How To Plumb a Basement Bathroom

No basement remodel is complete without installing a bathroom. The hardest part — by far — is installing the bathroom piping and drainage, tying into the drainage system, and establishing proper drainage and venting. This project is relatively inexpensive, but not getting it right the very first time could end up being very costly. This How-To guide will get you started on your basement bathroom renovation the right way.
Few Hours
Less Than $100


To do it right, it all starts with a plan. The design of your basement's new bathroom determines the scope of the project. Are you adding in a shower, for instance? What size are you looking at? Where in your basement is the best location? Your bathroom design will also affect whatever permits you require. 

About permits: Local municipalities have different permit requirements, but for a sub-level bathroom, you will need an inspection of the finished plumbing and electrics during construction. Consult your local municipality's website for the required information. 

Most recently, built homes have plumbing stubs pre-installed making installation much simpler. But not all homeowners are that lucky. For these situations, the biggest time saver is to build it under a first-floor bathroom. This provides easy access to existing utilities and drainage, which will reduce both your workload and the permits required.   

Working With What You Have 

Gravity usually isn't on your side when building a basement bathroom. In many cases, you'll need an ejector in order to transport waste away from the bathroom. However, sometimes the sewage lines will be far enough below the floor to allow you to plumb the bathroom without an ejector. (This is also dependent on local building codes, of course). You can gauge the position of a sewage line by using a locator; also, make sure the drainage lines are at least four inches in diameter for toilet and shower lines. 

Drainage, Drainage, Drainage.

Not surprisingly, the biggest issue with basement bathrooms is the drainage. Above-ground plumbing can count on gravity to drain away the sewage and wastewater. In a basement that is problematic, as some drains don't have the adequate slope, or as it's called in the trade, "fall", to drain away what you want to be drained away.  

This slope, or fall, is also a local code requirement, but the standard downhill slope is 1/4 in. per linear foot minimum. This is something you don't mess around with because otherwise you're in for a big mess to clean up — and a major repair. 

Plumbing Rough In Step-By-Step

  1. 1

    Shower & Sink Drain Location

    Divide the total size of your shower in half to locate your drain. Let's say you're installing a corner shower and it is going to be 4 ft by 4 ft. Measure 24 in. out from the corner on both walls. Mark lines 90 degrees out from those points and where they meet is the location of your shower drain. If you haven't installed drywall, add ½ in. For the sink, you tie that drain into either the drain pipe for the shower or the toilet, depending on your bathroom's design.
  2. Steps:

  1. 2

    Toilet Flange Location

    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.

  2. Steps:

  1. 3


    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. 

  2. Steps:

  1. 4

    Finding the Drain Pipe

    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.

  2. Steps:

  1. 5

    Hooking Up the Pipe

    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.

  2. Steps:

  1. 6

    It's a Trap!

    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.

  2. Steps:

  1. 7

    Finishing Your Rough In

    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.

  2. Steps:

How to Terms

Tools, products, materials, techniques, building codes, and local regulations change; therefore, Lowe's assumes no liability for omissions, errors, or the outcome of any project. The reader must always exercise reasonable caution, follow current codes and regulations that may apply, and is urged to consult with a licensed professional if in doubt about any procedures. 

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