ASK THE INSPECTOR: Drain water out of new basement now

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This article was published 12/06/2010 (5628 days ago), so information in it may no longer be current.

QUESTION: I enjoy reading your column every week for the practical advice you have. I am hoping for some advice on a new condo that I recently pruchased that is currently in construction.

It is a townhouse-style condo that is two stories, plus a basement. From the very beginning, the basement floor has been covered in what looks like at least a foot of water. I was told by the real estate agent that is selling these units for the builder that it will eventually be pumped out and the basement floor poured. I recently visited the condo again and it now is at the stage where the electrical and plumbing are done and the drywall is up. However, there is still the same amount of water in the basement.

Is that normal protocol? I am not concerned about the water as long it is removed and a proper floor poured, but is there a concern about mould forming in the walls and floors with so much humidity from the stagnant water in the house sitting in the basement all the time?

Thanks for any advice you might have on this. Susan Tsang, Winnipeg

ANSWER: I thought your question would be a good one to answer after the recent heavy rains that have flooded dozens of basements in our area. While the situation may be different in your condo under construction, the sources of the water may be the similar and the outcomes have the same concerns.

There are two main issues with the large amount of water sitting in the unfinished basement in your condo. Firstly, the question often arises in this situation of the effects of the high moisture content of the soil underneath the newly poured basement-floor slab.

I have heard builders and others tell customers that having the water in there is a good thing. The theory presented is that the soil is saturated and at its maximum density, so that when it dries out it will shrink and prevent the typical floor slab heaving and cracking seen in many new homes.

Unfortunately, this is not true and excessive moisture may increase the risk of cracking and heaving in the floor slab. Because the soil in this area is saturated, it will shrink excessively as it dries, which will increase the chance of further expansion when resoaked. If the soil is damp, but not completely saturated, before pouring of the slab it will minimize the large swings in shrinkage and expansion and prevent damage to the slab.

The next concern is the one that you have identified in your question. Having the drywall complete in the home with a small lake in the basement will increase the chance of mould growth due to high humidity. This will be compounded once the drywall is taped and painted, which adds considerably more moisture to the home as these materials dry.

Normal building materials will release a significant amount of water vapour into the environment of a new home, but this will be prevented from drying due to the additional moisture in the basement. Once the water is finally pumped out and the foundation insulated and covered with a plastic air/vapour barrier, moisture will be trapped in this wall cavity and may have difficulty escaping. This will be accompanied by the moisture that evaporates from the freshly poured concrete. It is quite common to see areas of mould and mildew behind the fibreglass insulation in basements in new homes for this reason.

Another issue that you have not inquired about should also be addressed. The plumbing drains and weeping tile pipes are installed on top of or slightly embedded in the soil below the basement floor slab and they may be affected by the water. With that much water in the basement, it will be very difficult to near-impossible for the plumber to install the proper plumbing drains and connections. The water will have to be removed, almost completely, before this can be completed. If the ground is swelled to its maximum due to the standing water, these pipes may shift significantly if installed immediately after the water is removed. This may prevent them from having the ideal slope for proper drainage, which can cause the drains to be sluggish.

It is likely that the water in the basement is primarily coming from the weeping tiles, but they should be terminating in a sump pit so that the water can be pumped back outside the house. Your builder may be reluctant to install the pit and pump before the concrete floor slab, for good reason. Because of the high moisture content of the soil, a newly installed plastic sump pit may heave and pop right out of the ground if it does not have the weight of the concrete floor slab and pea gravel on top to hold it down.

Regardless, a temporary pump and pit could be easily installed to get rid of the excessive moisture or the proper pit temporarily held in place with braces that are removed after the floor is poured. Also, the drainage-tile piping installed underneath the floor slab should be installed with sufficient slope for drainage, which may be adversely affected if the soil heaves or shrinks too much.

To briefly summarize, and answer your direct question, the water should be pumped out as soon as possible to allow reasonable drying of the soil prior to completion of the insulation, drywall and floor slab. It doesn’t matter whether this is done with a temporary pump or with a proper sump pump and pit but you should put pressure on the builder to do it now, either way.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors — Manitoba (www.cahpi.mb.ca). Questions can be e-mailed to the address below. Ari can be reached at (204)291-5358 or check out his website at www.trainedeye.ca.

trainedeye@iname.com