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Extension > Environment > Housing Technology > Moisture Management > Living on water

Living on water

By Richard Stone, Extension Educator in Housing Technology
Reviewed 2010

Photo by Richard Stone, University of Minnesota

My brother has a cute plaque at his lake home that says "A Place on the Water is a little closer to Heaven." Everyone who has been there agrees that in his case, with the home sitting up on a bluff overlooking the lake, that statement couldn't seem more true no matter what the season. Unfortunately, not every property owner has a lot with those same features or has had the same results.

A few years ago, soon after ice-out, I was asked to visit another lake home regarding water in the crawl space. When I arrived, I could see the water was pretty far up onto the front lawn. The homeowner who had invited me there opened the trapdoor to the crawlspace and asked if they should be concerned about the water level. It was not very far below the floor joists. Pumping was not an option, as they would be trying to drop the level of the entire lake. This homeowner was not looking at a very good menu of options. Sometimes, a foundation should just be set a little higher to avoid this type of future dilemma. The best time to make those decisions is before the new home is first built.

Flooded basement

Photo by Richard Stone, University of Minnesota

Water just under slab

Photo by Richard Stone, University of Minnesota

Gravel in dry site

Photo by Richard Stone, University of Minnesota

Sump

Photo by Richard Stone, University of Minnesota

Drainage board and tile

Photo by Richard Stone, University of Minnesota

Lakeshore lots are not the only place where this issue comes up. As better and drier building locations are usually selected first to be built out, the last remaining properties may have a variety of challenging conditions that need to be addressed. One of those challenges could be a water table that is high enough to impact the foundation choices that are available. On occasion, I have been contacted for advice about placing footings that will be below the water table. One builder had the excavator dig the hole per specifications for a basement, but not having checked the ground water level in the design stage he now had some serious issues to work through. Even if it is only a "seasonal" high water level, building in the water seems risky at best. Building that close to the water table would seem to be inviting customer callbacks or worse. The water table is not necessarily where you think it might be by looking at the contour of the land.

Do we know what we are building on? Is the grade original or has the landscape been changed, moving the water features around and perhaps leaving an old, established pond buried under grade that could still be intact enough to trap water and flood a basement? A builder should know what is under the surface and design the home to fit the circumstances. Once the design is sold and the hole is dug, it will be difficult to change course. No one wants to end up with a basement full of water, especially after the house has been completed.

If we build during a relatively dry time, will the ground water level be higher at a future point in time? If the long term ground water level has been established, the design used for the lot can be selected with confidence and the foundation elevation can be set safely. Many homes near water, either on natural lakes or ponds, or on created water features (storm water retention ponds) are built with slab-on-grade foundations or walk-outs that are not very far above the surface water elevation of adjacent water features. As shown by the example given earlier, the high water potentials must be known and respected to stay high and dry. Building just a little above the water level may not be much of an improvement when it comes to keeping the lowest level of the home as dry as possible.

To avoid risk, we can employ effective water management strategies to reduce the possibility of water damage that could result in the loss of both dollars and reputation. As described above, knowing the location of the ground water below a building lot is a great place to start. Next, the excavation should be well drained. A layer of washed rock several inches thick under the slab will help keep the slab dry by directing the water to either daylight drainage or into a sump basin. The air space in the gravel field acts as a capillary break and at the same time provides a good base for a soil gas system. If drain tile is placed around the inside of the footings and connected to a sump basin and all interior footings have crossover pipes through them, the basement is two steps on the way to an effective soil gas system. To keep moisture and other pollutants from entering the home and to prevent the "short circuiting" of soil gas system air flows, slab edges and all slab penetrations including tub openings, the sump system cover, and pipe openings should be sealed.

On the exterior of the foundation, drain rock or drainage board should be placed to move any surface water quickly down to the drainage tile system at the footings. It may be a good idea to keep the interior and exterior drain tile separated if the exterior part of the drainage system runs to daylight. The soil gas system on the interior may function more effectively if it is contained inside the perimeter footings and not connected to drainage tile that's open to daylight. After carefully placing all the underground drainage components, make it easier on them by sloping the final grade about five percent down and away from the building for at least ten feet and use gutters and downspouts to direct water from the roof at least ten feet out from the foundation so it will run away from the building. If the home is cut into the side of a slope, a swale should be designed and cut into the yard at least ten feet up-slope from the home to direct surface run-off away from the foundation.

Water is a major contributing factor to both durability and indoor environmental quality issues in homes. By aggressively addressing ground water issues during both the design and construction processes, builders can take control of the situation and reduce risk.

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