Can You Pour Concrete Under a Toilet Flange?

Toilet Flanges

• The proper installation of a toilet requires a level floor foundation. Concrete as a subflooring or flooring material can work well as long as the toilet flange fits securely in the floor. The flange connects the toilet to the drain piping that carries waste into a home’s drainage system. It has a circular shape with an outer ridge or lip that fits around the drain pipe and toilet pipe. The outer ridge includes bolt holes for securing the flange to the floor. Older homes may have flanges made out of metal or brass, while newer home constructions use PVC plastic.

Pipe Fittings

• When pouring concrete for a bathroom floor, ensure the drain pipe leading to the toilet is extended or, at least, accessible to the toilet flange fitting. Sizes for toilet flanges correspond with 3- or 4-inch drain pipe diameters. Glue or solvent adhesives secure the flange to the pipe. Since flange materials come in metal or plastic, the type of adhesive to use depends on the type of flange. To ensure the pipe extends far enough to attach to the flange, use an extended length of pipe and allow it to set as the concrete dries. Once the concrete has dried, cut the pipe flush with the floor to help ensure a level flange-to-floor fit.

Flange Fittings

• Since the flange fitting anchors or bolts into the bathroom floor, setting the flange in place while the concrete dries may provide a secure fitting for installing a toilet. Hanging the bolts through the flange as the concrete dries allows the concrete to secure the bolts and the flange in place. In some cases, this preset approach may cause unexpected problems when it comes time to attach the toilet drain pipe onto the flange fitting. The position of the flange may not line up with the toilet pipe, or the floor drain pipe may be too short to insert into the toilet pipe. Ensuring the toilet pipe aligns with the flange setting prior to pouring concrete can help avoid these problems.

Spacing

• As concrete tends to compress as it dries, spacing may become an issue in cases where a flange-to-concrete fitting sits too low to reach the toilet drain pipe. Installing flange spacers on top of the fitted piece can compensate for the lack of height. Flange spacers sit on top of a flange and also can sit on top of each other, though no more than three spacers should be used. Securing the spacers in place requires a sealant material, such as silicone caulk or solvent adhesives. Once the spacers are in place, long bolts or anchors are necessary to securely bolt the top spacer to the concrete subfloor.