How To Rough In Toilet Plumbing In Concrete Slab

How To Rough In Toilet Plumbing In Concrete Slab
  • Author: Amanda Arnold
  • Posted On: October 11, 2022
  • Updated On: August 21, 2023

Rough plumbing is when the vent system is in place, and all the water and drain pipes have been put through the stud-hole openings. Additionally, every pipe connection has been completed, sometimes through the foundation, but no sinks, facets, or other fixtures have been put in.

The main goal is to get the framework in place, followed by insulation, sheet rocking, and concrete pouring if a slab-on-grade building. Plumbing drains and pipes are typically placed while a building is being constructed before the concrete for the building foundations is poured.

In other words, most of the plumbing connections are located inside the foundations. Therefore, you will have to cut through the concrete foundation when replacing or fixing a damaged toilet drain or installing a new one.

Considerations Before Starting Rough In Toilet Plumbing

A basic understanding of every aspect of home construction is necessary to be a skilled plumber. So that you know what can be cut or drilled without endangering the framework, you need to have a fundamental understanding of how wood is framed.

To prevent placing pipes in the path of fixtures or ceiling can light, you must be able to read electrical drawings. To avoid stepping on ductwork, you also need to comprehend mechanical designs.

Steps To Install A Toilet Flange In Concrete Slab

One rather simple and seemingly complex bathroom job is replacing a toilet. As long as there are no issues, even inexperienced do-it-yourself plumbers may replace an old toilet in less than an hour.

It frequently happens when changing a toilet that the old toilet’s flange is incompatible with the new toilet. It can be rusty, broken in half, or just too filthy after being exposed to water and stress for so long.

Cut And Remove Old Flange

You will probably discover an existing toilet flange after removing the toilet. Before installing the new flange, you must remove the old flange and any obstructions. The old flange will be fastened with bolts buried within the slab on concrete slabs.

You can use the old bolts to secure the new flange if it is in good shape and aligns with the new flange. Cut off the bolts and other sticking-out objects using an angle grinder or a multi-tool with a metal-cutting blade. Be sure to cut down each piece completely flush with the concrete.

Remove Old Wax Ring

The old wax ring around the drain opening will remain in place even after you clear the blockages. The wax will probably be thick, gooey, and tarnished with sewage. It takes persistence, a putty knife, and latex gloves to remove the wax ring. Remove the old wax ring piece by piece and place it in a garbage bag for disposal.

Purchase A New Flange

Take particular note of the size of the drain pipe that has just been exposed; your new flange must fit this pipe. If you want to buy a replacement flange, measure the diameter or bring the original flange with you to the store as a reference.

Install A New Flange With T-Bolts

The threaded shafts of the T-bolts should be standing up when inserted into the new toilet flange. You can more clearly picture where the toilet will be placed once the bolts are set. Utilizing the T-bolts as a point of reference, press the toilet flange into the drain aperture.

To guarantee that the toilet tank is parallel to the wall, ensure the two bolts are spaced equally from the back wall. You can spin the flange in either direction until you find the desired position.

Even though you will have a little room for rotation once the toilet is in place, it is still preferable to get the position as straight as possible before fastening the flange. Ensure there are no gaps and that the flange fits snugly.

Firm The Flange On The Concrete

The toilet flange should be secured to the concrete slab using Tapcon anchors or a comparable masonry or concrete screw. When utilizing a plastic flange ring, take care not to overtighten the anchors should the ring shatter. After installing the toilet flange, you can now install the toilet with a new wax ring.

Even the most patient do-it-yourself may find it difficult to remove the old toilet flange since it is a messy job. Many DIYers have never drilled into concrete or installed fasteners, two specialty sub-projects. You might wish to contact a trained, licensed plumber in any scenario.

Seal The Concrete

Seal the cut foundation with fresh concrete after installing the new toilet drain. Consult your structural engineer to ensure the cement mixture has the same strength as the original concrete slab. Concrete must always be cut to fix malfunctioning plumbing.

If possible, arrange the installation of new toilets or other bathroom improvements so that the drain line can be run outside the foundation and connected to the yard’s main drain line.


This plumbing work can be challenging and require a high level of knowledge or competence, even with a step-by-step guide. This and the fact that you are inside a partially incomplete structure during this project make safety a concern.

In many circumstances, hiring a professional may be beneficial—particularly given that the smaller plumbing installation is only a stepping stone for the rough in plumbing.

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Author: Amanda Arnold

Amanda has been working with ConstructionHow since 2021. Her experience spans over 5 years in the creative niche such as home decor and trends, landscaping, renovations, and custom architectural values. As a home designer expert, she has a keen eye for the latest home improvement trends with accurate facts that readers find impossible to ignore. Being invested in home-building trends is how she has gained her lucrative expertise exploring more to bring a positive ambiance for all homeowners (and even tenants!). Currently, she lives in a beautiful beach home, a source of fascination for her.