Sump Systems and perforated basins.

perforatedPerforated sump basins used for a standard foundation drainage system are the wrong tool for the job, yet are common in new construction in the Toronto, Ontario, Canada area. They are full of holes (no surprise!) so water entering the basin will escape to surrounding soil under the slab, until it is fully saturated. When I test these basins by pouring water in, it will often escape and not activate the pump. The pump will only activate if the surrounding soil or other materials acts as a well and allow the water to rise.

The codes (OBC) and common sense say that storm water may discharge to a sump or drywell or to “daylight” ( outside to a drainage ditch) but NOT to the underside of the slab.  The codes do not specify what type of basin is to be used, and unfortunately City building officials accept the perforated basins.  This doesn’t make it right. Rather it shows weakness in their understanding of the requirement and interpretation of the code.

Ontario Building Code 9.14.5.  Drainage Disposal:  Drainage Disposal (1)  Foundation drains shall drain to a sewer, drainage ditch or dry well.  Sump Pits (1)  Where gravity drainage is not practical, a covered sump with an automatic pump shall be installed to discharge the water into a sewer, drainage ditch or dry well. NOT UNDER THE SLAB

Collecting all the water from the perimeter of the foundation drainage, and discharging it all under the basement slab close to the footings just is not a good idea. There is a code statement that could apply to this:

Ontario Building Code Section 9.16.  Floors-on-Ground: 9.16.3.  Drainage:  Control of Water Ingress (1)  Except as provided in Article or where it can be shown to be unnecessary, ingress of water underneath a floor-on-ground shall be prevented by grading or drainage.

A solid basin is the right tool for standard foundation drainage systems. The perforated basins are intended for collection from problem soil areas, and would typically be installed in a central location, surrounded by granular fill that facilitates water flow towards the basin.  The perforated basin may also act as a well and collect water from deeper than necessary. Most basins are about 24 inches deep. Digging a 24 inch hole under the house is likely to attract water from the surrounding soil, and this can result in significant and unwarranted water collection.