How do you replace a subfloor? (With step-by-step pictures)

You can stop rot easily, but repairing the damage may call for nothing short of surgery.

Unlike typical home ills such as broken windows, fading paint, and sagging doors, infestations of rot in floors aren‘t obvious. You might not even detect the problem until you feel a soft spot in the floor, hear a suspicious squish as you step down, or find your toilet tilting and beginning to rock from side to side You also might see telltale brown stains showing up on the floor surface.

Unfortunately, by this time, the rot has already done some damage, so a complete cure requires two tasks. First, you have to eliminate the conditions that caused it — moisture in wood. And second, you’ll have to replace any weakened wood.

Step-by-Step: How to Repair a Water-Damaged Subfloor

  1. Remove the toilet and other obstructions like the vanity to expose the floor and make work easier.

  1. Probe for soft spots in the wood floor with a screwdriver, removing tile as you go, to find the extent of the rot damage.

  1. Cut through the vinyl and underlayment along the tub with your power saw, finishing the cut with a chisel at the ends. Then pull up the entire underlayment and vinyl.
  2. Probe again with a screwdriver to determine the extent of rot damage to the subfloor. When cutting out the rotten sections, plan the saw cuts so they fall on the center of joists.

  1. Remove the subfloor section to eliminate all soft, rotting wood. Pull nails before cutting along joists.
  2. Allow rotted areas to dry out, then apply a coat of wood preservative to damaged and discolored areas.

  1. Fasten a 2x4 to the damaged joist with 3-in. galvanized drywall screws to supply a solid, flat nailing surface.
  2. Toenail or screw 2x4 blocks to the joists to support the edges of the subfloor and the waste flange.

  1. Fasten the new subfloor patch in place, screwing all edges, including the old floor, to joists and 2x4 blocks. Use 1-1/2 in. screws spaced every 6 in.

Your floor can get wet in many ways, but you won’t have to worry about most of them. Occasional spills, and puddles left from wet feet and floor washing won’t cause trouble. because they soon dry up. Rather, look for sources that regularly wet the floor and never give it a chance to dry out. That’s what causes rot. It most frequently occurs around leaky pipes or drains, showers and bathtubs, and toilets.

You might think that replacing the rotted wood would be easy — just cut out the damage and patch it. But unfortunately, rot usually occurs in hard-to-reach places — under the toilet, next to the vanity, under the kitchen sink or along a wall. The job gets tougher when radiators, sinks or other fixtures have to be moved.

Complications also increase when rot extends under walls and weakens the supporting structure of your house. If these repairs are beyond your ability, it‘s vital that you call your building inspector and ask how best to proceed.

Is floor replacement a big job?

In this story you’ll see how to repair the rotted floor around a toilet. The rot was caused by two moisture sources:

  1. condensation running off the cool bowl and tank onto the floor during humid summer days, and
  2. an old leaky seal between the toilet and its soil pipe (Photo 1).

If the signs of moisture around the toilet base were ignored early on and reset the toilet with a new wax ring and solved the tank sweating problem, it would have been possible to avoid trouble. No moisture, no rot. It’s that simple.

But with the damage done, now it’s necessary to tear out and replace the rotted wood floor.

A novice can complete the carpentry in this project with a circular saw, jigsaw, and a 12 in. variable speed drill for driving screws. You can remove and reset the toilet with an adjustable wrench. Because you’ll be removing the bathroom fixtures, repairing the rot and laying a new floor, you can expect your bathroom to be out of action for about two days.

However, if there are leaks in the toilet drain line (Photo 5) or if it needs replacement, the project will get more complicated. You’ll need to consult or hire a professional plumber to make sure your drain connections are leak-proof.

Getting rid of the rot

To get at the rot problem in this bathroom, it was necessary to remove the vanity and toilet. To do this, first shut off the valves where the water supply pipes emerge from the wall and then disconnect the feed tubes. If your bathroom fixtures don’t have valves, shut off the entire house’s supply at the water main. Then turn on your sink faucets to allow the water pressure to release, before un-hooking the connections under the sink.

You can remove the toilet by un-screwing the hold-down nuts on either side of the base and lifting it up (Photo 1). Be sure to flush it first, though, to empty the water from the tank, and then sponge out the excess.

With the floor cleared for action, you can now probe it with a screwdriver to find the salt spots and plan your strategy (Photo 2). Most floors have two layers of wood: an underlayment over a subfloor. The subfloor usually runs in a continuous sheet under all the walls. It can’t be removed easily. The underlayment, however, was put in after the walls were up, so it fits each room and is tailored to the type of floor covering being used, The underlayment, therefore, is easy to pull up and remove.

In this case it was decided to replace both the underlayment and the floor tile in the entire bathroom. The tile was in poor shape, the patch would have had to be large, and we couldn’t make a good color match anyway. Plus, it would take too long to smooth and prepare the old underlayment for new tile.

To remove the underlayment, we had to saw through it (but not through the subfloor!) along the tub (Photo 3), since it extended underneath. Then it was raised up with a pry bar.

Removing rot is a dirty business, so put on your gloves when you attack it. Also. stuff a rag into the toilet flange (Photo 4) to keep sewer gas from escaping into your house and to keep debris from falling down the drain,

Repairing the floor

With the underlayment gone, you can now assess the damage to the subfloor and remove the rotted section. Working around the toilet flange is always tricky. because you have to shove your new subfloor and underlayment under it to give it adequate support.

If you remove the old flange, though, you’ll have to do some complicated plumbing to reinstall it. Use a jigsaw to cut out the circles as shown in Photos 9, 10 and 11 to make the floor fit under the flange.

If you’ve had a recurring leak, your floor joists might be rotted and weakened, too. In that case, you’ll have to insert an entirely new joist and screw it to the solid wood of the old one This can be a big project – you may have to open more floor to insert the new joist, move electrical or plumbing runs, or raise a sagging floor. If you’re unfamiliar with these complications, hire a professional carpenter to help out.

Fortunately, this project’s joist rot caused only minor surface damage. which was repaired by screwing a 2x4 to the joist to provide a sound top edge (Photo 7).

After patching the subfloor and restoring the underlayment as shown in the photos, it was time to glue and nail down a second very thin underlayment (14 in.) to assure a perfectly smooth surface for the vinyl floor.

TIP: When you buy thin material, make sure you get underlayment-grade plywood, which is better than regular grades.

You’ll get a smoother, stronger floor if you stagger your seams along different joists, so the edges of one layer don‘t fall directly over those of another. Vinyl floors show all faults and irregularities beneath them, so the rough joint between the old and new underlayment along the bathtub would have shown through and looked bad if it weren’t covered. However, other floor coverings, like ceramic tile, won’t need that extra 14 in. underlayment.

You can’t always get away with adding extra layers to your floor without adjusting the toilet flange, though. In this case the 14-in. underlayment was laid around the flange, not underneath, because it wasn’t possible to raise the flange above its old height.

The wax ring put on the toilet still sealed the toilet to the flange, but it wouldn’t have if the floor had been built up much higher. This is an important detail to confirm in advance when deciding on your new floor covering.

Solutions to a sweating toilet

Next time you see a puddle of water underneath the toilet on a hot and humid day, don’t automatically blame your toddler. Check the outside of the tank and toilet to see if “sweat” is running down the sides and onto the floor.

Of course, toilets don’t actually sweat; water doesn’t seep through their impervious glazed-china walls. The beads of water are condensation caused by humid air meeting the cool toilet sides.

Normally the toilet would be at room temperature, the same temperature as air. But when the toilet’s used frequently, cool water from the water supply underground can cool both the tank and bowl. That’s why on a humid summer day you’ll often find beads of “sweat” building up and dripping to the floor. Of course, as the tank and bowl warm to room temperature, the condensation will evaporate.

Occasional sweating does no harm, but persistent condensation will continually wet the floor and allow rot to begin. It also weakens the glue under the tile and causes plywood to delaminate. Surprisingly, the best solutions aren’t all that simple:

  1. You can reduce the humidity level in your house with a dehumidifier or air conditioner, but that’s expensive if your only goal is to stop toilet condensation.
  2. You can glue a thin foam liner to the inside of your tank to insulate it. It works well and costs only about $25, but it can be difficult to install. You have to remove the tank mechanisms, so you’ll need a wide wrench and perhaps new rubber washers. Also, if you buy a new toilet, you can get one that includes a liner.
  3. You can hook a hot-water line to the cold through an adjustable “anti-sweat” valve, letting you warm the tank water during warm, humid weather. The valve costs $40 and requires that you run a hot-water feed line.
  4. Insulate your tank with a tank cover. You can find these where you buy linens, matched to your towels or carpet. Launder them often to keep them sanitary.
  5. Simply lay a towel under the tank to soak up moisture, if it only occurs a few times a year.

The DIYer will probably find the tank liner easier to install than the valve. After the initial work, it requires no maintenance and no additional expenses. To install an anti-sweat valve you have to run a hot-water line from the nearby sink hot-water supply. Then you have to adjust the hot/cold water mix as needed. Of course, you’ll pay a small energy cost to heat the water.

You can find both the tank liner and mixing valve at most plumbing supply stores, or they can order one for you.