How To Plumb a Basement Bathroom in 7 Simple Steps

Whether you love nothing more than taking on a new home DIY challenge or whether you’re just keen to save money, plumbing your own basement bathroom is a challenging but rewarding job.

Adding a basement bathroom to your home will not only make your life more convenient but it will also add value to your property making it an excellent long term investment.

In this article, we look at how to plumb a basement bathroom and the various things you need to bear in mind before you get started on your project.

How to plumb a basement bathroom


What Tools And Equipment Do I Need?

Before you begin your plumbing DIY project, you should ensure that you have all of the key tools, materials and pieces of equipment to hand in order to help you to save time and inconvenience.

Here is your guide to everything you will require:

  • A screwdriver (four in one)
  • Cordless Drill
  • Spirit Level
  • Hammer
  • Reciprocating Saw
  • Miter Saw
  • Safety Goggles
  • A Shop Vac
  • Sledgehammer
  • Ratchet/Socket Set
  • Stepladder
  • Spade
  • Trowel
  • Tape Measure
  • Torpedo Level
  • Pipe Snapper
  • 2x4s
  • Cast Iron and Plastic Pipe & Fittings (2″, 3″ and 4″)
  • Pipe Glue
  • Concrete
  • Band Couplings

Basement bathroom piping


How To Plumb A Basement Bathroom:

Step One – Find The Main Drain

The new drain lines must be connected to the existing line, and this must be found before you can plan your basement bathroom. Make sure to identify the location of your main line and break the concrete to ensure that you are correct.

You also need to check that it is of an adequate depth to accommodate enough of a downhill slope for the new drain line.

  • Find the large vertical pipe which runs into the floor of your basement. This is called the main stack. While it may run directly to the street, it could be at an angle. A good way of finding your main stack is to spot the cleanout plug which will be along the basement wall which faces the street. This will indicate where the main line leaves your property.
  • For homes with private septic systems the main line runs towards the drain field. One way of finding it is by punching through the basement floor in the location where you believe it is, or call in a plumber to help you.

Bathroom drain

Step Two – Making A Downhill Slope

A drain line will need the downhill slope to be a minimum of 0.25″ for each linear foot to enable the waste to flow properly through the pipe.

You will need to take some measurements to ensure your plan has allowed enough depth.

  • Measure the depth of the tie-in point at the main line’s center.
  • Measure the planned depth for the pipe below the drain.
  • Subtract the planned pipe depth from the tie-in point depth and then multiply the answer by four. This will give you the maximum drain line length extending from the end of the horizontal pipe to the main.
  • If you find that the main line does not have an adequate depth the fixtures will need to be located in a closer position to the line. Alternatively, you could look at installing a sewage ejection pump.

Step Three – Planning The System

Once you have completed steps 1 and 2, the next stage is planning our your entire bathroom.

Use a pencil to mark the location of the key fixtures on the floor of your basement making sure to include the shower, sink, toilet, drain lines and walls.

Remember that nothing is set in stone as yet, and changes are highly likely at this point.

Step Four – Cutting The Pipes

The easiest way to cut the waste line if you have cast iron pipes is to use a pipe snapper. This is a tool which uses a cutting chain that tightens till the waste pipe cracks.

You can rent one from a tool rental store.

If your pipes are very old, however, they may crush instead of cracking, and in this instance you will need to use a reciprocating saw. A reciprocating saw can be used to cut easily through plastic pipes.

Step Five – Trenching The Floor

You can use a standard sledgehammer to break up your basement floor. If you break through where the tie-in point is located you can make a starter hole.

If you aim the blows correctly, you can create a trench line by picking out big concrete chunks as you work. Try to keep the trench with just enough width to use the spade.

When digging out the dirt, keep it separate from big concrete chunks as you will be using the soil later as backfill.

Plumbing water tap

Step Six – Building A Drain System

Once the trenches are complete, you must install a Y fitting by cutting into the main line. While the main line is cut open, make sure no water runs in the property.

  • Begin your drainage system by cutting into the drain main line and installing a Y-fitting. You can either use a cast iron or plastic Y fitting, however if you use plastic, you will need to glue some short pipe sections into the fitting so the rubber couplers can be accommodated.
  • ABS plastic or PVC can be used for the DWV system and both are equally simple to join and cut.
  • Use a torpedo level to check the pipe slope to ensure that it is adequate to accommodate drainage.
  • Once you have completed a section of piping, you should pack dirt around and under it as this will prevent shifting while other sections are being built.
  • Make sure you are aware of the toilet’s “Rough In” i.e. the distance to the center of the drain from the wall, accounting for the drywall and framing thickness.
  • Once the drain system has been capped and completed, you should ensure to have it approved and inspected by a building inspector. You should do this before your backfill the trench.
  • When backfilling the trenches, take care to tightly pack the soil as this will help to prevent later settling. However, while tamping the soil you need to ensure that you do not accidentally move the pipes.
  • Fill the holes to 3″ below the basement’s floor and pour in 3″ of concrete in order to replace the floor and level it.
  • Avoid concreting the area around the shower drain until the shower pan has been framed in just in case repositioning is necessary. Once you are certain the positioning is correct, you can finish concreting.

Step Seven – Building The Vent System

Once you have framed the walls of your basement bathroom, you will need to assemble your vent lines. These run below the joists of your floor and can be hidden by framing a lower ceiling.

Alternatively, you can keep the ceiling height by running the pipes through your joists, although you will need to bore larger holes and this could cause joist weakening.

The vent system is necessary to let air in and without one, the sewage will race through the waste lines creating a vacuum and air pressure inside the pipes. This will result in gurgling, noisy drains and may even result in sewer gas entering your property.

In many basements, it is possible to tie the vent system into the laundry sink venting line. Make sure to check your local plumbing codes as some areas have extra leniency on issues such as fitting choice and vent sizes.

  • Using a T-fitting, you can connect the vertical pipes to the horizontal pipes and join vent lines together.
  • You will require a 3 inch drainpipe for your toilet, but a 2 inch drainpipe is adequate for your other fixtures. You are not allowed to use a pipe smaller than 2 inches underneath concrete slabs.
  • Each drain requires a trap and each trap requires a bent. You will need to have a different maximum distance between the vent and the trap depending on the pipe’s diameter. If you have a 1.25″ pipe, you will need a maximum distance of 30 inches. If you have a 1.50″ pipe, you will need 42 inches. A 2″ pipe needs 5 feet, a 3″ pipe needs 6 feet and for 4″ pipe, you need a maximum distance of 10 feet.
  • Toilets already have an inbuilt trap, so one is not required in the drain line, however a vent is still necessary.
  • Vents are able to run horizontally, however they must be a minimum of 6 inches above the level that water will flow over the toilet, tub or sink’s rim. This is known as the spill line.
  • Most bathrooms require a 2 inch vent. While it is possible to use smaller pipes running to the shower or sink, it is usually more convenient to use a single size across the entire system.

Although plumbing your own basement bathroom is a large task, it is a rewarding project for any homeowner.

If you take care to follow these steps, you should achieve an excellent result while saving yourself the expense and hassle of arranging a professional plumber to come out and do the job for you.

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