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How to Build a Sub-Floor and Wall in a Bathroom

DIY experts show how to build out a sub-floor in preparation for a tile installation and build a partition wall as part of a bathroom renovation.

More in Bathroom

  • Time

    Weekend

  • Price Range

    $250 - $500

  • Difficulty

    Moderate to Hard

Highlights:

Step 1: Build Up the Sub-Floor

In this bathroom renovation, we tore out a concrete floor to the wood sub-floor and stripped the walls down to the studs. Since we found the sub-floor beneath the old shower had been built up and wasn't even with the rest of the floor, we built up the remaining floor to get a uniform height.

To build up the sub-floor, install a layer of five-quarter 1"x6" boards to the base. Despite their confusing name, these boards are exactly one-inch thick.

Using safe cutting techniques, cut the 1x6 boards using a circular saw. Position and balance the board over the two sawhorses, making your cut outside the sawhorse (Image 1).

Note: If the cut is made between the sawhorses, the boards will drop after the cut is made, and the free end can kick, causing injury.

Apply a layer of common roofing paper over the existing sub-floor to provide a vapor barrier and avoid creaky floor boards (Image 2).

Check the end-grain of each board before nailing it down. Position each board so that any slight curve found in the end-grain is face down.

Note: It's important to position the boards this way to avoid cupping which can cause cracks after tile is installed over the sub-floor.

Using ring-shank flooring nails, which ensure a secure bond, nail the boards to the existing sub-floor.

Step 2: Install Plywood

Use 3/4" plywood (or a thickness appropriate to your project) to bring the floor to level position against any adjoining subfloor. Once the five-quarter 1"x6" boards are all nailed down securely, cut and place the plywood in position (Image 1).

Again, use ring-shank flooring nails to install the plywood (Image 2).

As applicable, check to make sure the floor is level against any adjoining subfloor.

Step 3: Install Cement Backer Board

Once the sub-floor has been leveled, prepare the floor for tile installation by laying down 1/2"-thick cement backer-board. Cement boards come in 3'x5' sheets. The boards are relatively easy to handle and much simpler than the old method of laying down concrete. Lay out the sheets on the floor and measure the boards (Image 1) against the floor plan.

To cut backer board, score the line of your cut using a utility knife (Image 2), then snap the board along the cut.

Once cut to fit, the best method for securing backer board is to apply pre-mixed mortar to the wood sub-floor, then screw the backer board to the sub-floor. The mortar is the same used later for attaching the tile. Use a 1/4"-rib trowel to spread the mortar over the wood surface (Image 3).

To ensure the best bond with the tile, lay the backer board on the mortar with the rough side up (smooth side down). Secure the backer board to the floor with screws (Image 4). Or, use nails as long as they are placed 6" apart.

Repeat these steps until the entire sub-floor is covered with backer board.

Step 4: Build the Partition Wall

The next major step is the construction of a simple partition for the wall. In our project, the frame will become one of the shower walls. The partition frame consists of a top plate, floor plate and vertical studs, all made from 2"x4" lumber.

When framing a wall, it's easiest to assemble the partition first, then fit it into place, provided accurate measurements are taken beforehand and followed precisely. Make the appropriate measurements and using the circular saw, cut the boards to length.

Note: It's important to place the studs 16" on center meaning the 2x4s are placed exactly 16" from the center of the one beside it.

Once the boards have been cut to length, lay out the partition on an open floor for nailing (Image 1).

Once the partition is assembled, move it into position (Image 2).

Using 3-1/2" (16d) nails, attach it to the floor and the existing header. In our project, the frame is installed in a hallway where a closet wall was and the header was previously part of a closet door. Nail the frame to the 2x4 studs on either side.

In our project, the new frame becomes the foundation of the shower walls.

Step 5: Install the Drywall

Install water-resistant drywall on the partition inside the bathroom and install regular drywall on the side facing exterior walls. In our project, it was the hallway.

drywall installed to both both sides of partition

Courtesy of Jeffrey Rowe

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