Site Conditions and Constraints


How does a small lot limit my on-site system? 

Some lots have an area that is too small to fit a conventional leaching bed. In cases such as these there may be alternative systems that require a smaller area which can be considered such as filter beds or shallow buried trench systems. The use of an advanced treatment unit can also reduce the foot print of the soil absorption system by up to one third.

How does a high water table affect the installation of an on-site system?

All absorption trench systems must meet a minimum vertical separation distance to the seasonally high water table of 900 mm from the bottom of the trench. In areas where the water table does not permit the installation of conventional inground absorption trenches, a raised system can be installed. This can be a raised conventional system, or a raised filter bed system. Alternatively, an advanced treatment unit with an area bed could also be installed as some area beds have reduced vertical separation distances.

What is an inappropriate soil and how does it limit my on-site system? 

An inappropriate soil for a conventional inground on-site system is one where the percolation time (T-time) is greater than 50 min/cm or less than 1 min/cm. (OBC Section In this case a raised bed must be installed. Alternatively, a shallow buried trench system may be installed in native soils with a T-time from 1 min to 125 min.

Are there any other limitations on my on-site system?

There are a number of limitations on the use of an on-site system including:

  • Cannot pave over the leaching bed
  • Should not plant trees in the leaching bed
  • Cannot drive heavy equipment over the leaching bed
  • Must meet all minimum distance requirements: to well; to property line; to house/structure; to river, lake or stream or spring as defined in the Ontario Building Code
  • Septic tank must be accessible

Why is there a loading rate for a raised bed?

The OBC require that the mantle area for a raised bed or a filter bed be designed for a specified loading rate. The OBC sets a loading rate to prevent effluent break out from the sides of the leaching bed or mantle area where difficult soil conditions exist. It has been the experience of enforcement agencies that the construction of raised beds and filter beds under such conditions may require that the mantle area be extended for more than the 15 meter minimum.

Which is better to cover the stone in an absorption trench: untreated building paper or a geotextile fabric?

Section of OBC requires that the stone covering the distribution pipe be covered with untreated building paper or a permeable geotextile fabric. The purpose of this separator is to prevent soil or leaching bed fill from entering the stone during backfill. Although both are allowed, the untreated building paper will decompose within a few months compared to a geotextile which will last the life of the tile bed.

Clearance Distances

The Ontario Building Code has strict requirements on the distances from the on-site system to any nearby structures, wells, watercourses or property lines. It is important for designers to take note of these at the original site visit and during the designing process.

clear Clearance Distances for Class 1, 2 and 3 Sewage Systems
Table Minimum Clearances for Distribution Piping
Table Minimum Clearances for Holding Tanks
Table Minimum Clearances for Treatment Units