How to reduce the risk of I/I in new sanitary sewers from the private side

construction pipe
It is essential that we reduce the risk of private side I/I when we build private side sewers. Photo Credit: Roman Milert,

By Barbara Robinson

The issue of inflow and infiltration (I/I) in new construction has been on the radar screen of engineers in Canada for some time. They have known for a long time that 50–60% of I/I in existing systems originates on the private side of the sewer system.

Until now, however, we’ve lacked the understanding of the underlying issues that would allow reducing the risk of I/I in new construction. With the increased intensity and frequency of storms associated with climate change, it is essential that we reduce the risk of private side I/I when we build private side sewers.

A recent publication, “Manual of Best Practices to Reduce Risk of Inflow and Infiltration in Private Side New Construction of Sanitary Sewers” provides practical, actionable solutions for building officials (and development engineering departments) to immediately implement strategies to reduce this I/I risk. The detailed research in this report was undertaken on the Ontario Building Code (OBC). However, issues discussed concerning specific Code references related to I/I appear to be the same as in the National Plumbing Code of Canada.

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Background and Methodology

This work is the result of five years of study of the factors contributing to I/I on the private side in new construction. The strategies presented are based on wide consultation with hundreds of stakeholders across Ontario and Canada, including municipal building and engineering staff, consultants, regulators, contractors, developers, drain layers, plumbers and other related groups. Stakeholders helped to identify gaps in guidelines, standards and codes, construction practice, inspection and testing, certification, jurisdiction, education and process, which contribute directly to the issue of excess I/I in new construction.

Consultation has included: formal surveys, training on the OBC, ongoing discussions with both a building officials stakeholder group and a municipal engineering staff stakeholder group (both developed for this work), direct questions on the Code and its interpretation posed to the building officials stakeholder group.

The work was also informed by the development of the CSA Z800 “Guidelines for Basement Flood Protection and Risk Reduction (2018)” and “Reducing the Risk of Inflow and Infiltration (I/I) in New Sewer Construction: A National Foundational Document (Standards Council of Canada, 2019)” and the direct engineering experience of myself.

This work is essentially an engineering assessment of the factors contributing to I/I on the private side, and recommendations to reduce this risk. In order to implement many of these recommendations, building departments and engineering development departments will need to work together.

The Ontario Building Code (OBC)

It is important for engineers to understand that application of the OBC, and its associated inspections, is fundamentally different to what occurs on the public side. Construction of sewers on the private side is not subject to full time inspection as it is on the public side. Building inspectors only attend site during Prescribed Notices (e.g., at specific milestones in the construction).

The Prescribed Notice that governs the Sanitary Building Sewers (SBS) requires that “the person to whom the Building Permit is issued shall notify the chief building official (CBO) of readiness for inspection and testing of building sewers and building drains” (paraphrased from OBC). This means that the building inspector only attends site to review the SBS when they are specifically contacted to undertake this inspection.

This review takes place when the pipe is already laid and jointed (though not buried). Building departments are permitted to review only a fraction of the SBS in a subdivision (e.g., 10%) as a representative sample of all work on site.

In addition, a developer does not usually install all the SBSs at once, but rather a few at a time. This requires the building inspector to make multiple trips to the site. This can be overwhelming for building departments, particularly if a lot of development is taking place at once. Necessarily, some inspections may get missed.

Finally, building inspectors perform “inspection” per the OBC, but do not guarantee the work (similar to engineering inspectors: engineers review, but cannot be responsible for, construction, per Professional Engineers of Ontario). The only party who can be responsible for the proper and adequate installation of the SBS is the contractor installing it, who works for the developer. It is the developer who has a contractual relationship with the municipality, and he is ultimately responsible for ensuring that leak-free infrastructure is delivered.

11F112To provide adequate treatment of sanitary sewage and effluent.
F113To minimize the risk of injury as a result of contact with sanitary sewage or partially treated effluent.
13F132To limit excessive demand on the infrastructure.
F133To limit excessive peak demand on the infrastructure.

Table 1: Functional statements from the OBC

Primary finding

The most significant finding of this research is that PVC pipe on the private side is not currently being installed according to existing CSA and ASTM standards, referenced in the OBC. These standards, which fall under the National Standard of Canada B1800-18, Thermoplastic Nonpressure Piping Compendium, specify the requirements for installation of the SBS, known in engineering as the private side lateral.

These standards are the same as those used on the public side sewer system, that is CAN/CSA 182.1 and 182.2, both of which state that they must be installed per CAN/CSA 182.11, which covers the design, installation, inspection, testing and acceptance of PVC pipe. Manufacturer’s literature explicitly recommends installation to these standards.

Plastic pipe must be installed with flexible pipe principles that recognize that performance relies on proper bedding, haunching (embedment), backfill and compaction. Unlike concrete pipe (which has its own structural integrity, without the support of soils), PVC pipe relies especially on the embedment of the side soils (haunching) to keep it round against compression by the weight of overlying soil.

Figure 1 shows the haunching zone which is essential for the performance of PVC pipe, and the potential deflection that occurs if haunching is not adequately provided and compacted.

Engineers will already be familiar with these requirements, because they are specified in Ontario Provincial Standard Specifications (OPSS) and Ontario Provincial Standard Drawings (OPSD) for plastic pipe and widely used in Ontario and the rest of Canada. The OBC, while referencing the relevant standards, includes only two sentences about the installation of the SBS, which are vague and inadequate. Specifically:

–“Nominally horizontal piping that is underground shall be supported on a base that is firm and continuous under the whole of the pipe.” OBC 2018

–“Where piping is installed underground, the backfill shall be carefully placed and tamped to a height of 300 mm over the top of the pipe and shall be free of stones, boulders, cinders and frozen earth.” OBC 2018

It should be noted that the 2018 OBC references 426 external standards, many of which need to be purchased. Building inspectors have a complex job that requires that they inspect for many issues and cannot possibly be familiar with (nor would they necessarily have access to) all the underlying standards that make up the Code.

Since building inspectors have widely reported little or no understanding of I/I, its risks and impacts, they have not generally been looking for risk factors for I/I when inspecting the SBS. It has been recommended that the OBC include the specific requirements from the standards, right in the Code, to assist building inspectors in performing their work.

Furthermore, CAN/CSA 182.11 requires that testing be performed after the pipe is buried. Currently, the OBC calls for leak testing prior to burial, which cannot detect joints that shift or separate during backfill operations. This is not in alignment with the governing standards and needs to be examined by Code officials.

The OBC requires that air or water tests of the SBS be held for fifteen minutes with zero leakage. For most contractors, this would represent a substantial slowdown in production, and they are very reluctant to perform it. This leak testing is not currently being performed by the vast majority of building departments in Ontario.

And finally, contractors have started to use clear stone as pipe bedding. While it is more expensive to supply, the labour costs are lower as it does not compact. However, clear stone is not explicitly permitted, and is not recommended because it acts like a French Drain. This allows water to travel along the trench bedding to the next available entry point to the sewer. Use of clear stone increases the risk of I/I in sewer pipe systems and is not used on the public side.

Additional findings

In addition to improving how we install PVC pipe on the private side, a number of other important factors have been identified as contributing to I/I risk on the private side.
The use of solvent welded pipe systems on the private side does not appear to have been approved by any of the standards agencies. All citations and testing in the background technical specifications refers to gasketed pipe, so it is unclear that solvent welded pipe is suitable for underground installation.

Furthermore, a two-step solvent weld system, whereby the solvent dissolves the PVC slightly and the cement then bonds it, is recommended. It is absolutely required in cold weather applications. Since the building inspector is not on-site during assembly of this pipe, the risk of poorly constructed joints is high. Initial set times for solvent weld joints range from one to twelve hours, depending on temperature, before the joint can be carefully handled. Therefore, using a solvent weld joint represents a significant risk of joint failure in the SBS.

The OBC still contains provisions that permit the discharge of foundation drains and storm sewage to the sanitary sewer. These should be removed from the Code to align with modern design and sanitary sewer use bylaws.

The inspection of the SBS to the sanitary sewer lateral at property line is implied in the Prescribed Notices, but not explicitly identified. Given that this location is a common source of I/I in older systems, it is recommended that this inspection be made more explicit to assist building inspectors in prioritizing this inspection.

Currently, many building departments in Ontario are not inspecting storm infrastructure on the private side. Storm drainage in new construction is often hung from the side of the foundation wall prior to backfill. This means there is a risk that inadequately installed storm sewers could leak down the basement wall and get into the sanitary sewer (or cause the sump pump to cycle). This inspection is required in the OBC, so it is recommended that building departments undertake it.

Another issue of concern identified in this work is the installation of backwater valves (BWV) on the sanitary building drain. Standard backwater valves used in Canada require a 2% slope across the valve to function adequately.

However, both the sanitary building drain (pipe under the concrete floor) and the sanitary building sewer, are currently only required to be laid at 1%. If BWVs are to be installed, the slopes on the upstream and downstream pipes need to be increased to 2%. Failure to adjust these slopes increases the risk that the BWV will become hung up with solids and will not work when flooding occurs.


The recommended best practice to immediately reduce the risk of I/I in the SBS uses the power of the Building Code itself. Division A of the Code outlines Objectives and Functional Statements which lay the groundwork for what the Code is trying to achieve. The Functional Statements that are of primary interest in reducing I/I risk relate to health and safety and demand on the infrastructure, and are summarized in Table 1.

Based on the Building Code Act, the building department cannot ask for requirements that are more stringent than the building code itself. Division B of the OBC contains “Acceptable Solutions”, which are sample ways in which the objectives can be achieved for each construction item. The CBO has the authority to accept an Alternative Solution provided that it achieves the same level of performance called for in the Functional Statements.

It is recommended that building departments offer the Alternative Solution of installing gasketed, SDR28 pipe that is haunched and installed without clear stone. Although the material costs for this Alternative Solution will increase, it will be offset by the time saved in holding solvent weld joints for the required amount of time and set up and performance of the leak test. Ultimately, it will cost less for contractors to use this Alternative Solution than to install the SBS as explicitly required in the OBC.

How can development engineering departments help?

Reducing the risk of I/I in private side sewers can be achieved immediately, if the will exists within a municipality. It will, however, require that building departments and development engineering departments work together, which is essential to reduce I/I risk.

Development engineering departments typically negotiate the terms of the subdivision (or site plan) agreement, and outline requirements in a development manual or equivalent. If developers could be advised as soon as they approach a municipality that they will be required to meet the OBC explicitly, or use the suggested Alternative Solution, it would save headaches for building inspectors. In municipalities with significant capacity issues, or under a Ministry of the Environment, Conservation and Parks order to reduce I/I, this approach will be invaluable.

The time to reduce the risk of I/I from the private side is now. We have the information and tools available to make these changes immediately.

Barbara Robinson is with Norton Engineering Inc. Email: or visit

Read the full article in ES&E Magazine’s April/May 2020 issue below.

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