This topic is incredibly important for any basement renovation. Often while inspecting finished basements, one of the biggest mistakes I would find are improperly positioned drain, waste and venting systems (DWV). This typically results in poor performance or expensive renovations or both. I've seen this in basements that are used simply as recreational spaces as well as ones that are actual second suites (both legal and illegal).
The reason this is a costly mistake is simple. Accessing DWV in basements involve opening up your floor slab to get to the drain, and going upwards through the house to get to the venting (Note: Don't confuse venting with ventilation - this type of venting are the pipes you see sticking out your roof). Air needs to get in and out of the drains in order to properly get rid of the nasty stuff without a hitch. A good analogy is pouring canned apple juice where you would typically punch 2 holes instead of 1.
The two most common violations I see are 1). Missing venting entirely (which will likely result in more backups and clogging) and 2) Fixture that are very far away from the main drain (this is more of an expense issue rather than a violation).
So here are 3 things to do before planning the layout of your finished basement with respect to DWV. This can save you thousands of dollars and a potential drainage nightmare later.
1. Locate the main drain
You can locate the main drain by connecting an imaginary line between the cleanout with the main soil stack. Plan all of your drains (toilets, sinks, tubs, etc) to be as close as possible to this drain line. This will mitigate costly excavations to access this drain, often at several hundreds of dollars a linear foot.
2. Utilize existing venting
Next try to position your fixtures as close to the main stack as possible. If you're able to do this, you may be able to utilize the venting capabilities of the main stack. More than several feet away and you'll have to create a new vent pipe, or if you're lucky you may be able to piggy back off of a kitchen or bathroom vent pipe above.
3. Get a video scope
Also, get a video scope of your main drain to see if there are any major issues before tackling the major reno. An old clay pipe might have tree roots clogging it, or it may be partially collapsed. And you might not know about it until the increased capacity of additional fixtures. The last thing you want is to replace a portion of your drain after the floor has been finished.
Note: Poor DWV configurations are not generally an issue on main and second floors because they've typically already been set in place. Supply plumbing is also not a big issue for basements since they are overhead and can easily be reconfigured.
Those are the 3 simple things to be aware of when designing your finished basement in terms of DWV. The issue sometimes with architects and designers are that they typically focus entirely on spatial design first, and force mechanical systems to work around that design.
In my opinion, I feel that design should work around mechanical systems first where possible, as those are where the major expenses are. This is where it helps to have an initial assessment with an experienced contractor or reno consultant who has a good holistic understanding of all aspects of the house first before any major design decisions are made.
If you need any help, let me know. This is where my experience in building science, home inspection, energy auditing, renovations and basement suites can save you thousands or tens of thousands down the road. Not to mention a major headache.
In a future article, I'll discuss some of the other functional issues of basement spaces, such as HVAC and electrical considerations. Stay tuned for that.