Plumbing question- 3 or 4 inch for basement toilet?

3" is fine for a bathroom group, however the lower bathroom should be protected from backflow from the upstairs fixtures should you ever have problems downstream of the new tie-in point, which means you should add a 3" normally closed backwater valve. Also don't forget to vent your new fixtures, you should try to get a vent dropped down from somewhere up above, depending on distance 1.5" vent should be fine. I am off to work, I will check this thread this evening, also I should note all my advice is based on the Canadian Plumbing code.

My whole house is plumbed with 3".

bjorn773 said:

I do have the vents planned. I have a utility sink in the basement next to the new bathroom. It has a vent that ties into the main stack above the upstairs bathroom. I will vent my shower and sink drains into a 1 1/2" vent pipe. I planned to tie the new vent into the existing utility sink vent pipe a couple feet above the overflow of the sink in the lower bathroom with a long turn elbow.

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Sounds good, your getting good info from somewhere, your not just attacking this blindly like I see so many do!

Highbeam said:

My whole house is plumbed with 3".

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3" is fine until the third water closet, at the point the 3rd water closet (toilet) connects to the building sewer, the pipe must be increased to 4", based on the Canadian Plumbing code anyway.

Ah, I only have two toilets so that must be the deal.

Rain water should not be entering the sewer systems in new development areas, some old parts of old towns (Charlottetown for instance in my province) still have some combined areas (sewer and storm) which could be an issue with rain water, but for you it obviously isn't. With that said, you could still run into problems if (and its a "if", but could definitely happen some day) but if you ever had your main sewer plug between the basement and street (or septic system) this gives a potential for the upstairs to drain through the downstairs fixtures onto your basement floor (it would be out of your floor drain in your case). Flush the toilet a few times, drain the bath, run the dishwasher and do a load of laundry (my worst case scenario of fixtures mentioned are all on upper level) You could have quite a stinking mess in the basement if all the above happen before you go downstairs. If lower fixtures are all protected you would prevent this from happening, things would stop draining upstairs and you would be calling a plumber to out why,

In another scenario, depending on how local grades work out between you and the sewer system, if there was ever a blockage out in the municipal sewer system downstream of your house, this could allow homes upstream of yours to have their effluent drain into your basement, not a pretty site to walk downstairs to 3' of sewage

Here where I live, all plumbing systems are required to vent from septic/municipal sewer straight through your main stack on the roof without obstruction, what we install for house backflow protection in houses at risk is a "normally open backwater valve", these have a flapper that lay down with floats on the sides, in a back flow situation the flapper floats up stopping back flow into your basement from outside sources, that is if the proper yearly maintenance was done on the valve, check yours if you have one, make sure nothing is built up on top of floats preventing the flapper to flip up, manufactures say to check annually, but no one ever does.

Now, every lower branch with fixtures flowing by from upper floors also need to be protected. A normally closed backwater valve is quite a bit cheaper and generally used for this application. All the above is once again based on the Canadian Code, I imagine they are similar in both Canada and the US, any questions let me know.........hopefully I explained things alright.

A general note with regards to backflow from outside sources, most insurance companies exclude this from homeowner coverage unless there is an added rider.

Many municiapl systems are overloaded and it only takes one pump to shut off at the worng time for a backup. Years ago I was surveying and found an open waste discharge from a large mobile home park that was supposed to pump back to a city sewer. The sewerage was from an overflow on the pumping station in case the pump failed. After the city inspector visited, the park owner fixed the pumps and bricked in the overflow. I expect the next time there was heavy rain at least a few of the trailer had geysers coming out of their toilets.

I think there is some info on the net that 3 inch is better for the newer low flush toilets. They don't dump as much water as the old ones. Also they don't have enough water to float stuff down a bigger 4 inch pipe. With a short run this might not be a problem.