PART 4 - WET FLOOD PROOFING CONSTRUCTION AND FEATURES
In Part 3 of Designing A Flood Proof House, I explained the range of dry flood proofing measures that can be taken both on your property and in your house itself. In severe flooding, the water may rise high enough that dry flood proofing measures are no longer workable. If the water breaches your last defense, your house must be designed to survive water flowing into it to minimize flood damage. This design approach is called "wet flood proofing".
Building Configuration - Wet Flood Proof
A new house is best positioned for surviving a flood of any type because the structure and the spaces can be arranged to cope with floodwater. This is the first half of wet flood proof strategy. Assume that everything below the flood level will be damaged, and prioritize what should be relocated higher up.
A basement will always be much more at risk of flooding than any other part of the building; most of the deaths in the New York deadly floods caused by Hurricane Ida's storm surge were tenants in basement suites. Avoid a basement unless it's highly important to your needs. If you do have a basement, relocate all equipment out of it, to a higher floor.
Consider also designating the ground floor of a house as the sacrificial level that allows water to flow through it. This may be a mostly uninhabited space or garage, or a very open living space with patio furniture designed for exposure to water. You do not need to design a house on stilts as ancient communities historically did in Southeast Asia and other coastal flood-prone areas. Avoid interior walls, and design the loadbearing exterior walls to withstand water pressure and also impacts by floating debris. Effective "wet flood proofing" not only allows water to flow in but to flow freely through the level and exit the home; better flow and less resistance decreases the risk of structural damage.
The structure should be designed with stronger joints (e.g. strapping or tie-downs) that help keep the structure as a whole together. A portion of a wood frame house could easily be pulled out and cause the roof to collapse. Foundations must be built deeper; soil washed away near the surface will undermine shallow foundations and cause partial or complete collapse of your house.
Wet Flood Proofing Features
Whether you fully inhabit the ground floor level or not, ensure that all equipment and especially electrical wiring is located well above the ground. There's no single appropriate height since the water levels are specific to the location and to the particular flood event. The higher the installation, the lower the risk of flood waters reaching the equipment; 1.5m or 5' is likely as high as practical for electrical outlets.
Drainage piping (i.e. stormwater and sanitary sewer pipes) cannot be located above ground, so install backflow prevention valves. They allow normal flow from the house into the sewer system but prevent contents of flooded sewage pipes being forced back up into the building.
In contrast, to help allow surface floodwaters through walls above the ground, use devices called flood vents - also known as flood ports or flood gates - that are normally tightly closed but when submerged will pop open.
Reduce clutter. The first reason is that fewer possibly dangerous items (e.g. air conditioning units, small appliances) carried around by floodwater reduces the amount of damage and the time spent sorting through flotsam for important documents. The second reason addresses the other half of wet flood proofing strategy: reducing the cost of repair and cleanup by picking the right building materials.
The ground floor walls should be solid construction -- concrete, concrete block masonry, structural brick, or even cross-laminated timber (CLT) panels. The cavity walls (e.g. wood studs) in a wood frame house whether insulated or not become reservoirs for water, and wood framing will very quickly develop rot in the enclosed spaces. The face of the walls should also be durable materials such as tile, plaster, resin panels (plastic), or cement board (aka "green board"). Drywall (gypsum board) on the other hand easily absorbs water, swells, and begins to disintegrate. Typical MDF baseboards also will be destroyed.
Cabinetry and doors should be constructed of solid wood and plywood. Typically, the structure of cabinets is medium-density fibreboard (MDF) or high-density fibreboard (HDF / Masonite / hardboard), these materials will not survive being submerged. Doors are typically "hollow core" framed by HDF and will be damaged by flood water.
Floor materials should likewise be solid, durable construction. Stay away from wood flooring, laminate flooring, or carpet. Concrete floor slabs can be topped with ceramic or porcelain tile, appropriately installed sheet vinyl or linoleum, terrazzo, or the concrete itself can be polished. Rugs both large and small can be moved out of harm's way but otherwise provide a more comfortable floor.
The durable floor and wall finishes are far less susceptible to water damage and to staining, but they are also relatively easy to clean. Flood water carries not only suspended sediment (mud, silt, sand) but also hazardous chemical and biological contaminants.
After the area is cleaned, a forced ventilation system greatly helps surfaces dry out as it removes damp air. If you do have a basement, providing windows around the perimeter further encourages natural ventilation that will help dry out the space.
Proper Planning Can Save Your House
Designing your house to withstand, survive, and even remain mostly untouched by a flood event requires planning and careful thought. While some features and devices are important parts of a flood protection strategy, they are only pieces and must be incorporated into a larger strategy that will prepare you for almost any flood risk as natural disasters caused by shifting weather patterns and expanding urban development increase in frequency.
My SAPPHR Strategy™ includes these strategies. If you would like to discuss what the flood risk is to your house or how your new or remodeled home could include these techniques to prepare for a flood, please book a free call with me using the button below.