How To Plumb a Basement Bathroom

Most toilets drain holes need to be centred on the one-foot point from the finished wall. Be sure to include the extra ½ in. for the drywall. Mark that spot as it is the location of the centre of your toilet flange. 

Note: The minimum side-to-side distance for a toilet is 30 in. so plan your wall design accordingly.

No bathroom plumbing system is complete without a vent. When you flush or take a shower, wastewater pushes the existing air in the pipes and can form a water lock if there is no air available to stop a vacuum from forming. If you are installing your bathroom under your first-floor bathroom, you can tie into the existing vent. If not, you will have to get a pipe outside and above grade. 

You need to uncover your existing drain pipe to connect it to your shower drain and toilet flange. This will require breaking concrete. Basement slabs are generally thin, so it's not as hard as it sounds. For a shortcut, use a masonry drill to get you started, or rent a concrete cutter. 
Once you've exposed your drain pipe, you need to hook up to it. In newer installations, this is a simple connection, usually at the cleanout. In older homes, use a diamond-plated grinder wheel to cut through cast-iron drain pipes and use a mechanical coupling to hook up.

By far, a 2 in. pipe is the easiest and best solution for basement bathroom installations. It's easy to cut and connect. Dry-fit the piping and connectors until your measurements are perfect.

Traps are used to stop sewer gases from the drain pipe from coming into your home, so these are essential. Install the traps at the end of each pipe — except the toilet. Toilets have their own trap built in. While doing your dry fit, make sure your pipes slope ¼ in. inch per foot to ensure proper draining. 

Tip: You don't want your traps and flange to move when you backfill and during the concrete pour. Drive a 1 ft. piece of rebar on either side of your pipe and duct tape it to your PVC.  

Call for an inspection at this point.

Be sure to cover the toilet pipes with a cap and wrap the shower drain in plastic. Next comes backfilling the area you have excavated. Don't use the dirt you excavated, as you are never going to compact the original soil the way Mother Nature had it before you went in. In six months to a year, after a few rainfalls and after the groundwater table rises and falls, that ground is going to settle and you're going to have a void under the basement floor, which then can be a source for cracks. Backfill with gravel instead, which will not compact. Mix your concrete and pour your patch, making sure that it is level.