Reduce the risk of basement flooding...

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Option 13: Lot grading, backfilling and swales

Lot Grading

If you’ve ever had a look around newly constructed homes, you might notice that yards are gently graded away from houses. When a yard is graded in this manner, it helps keep water away from the home and foundation, and helps keep the basement dry. Most municipal governments in Canada have bylaws that require lot grading that directs water away from the home, as a properly graded yard can substantially reduce basement flood and moisture problems.

Over time, however, yards can lose this slope for a number of reasons. For example, landscaping work can impact lot grading, and over time, soil may settle and result in lot grading that directs water toward the home and foundation. New decks, additions to homes or alterations to your neighbour’s properties can also affect lot grading around your home.

When regrading lots to direct water flows away from the home, the following points should be considered:
• A drop of 10 to 15 cm over 1.5 metres from the wall of your home may be required to adequately direct water away from the home.

• Lot grading should direct water to an acceptable drainage outlet that can direct flows to municipal conveyance systems. Consult your local government to ensure that water from your lot is directed to the right place.
• Ensure that lot grading does not direct water in such a way as to create ice build-up on neighbouring properties or sidewalks.

• Ensure that water flows from your lot do not negatively affect neighbouring properties.

Swales are grassy depressions, like a shallow ditch, that help direct the flow of water away from homes. Swales generally direct water toward drainage infrastructure, including roadways or storm sewer catch basins. The same factors that affect lot grading can affect swales. For example, landscaping and the settling of soil can both damage swales.

Sheds and fences should not be placed in a manner that blocks drainage paths or catch basins. Consult your municipality to see if a drainage plan for your subdivision exists, and ensure you maintain the intended drainage plan if you are going to alter your yard by building fences or sheds.

When your home was constructed, the basement was dug from undisturbed soil. After the foundation was completed, soil was pushed back into the gap between your foundation walls and the undisturbed soil. This soil is not as compact as the rest of the undisturbed soil surrounding the foundation. This less compact soil is more permeable, and allows water to infiltrate into the ground beside the foundation.

Permeable soil next to the foundation can increase the chances of infiltration flooding in the home, and can increase flows into the weeping tile. If the weeping tile is connected to the municipal sewer system, excess water can increase overland flooding and sewer backup risk in the neighbourhood. If the weeping tile has been disconnected from the municipal system and is drained into a sump-pit, the sump-pump will have to run longer to pump excess water flows from the weeping tile.

As part of the lot grading improvements, a compact soil should be used to fill in any depressed areas directly beside the foundation wall. A compact soil, such as clay, should also be used to "cap" the backfill area, and should be properly sloped to keep water away from the home.