Building on a Wet Site

Ramakant writes: We are in discussion with a builder to buy a lot to build our new home. Currently it’s a wooded area. However we discovered that this particular lot had a pond at one time covering almost 50% of the lot.

Is it safe to build a new home on this lot? If so what are the things we need to consider?

The builder says that we just need to stabilize the foundation with more concrete and other things. He says that this is nothing unusual and that they do very often.

Steve Bliss, of, responds: I would proceed with caution because a wet building site could bring you headaches for years to come. If any part of your foundation or septic system (if needed), is below the water table for part of the year, then you will have problems. It is nearly impossible to keep water out of a structure built below the water table. Also, building on wet ground can raise moisture levels inside the house, leading to mold and mildew.

If you are building on a part of the site that is “high and dry” and well above the surface level of the former pond, then your house should be fine. In fact, it’s possible that the pond could be restored and become a positive feature of your land. However, you still need to address the pond area and understand why there was a pond there and whether it will return – or possibly remain a wet and swampy portion of your land.

Whether natural, man-made, or a little of both – that is, a natural spring or pond that was enlarged by a previous owner – any pond needs a water source. The source is either subsurface water or surface water, or a combination of the two. Subsurface water is either the water table or a spring, while surface water is either a stream or rain and snow runoff from a large area. In either case, you can expect that the water source will increase and the water table will rise during the rainy season or after the snow melts in cold climates.

Since there was once a sizable pond here, there are still water sources and the water could certainly end up in your basement or crawlspace. A wet site can even cause problems with a slab foundation as the excess moisture in the soil can pass through the concrete and into your home, resulting in high moisture levels and possibly mold and mildew.

Homes can be safely built on wet sites, but special drainage around the house and foundation and an effective moisture barrier below the foundation will be required. As for the type of foundation needed, that depends on the type of soil and the weight of the structure. Again, a house can be built on any type of soil, but the weaker the soil, the more difficult and expensive it is to build a suitable foundation.

Just because something is possible, it does not mean it’s a good idea. Before proceeding, I would get the opinion of an engineer who specializes in this area – called a soils, geotechnical, or civil engineer. In an hour’s time, they could tell you whether it is feasible and wise to build on this site. Also be aware that special zoning rules may apply if the land is legally considered a regulated wetlands.

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