Floor Drains – Not just a hole in your basement floor

Hi folks, I’m a little late this month, and would like to apologize to my loyal readers, as well as thank you all for your emails.  It’s nice to know that my blogs are being read and appreciated, since although contrary to what it may seem, I do put some thought into them.

This month I choose to speak about floor drains.  Most city-dwelling people have them in their basements, and yet many don’t even know what they are and what they do.  If you don’t know what a floor drain is, don’t worry as you’re not alone.  You may have seen it in your basement.  It’s basically a round opening in your basement floor, a few inches wide, and normally has a small grill over it.  Its purpose is just what the name implies; it serves as a drain for your basement floor.  If you’ve ever had your water heater burst, or your washer overflow, you can appreciate the importance of having a functional floor drain.  Without it, your basement can turn into your very own indoor swimming pool!  This is particularly unfortunate should you have boxes full of precious photographs and other keepsakes, being stored on your basement floor.

Your basement floor drain is considered a sanitary fixture, just like a sink, tub, toilet, or shower.  The drain is therefore connected to the sanitary main, and thus has a p-trap in it, similar to all of your home’s other sanitary drains (hopefully).  A p-trap is that u-shaped bend in the drain pipe that you often see beneath your sinks.  The purpose of a p-trap is to hold water in the pipe.  This water acts as a seal, preventing foul and flammable sewer gases from exiting the drain, and entering your home.  The problem with floor drains is, because they are rarely (if ever) used, that the water in the trap has a tendency to dry out.  This is why I always recommend to my clients to prime your floor drain once every six months or so.  Priming is basically just pouring water into your floor drain; about a litre should do it.  The idea is to flush out the old stagnant water, and top up the trap with new, fresh water.  If your trap dries out completely, you will start to notice a foul odour of sewage in your basement.

That brings me to my next point… do not block off your drain!  You’d be amazed at some of the things I’ve seen jammed into floor drains over the years.  Socks, underwear, teddy bears, rags, tennis balls, etc.  I’ve even seen a couple of drains which were permanently sealed with concrete!  What tends to happen is that the traps dry out, and the unsuspecting homeowners begin to smell a foul odour in their basement, which they quickly trace to the floor drain.  Not knowing they simply need to poor some water in there, they proceed to “seal” the drain with whatever they can find.  And no, I wasn’t kidding, there have truly been a few instances of people actually pouring concrete in there to permanently seal it up, not realizing that the drain is there for a reason.

My next point is backflow.  This is what happens when the city sanitary sewer blocks and surcharges (backs up), and you end up getting your neighbours’ sewage entering your basement through your floor drain.  Although backwater valves are now commonly installed on the storm main to protect the footing drain from flooding over, backwater valves on the sanitary main are still a rare occurence.  Realistically, I’d estimate that approximately only 1 in 300 modern homes have a backwater valve on their sanitary main.  Although I don’t know for a fact, my personal (and professional) belief is that they are rarely installed because of the fact that sanitary sewers do not tend to back up nearly as often as storm sewers do.  There’s a reason behind this, and stay tuned for my next article, which discusses in detail the different sewer systems, and your home’s main sewer discharge pipes (assuming you live in the city and don’t have your own sump and septic systems).

So as I said, your sanitary main is likely not protected from sanitary sewer blockage.  Unfortunately, your floor drain tends to be the closest exit point for the sewage that is making it’s way backwards, back you your sanitary main.  So, in the extremely unfortunate case that your local sanitary sewer has blocked up, you will quickly find the contents of your neighbourhood’s toilets, showers, and sinks, flowing out of your floor drain, slowly filling up your basement.  Yes, I agree, it’s a scary thought.  Now, before you all dash out to buy backwater valves, keep in mind that it’s a bit of a process to install a mainline backwater valve, and therefore I’d recommend that it be left to a competent professional plumber.  However, the good news is that it’s relatively inexpensive to do so.  A basic inline backwater valve retails for around $50 or less.  These are the ones that builders tend to install for the storm main.  Personally, if I was to retroactively install one for my sanitary main, I would go for the top-of-the-line model, which is still only around $100 or so.  As for installation, well, like I said, it’s a bit of work, but actually not too complicated if you’re good with plumbing… and smashing concrete.  The thing is you first have to get to the sanitary main.  The easiest way to locate it is to look for your primary clean-out.  In the vast majority of modern homes, you will find adjacent to the water supply (the water meter, supply pipe, shut-off valves), two 4″ black ABS plastic pipes sticking out of the floor, with round plugs sealing them up.  Those are your primary clean-outs.  One accesses the storm main, and one accesses your sanitary main (again, I will discuss this in greater detail in my next blog).  Once you’re located the clean-out for your sanitary main, simply go ahead and break up the concrete directly in front of that clean-out.  You will eventually uncover the 4″ sanitary main.  Once you’ve cleared away enough of your concrete floor, you then proceed to cut out a section of the pipe (measure the distance between the inlet and outlet connections of your new backwater valve).  Once the section of pipe is removed, proceed with slipping on your backwater valve and cementing up the connections.  Once the valve is installed (and tested, very important), you can proceed with pouring in new concrete around your new valve (make sure not to cover up the valve box’s access hatch with concrete).  So like I said, it’s a bit of work, and a bit complex if you’re not particularly good at plumbing, so you’re best to leave it to a competent pro.  Also, if you mess it up, you’ll end up with a sanitary leak… yuk!

Although it may seem like a lot of work, for a competent pro, it’s not that much work to install an inline backwater valve.  Between the cost of the valve, and the labour, you’ll probably spend around $500 (give or take).  When you consider the savings you may get on your insurance, plus the peace-of-mind that you won’t turn your basement into a giant toilet, it’s money well spent.  I should also note that there’s a “middle-ground”.  You can install, what I call, a floor drain ball.  This is a small backflow preventer that fits directly into your floor drain.  They retail for around $20 or so.  It basically consists of a circular rubber flange, that pressure-fits into your floor drain pipe, and attached to the centre of this flange is a rubber float (“ball”), held open by a small spring.  If sewage was to come up your floor drain, the float (“ball”) would be pushed up against the flange, thus sealing your floor drain and protecting your basement.  However, this is just a temporary fix, as the sewage will continue to fill your DWV pipes, and eventually come out the next closest opening (usually your basement laundry sink, or your basement shower, if you have one).  But as these devices only cost around $20, and installation is pretty much idiot-proof, it’s an easy buy.

In conclusion, floor drains are there for a reason, and can save you from a lot of grief, should you have an unforeseen plumbing emergency in your basement.  However, they do require a little bit of attention.  So do yourself a favour and give it a prime every once in a while, not only will you keep the trap full and clean, but you’ll also take comfort in knowing that the drain is functional in case you need it (or else the water you’re priming with would just spill out of the drain).  If you don’t have a backwater valve, or even if you’re just not sure, go ahead and install a backflow preventing float-ball in your drain.  Better safe than sorry!

Happy home-owning!

Eric Parent

2017-12-21T01:01:19+00:00 April 1st, 2016|Plumbing|2 Comments


  1. Mark March 6, 2017 at 10:24 pm

    Great article! Very informative! I like it! Well done! One thing I would like to mention is that these floor drains are a common source of radon. To prevent radon from rising up through the drain, one could install a retrofit floor drain that allows water to go down, but no gas can come up. Just an idea. For more information on this or anything to do with radon, contact us – http://www.simonairquality.com/contact

    • Eric Parent March 9, 2017 at 11:43 am

      Very good tip Mark! We always recommend automatic trap primers to make sure the drain trap is full enough to stop any gases from escaping the floor drain, which would prevent Radon that could have penetrated the floor drain pipe due to a crack. But traps due sometimes dry out, particularly when the condensate hoses are draining elsewhere then the floor drain. Also, over time gaps can also form between the drain sleeve and the slab, around the floor flange, so making sure that the perimeter of the drain flange is sealed, along with any floor cracks in the slab, is definitely wise. Thanks for bringing that up, Radon gas is definitely a concern!

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