How to flood proof your basement

Flooding may be the new normal – how can we protect our home?

By Charles Weiner

It never seems to stop raining! At first it was the corner of the basement wall, a bit of staining on the concrete, then a small pool of water on the floor. A week later they are calling for 50 mm. You go down the basement stairs and to your horror there is a foot of sewage water on the floor. You hear on the radio that because of the heavy rainfall, the city storm sewers are backing up and the main sewage system is overcapacity. The excess water has nowhere to go but straight up the sewage pipe and into your basement. Welcome to the future.

Fortunately with a bit of foresight all scenarios that involve flooding in your basement can be avoided, including the backup of sewage.

Most homeowners have or debated having their house waterproofed, but the term is misleading. What contractors call waterproofing is what engineers call damp-proofing. Under normal circumstances, damp-proofing your home by installing membranes and drainage is good enough, but these are not normal times. Now a homeowner (including an owner of a new home) needs to flood proof their foundations.

There are two circumstances where flood proofing your home can be difficult. One is if you have built on a flood plain, and the other is if the water table rises in your area and cannot drain away. This is more common in rural areas especially along the Ottawa River. Other than these scenarios, there are simple, affordable methods of flood proofing your home.

The only way to prevent water from getting into your home is to lock it out. All the membranes in the world cannot achieve this. To lock out water you need concrete, and the best place to start is at the bottom of the house. Most older homes in the Glebe do not have exterior drainage and when water from excessive rain builds up it will enter the home either from below the footing or in the case of a stone or rubble wall, below the foundation walls. Water entering from below the structure will build underneath the interior concrete floor. The hydrostatic pressure forces the water out from cracks in the floor causing the water to pool on the floor. In rare cases the pressure can be so great that it lifts the floor or fills the basement cavity completely with water. This, as we know from recent history, is more common in flood plain areas.

Flood proofing steps

Creating a lock to prevent water from entering from either below the footing or foundation wall or between the footing and the foundation wall requires two steps. The first is to slope the dirt on the outside of the footing at a maximum of 45 degrees outwards from the house. I usually do not exceed 22 degrees so that even in the most adverse circumstances the foundation cannot be undermined. Concrete is poured over the sloped non-excavated soil forming in essence an extended footing that lowers the base of the wall by up to five inches. It also moves the water away from the wall, preventing it from working through the lower non-excavated soil.

A second bench footing is poured on top of this preventive footing to lock the water out from between the footing and walls. The top of the bench rises 12 inches along the base of the foundation and is sloped to further keep the water from building up along the original structure. Foundations made of blocks are extremely vulnerable to lateral weight so it is important that the weight of the bench is taken by the new lower footing. This prevents any extra weight being placed on the original foundation wall. I cannot stress how important this is, especially for deeper foundations or block walls. I use a structural or geo-tech engineer to outline exact specifications for the process in these cases.

Moving the water away from the base of the wall also requires drainage. Most older houses in the Glebe and surrounding area do not have exterior drainage tile beds. Depending on the lay of the land this may not prove essential but many homes will need it in the near future. The problem is that the city storm sewers can fill, as recent history shows, past capacity. If your drainage is connected to their system, water can back up into your home’s weeping tile bed.

A second reason that prevents homeowners from connecting their weeping tile system to the city is the exorbitant price they charge for the hook-up. This can run from $15,000 upwards. Still I would recommend the hook-up for homes with poor soil drainage.

I am not a fan of sump pumps because if one fails or the hydro goes out for a period longer than the back-up battery charge lasts, your basement can fill like a swimming pool. Water outside should stay outside. Again in rural areas the sump pump may be the only alternative.

We now come to the preparation of the above-the-new-bench foundation wall. After repairs, all walls are covered with two coats of a waterproof concrete-based coating called Thermoshield®. In some ways this functions as a parging coat but unlike normal parging, Thermoshield is 100 per cent moisture proof. It is the Rolls Royce of products and works on stone, concrete, block walls and repaired rubble walls.

The application of Thermoshield requires first grinding and washing off all tar residues from the existing foundation. Usually tar is not present on stone foundations. A coating of tar can be applied after the Thermoshield is installed. Following this, we can put ship-lock polystyrene on all concrete walls, block or rubble foundations 2.5 inches below grade. It will glue itself tightly to the tar backing and then is pressed in place by the soil. I do not use fasteners when possible so that there are no holes for water to penetrate. The bottom of the polystyrene is held in place by a thick cover of polymer cement. I use a special product made to repair seawalls and dams. Then the base of the polystyrene and all surfaces below, including the top of the first footing and bench, are covered with Thermoshield.

Finally, if you live in a heavy clay area, backfill can be replaced with a fast draining aggregate like sand. This is usually recommended for weakened foundations such as rubble or block where wet clay is a greater structural burden on the wall. This is not needed in 99 per cent of cases.

All houses are unique and require examination and calculation before the correct procedure to flood proof is employed.

Preventing sewage backup

Oh yes, as for the smelly stuff? Put a back up stop valve on your sewage pipe. This is done right at the interior front wall of the basement. It requires opening a small area of the floor but the job is done in a day and will prevent anything unwanted from making its way backup the pipe. Every home should have one.

Charles Weiner is an Ottawa structural expert and foundation repair specialist. He can be reached at or 613-915-8377.

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