When a new subdivision is designed, one of the most important tasks done by design engineers is the assignment of lot grades. Proper lot grading ensures that sudden changes in elevation within a subdivision are minimized. More importantly, lot grading ensures that proper drainage will occur in new subdivisions.
Lot grades are established to run towards the public roadway or rear lane. Inadvertent water runoff onto adjacent properties is not desirable. Because of these important factors, lot grades for individual lots cannot be changed by developers or homeowners. It is the homeowner's responsibility to ensure that future developments do not interfere with proper drainage and cause water runoff onto adjacent properties and create problems between neighbours.
Failure to adhere to designed lot grades can result in the homeowner being responsible for constructing a retaining wall or other drainage facility costs.
Homeowners are advised that when building a rear garage or shed, lane elevations should be determined so that the structure can be built to an elevation that will prevent future drainage problems.
The following are definitions of lot grade terms used in site plans:
Lot Grade at Building:
Finished landscape elevation after construction is finished, includes lawn.
Sewer Invert at Property Line:
Elevation of the sewer service at the property line.
Full Basement Requirements:
There are two general reasons why full basements are required on some lots:
- Lot is leveled with significant fill during initial grading. Full basement ensures the house footings get put down in undisturbed soil.
- Full Basement design may be put on a lot with a significant slope from front to back or back to front. Again this is so the footings get down to undisturbed soil and to allow for sufficient frost protection.
Placing footings on undisturbed soil minimizes settling and shifting of the structure.
Walkout basements are viable when the difference between lot grades at building and the finished elevation at the rear of lot is a minimum of two metres.
Direction of Drainage:
Positive drainage away from any building on a lot must be maintained, but should not be excessive. Grades from foundation wall to property line should not exceed 6 percent, especially in sideyards. Over a standard 1.5m sideyard, the change in grade should not exceed 9cm.
On any lot where a portion of the lot is higher than the lot grade at building, the drainage directions on the subdivision elevation maps shown must be maintained.
Certain types of lots accommodate certain housing types. For instance, grade discrepancies can be minimized if split level homes are built with the high side of the house on the high side of the lot.
Lot grades at infill sites should be maintained at the same elevations as established for the site at the time of subdivision. New structures should be constructed so that may not cause any drainage problems.