When can you use a gravity type septic system?

Feb 4

No - this isn’t one of ours…can you spot it?

Updated December 9, 2020

Homeowners often ask if their site is suitable for a gravity distribution system. In fact, many people think that gravity dispersal is no longer allowed. Well, that not the case. Gravity is still a suitable system in BC, but just not in all applications.

Most homeowners are interested in gravity systems because that is what their existing system is and it’s what they know. The idea of adding additional equipment like pumps and controllers makes them nervous — both from a cost and future maintenance perspective. If it were up to them, they would have the exact same system type installed. So why are we seeing less and less gravity systems?

Standard Practice Manual

Well, first you need to know that registered onsite wastewater practitioners (ROWPs) are restricted to designing systems in accordance with the BC Standard Practice Manual (SPM). It is based on research and evidence and is regularly updated (we are on version 3 at this time). As the manual has evolved, it has incorporated new research on the applicability of gravity dispersal systems and has provided certain restrictions on their use. Depending on your soil type, depth, slope, and other factors, a gravity system may no longer be a suitable system.

When you can use gravity

The SPM is very well laid out and provides excellent tables on when and where each type of septic system can be used. Case in point, for gravity systems, the SPM provides table II-5 “Situations where gravity distribution systems are allowed or not allowed”.

We’ve attached this table as an excerpt for reference. For the most recent version, please always check back with the online posted version at https://www2.gov.bc.ca/gov/content/environment/waste-management/sewage/onsite-sewage-systems/sewerage-system-standard-practice-manual.

Now this only gets you part way there. You will see there are a lot of soils and technical terms that you need to decipher. This is where your ROWP or professional engineer can help. But if you are curious and want to investigate on your own, the manual also does a great job of defining these terms and how you can go about doing things like percolation tests. But do please keep in mind that a lot more than this goes into determining the best-fit system for your site and use cases.

Is that it?

Not quite. Every site is different and has its own constraints. In some cases, a gravity system may very well make the most sense. In other cases, it may require a departure from the SPM, and only in certain cases so as not to risk public health.

Significant departures of any kind will typically require that you utilize the services of a professional engineer such as OwnerPro, as they are permitted to use different design manuals than just the SPM. Departures or not, we are here to help you better understand this process and design the system best suited for your site.

Update - what about soil depth?

One of our readers highlighted an important point - namely, what about soil depth? Your soil type works against you for some homeowners - either it drains too fast or not fast enough, and then gravity is not a suitable system. In many cases, your soil type is acceptable. Still, the required vertical separation (depth below the dispersal infiltrative surface) is prohibitively large - over 5 feet!

However, suppose your soil conditions are just right. In that case, the vertical separation can be reduced to 90 cm (3 feet), or even slightly smaller if including a blinding layer.

This isn’t super clear in table II-5, at right. We’ve now added another table, which presents this more clearly.

In all cases, we still need to figure out your systems length, which can throw another wrench into the planning.