How To Replace Rotted Bathroom Sub Floor

Be Sociable, Share!

    One of our friends asks for some help with a bathroom repair that went bad. It seems that a fitting in their second floor bathroom was leaking so they turned the water off at the valve and removed the toilet. At that point they decided to replace the toilet and went to the store to purchase one. By the time they got back they found that the valve leaked enough water to fill their bathroom and start leaking into the bathroom ceiling that is directly below. When they cleaned everything up they found they had floor rot around the toilet flange and are now expecting to replace a section of floor so they can replace the toilet.

    This is the worst kind of situation. You think you are repairing one thing and it ends up being so much more. In their case they were somewhat lucky because they discovered the problem pretty quickly. The water damage to the drywall of the ceiling below was minimal. I would expect that in the next week they will probably see some drywall tape peal and maybe a nail or two pop but at least the whole ceiling didn’t come down.

    Unfortunately because they have to replace some subfloor around the toilet flange that ceiling may need to come down to  complete the repair and to inspect it from below.

    It will also mean replacing the finished flooring in the bathroom if it is tile over plywood with a concrete backer board but if you have a mortar bed floor things just got so much more worse.


    If you need to repair sub floor under a mortar bed which is standard in older homes you might as well consider doing a complete bathroom remodel. If your bathroom is at that point where your sink and vanity and tub could use replacing then when you need to tare up the mortar bed to fix the sub floor there really isn’t a better time to do it. You can drop in a fiberglass tub and new sink pretty easy compared to the floor work you will be doing.

    If your home is new and you have vinyl floor over a plywood subfloor then the job is pretty simple. Yes you will still need to replace the vinyl and the plywood but you aren’t getting in there with a sledge hammer or rotary drill with a chisel head taring up an inch and a half of cement mortar.

    To replace the subfloor first remove the finished flooring. Next inspect the subfloor to decide how much has to come out. Any wood that is water damaged must come out and any that is black should come out. There is no sense leaving wood with mold in the floor and refinishing the bathroom.

    To replace the subfloor you need to buy subfloor plywood at your supplier. Then you will need to install some 2×4’s for nailing it to the opening you made. Install them 90 degrees to the joists where the new floor meets the old floor. Now you can glue and nail or screw the subfloor to the joists.

    if the floor is not level you will need to use layers of 1/8th inch thick Luan Plywood to build up the area then cover the entire subfloor with sheets of luan for a flat level floor that you can install your finished tile or whatever you want for your bathroom floor.

    Final Note

    The work is going to be laborious but not extremely technical. Concrete mortar beds are where most people get stuck because they don’t know how to remove them and today most contractors won’t float an entire bathroom with mortar beds they will only use it in the shower pan for best results.

    To remove a mortar bed means lots of hammering its just difficult work to crack it up. I have seen some contractors use 1 inch rotary hammers to do this work and others will use a short sledge hammer. It depends on your endurance.

    When you finally get to reattaching fixtures like your toilet you may need to make some modifications. Older homes may have a combination of PVC, Cast Iron and even lead pipe in very old homes. You may need to remove the ceiling in the room below to gain access and make replacements.

    Is this something you can do? Maybe but if you hire a contractor expect to pay quite a lot so doing it yourself is in your best interest.



    Be Sociable, Share!