Trained Eye


Concrete Spalling & Wet Basement


Mr Marantz

I have a couple of issues with my basement that I hope you can help me in pinpointing the problems and possible solutions. We are currently renovating our basement and ran into a few unusual items.

Some background on the house:

I have discovered that it was moved onto the current foundation around 1973. The front door is inset 6 inches from the outside wall, but the basement did not allow for this, so our final step to the front door is essentially part of the ceiling for the basement. We have had problems with floor moisture in the basement that we solved with a sump pump installation a few years ago.

The first issue is that we are getting pieces of concrete flaking off the walls. These are as large as a pocketbook, occur only above the ground line, and are most prevalent right beside the floor joists. I am unsure if this is moisture related or from the house move. Secondly, there is also a large amount of mildew on one wall, mainly beneath the doorway, which I understand, but also across the entire upper wall. I will be getting the front door relocated, but am concerned about the fact that there was mildew all across the upper wall. I also wonder if this is the only thing I need to concern myself with.

Thank you for your time.


The first thing to address before attempting any basement development for a Rec-room or living space is moisture intrusion. If a basement is very damp or has ongoing seepage problems, adding living space can be hazardous to the health of the inhabitants, especially if they have Asthma, allergies or sensitivities to mould. Finding the source of the moisture and taking measures to stop or minimize its entrance into the home is essential before renovations. Re-grading around foundations, replacing rotten basement windows, extending eavestrough downspouts and other maintenance items are critical. Installing a sump pump, such as the one mentioned, may also help to remove excess moisture from the soil around and below the foundation.

Seeing visible mould or mildew on bare foundation walls is a strong indication of a damp basement and problems with moisture. This may range greatly from a small amount of mould from condensation of warm house air on the cold foundation walls to major mould from other sources. Drying clothes on a clothesline in basements, disconnected dryer vents and downspouts, and cracks or holes in the foundation below grade are common sources of moisture. If a basement shower or bathtub is in place without a properly vented ceiling fan, moisture and mould are a certainty. If any of these issues or defects exists, they must be repaired or remedied to stop obvious sources of dampness.

If a white powder, called efflourescence, is seen on the bare concrete walls along with the flaking concrete observed, there may be a more serious concern. Foundation walls are normally waterproofed after curing on the exterior with a Bitumen-based compound that may wear after many years. This is a tar-like material that may lose its ability to provide proper waterproofing on older foundations. If this has occurred, proper grading, and downspout extensions may be enough to prevent seepage, all but in the wettest conditions. During heavy summer rains or from rapid snowmelt, moisture intrusion into the basement may still be experienced. The most common, yet costly remedial action for this problem is exterior excavation around the foundation. Once the soil is dug away from the concrete, it can be cleaned and new waterproofing or a membrane can be installed to replace the worn damproofing material. Older plugged weeping tiles are often replaced and cleared, at the same time, to allow for proper drainage.

The flaking concrete may be due to swelling of the floor joists if they are surrounded by concrete at the top of the foundation. If the floor joists are not well sealed at the ends on the exterior, they may absorb moisture and swell, causing the concrete to flake around them. The joists should be well insulated and sealed on the inside and checked to see that they are not exposed to rain and snow on the outside. If they are absorbing water from outside, they will eventually rot and cause a major problem.

The main area of mould is likely caused by water coming in under the improperly located doorsill. Once the door is relocated and properly sealed, the moisture needed for mould growth should be eliminated and it can be cleaned or removed. The mould along the top of the rest of the foundation wall is likely due to poor air sealing of the floor system, when moved on to the new foundation. This area between all the floor joists should be well caulked or sealed with blow-in foam insulation and properly insulated to prevent air leakage. Cold winter air coming in these areas will cause condensation of the warm house air when they meet. Conditions for mould growth will be ideal, due to the poor air circulation. Any mould found on foundation walls should be cleaned thoroughly with a 10% bleach solution, or with a commercial quality disinfectant cleaner. Once dry and clean, all foundation walls should have proper insulation and air-vapour barrier installed to prevent condensation, before wall coverings are installed.