco, the exterior wood beams are replaced with concrete grade beams at ground level and a pony wall is built to support the floor joists. Your pony wall can be built out of treated lumber, concrete masonry units (CMUs) or a solid concrete stem wall. The type of pony wall chosen is determined by engineering requirements, cost and personal preference.  Building a raised foundation using a grade beam and pony wall subjects you to more regulations and several potential problems, mostly moisture related. Recently, some builders have been building pony walls on a solid concrete slab poured right at ground level. This solves most of the moisture issues but generally is a more expensive technique.

The purpose of a raised foundation is to both raise the house above potential flood waters and to allow

What happens if we do have a water event that is deeper than the bottom of the flood vent? Obviously water will be trapped inside the grade beams, which could cause serious problems. Neither FEMA nor the cities have regulations that address this situation. As a simple solution, many builders have begun installing yard drains under their raised foundations. Once the flood waters recede, the water trapped inside the grade beam will be able to drain away.

The other moisture problem with raised foundations involves ventilation. Hot, moist air trapped inside the crawl space can cause significant damage to floor joists, the subfloor and even finished floors inside your home. With a traditional “pier and beam” foundation there is plenty of open space for air to flow through the raised foundation. However, when the crawl space is enclosed with a pony wall, air circulation is drastically reduced. Technically, the flood vents meet the building code ventilation requirement. Most builders, however, take additional steps to control moisture under a raised foundation.

The most effective and simplest first step is to cover the exposed dirt with a thick sheet of plastic. This retards the movement of moisture from the ground into the crawl space. Sometimes this plastic sheet is also covered with crushed gravel to ensure it stays in place. Additional measures include adding fans that cycle on and off, dropping an air conditioning supply vent into the crawl space, and using vapor retarding foam insulation. The finished floor inside the home should also be protected from moisture moving through the subfloor.

Building a home on a raised foundation gives it a look and feel that can never be duplicated with a slab on grade. However, the builder and homeowner have a shared responsibility to ensure that the foundation lasts for the life of the home. In our hot and humid climate, the builder must take extra precautions to protect against moisture problems under the foundation. The homeowner is also responsible for proper maintenance of drainage and flood vents to protect the home in the event of another Allison. As proven by the many older homes in Houston, a raised foundation can provide decades of service.