How To Replace Rotten Cement Board Underlayment

Person measuring to cut subflooring
  • 12-24 hours
  • Intermediate
  • 80-400
What You'll Need
Crowbar
Measuring tape
Nails
Sawzall or circular saw
Cement substrate flooring
Utility knife
Hammer
Drill
Sheetrock or concrete screws
Thin set mortar
Trowel
What You'll Need
Crowbar
Measuring tape
Nails
Sawzall or circular saw
Cement substrate flooring
Utility knife
Hammer
Drill
Sheetrock or concrete screws
Thin set mortar
Trowel

Cement board is essentially sheetrock with a moisture barrier seal used in places where water is prevalent. Like the bathroom. Or the bathroom floor. So in this article, we are going to give you the steps to replace the cement board that is the underlayment on your bathroom floor. If it is already rotten it can produce mold or mildew. Greenboard is no longer recommended for use in wet areas.

Cement substrate flooring is what you need to replace it with. This is like a big piece of sheetrock made out of cement with mesh in it to allow for flexibility. Before you go buy it, measure the flooring you are replacing and buy the sheets that are large enough to allow you to make as few cuts as possible.

Step 1 - Remove the Old Cement Board Underlayment

bathroom under construction

First, if you have any type of carpet or tile on top of this underlayment you haven't already removed, you must remove it. If you have carpet, just rip it up and make sure you pull up the nail strips around the edges. If you have tile, break it up and remove it with a scraper until the floor underlay is bare. Using your Sawzall, make a small cut about 3/8" deep in a place where there is not a joist or a stud.

You can use a stud finder to make sure if you don't know how to find it yourself. They are very cheap and you will use it again. Now put your flat edge of the crowbar in the hole you made and pull up the rotten cement board. As a note, if it is really rotten you may not have to do this. Remove all the flooring making sure not to punch holes in the sub-flooring below. Using the hammer or the nail puller on the crowbar, remove all the nails out of the floor.

Step 2 - Prepare the Underlay

Now that you have removed the old floor, make sure you clean it thoroughly. If it is not dry use a dehumidifier overnight to make sure the flooring is dry before going on to installing the underlayment. Using one sheet, lay it as close to the corners as possible. If it fits and you have no cuts to make, use the thin-set to put a light coating onto the back of the mesh side of the board. Then lay it down.

Now using the screws, firmly screw it into place, make sure the screws do not stand above the floor. If there are cuts to make on any of the sheets, you need to measure the area. Then using a straight edge mark where the cut is going to be. Using your utility knife, you can cut through the cement substrate. Or if you would rather, use the circular saw to make a clean cut. Once all your measurements and cuts are made go on to the next step.

Step 3 - Lay the Substrate Flooring

installing floor tiles

If you have already put down one piece, then go on to the next piece that fits on that one. Apply the thin-set mortar in an even layer. Screw it in place firmly. Place the next piece of flooring in place, use the screws to secure it. Always apply the thin-set to it beforehand. This will keep it firm without moisture going under it. Now you are finished and ready to put your flooring back on top.