Repairing a Bathroom Subfloor

If you’re finding that your bathroom floor is starting to creak and look saggy, it’s probably time to look into the steps you need to take for repairing the bathroom subfloor. In the bathroom, floor issues are usually caused by dry rot – which isn’t dry at all, but is a fungal growth caused by moisture.

This is a common bathroom problem – since it’s difficult to completely seal all the areas of bathroom flooring, it’s only a matter of time before small drips of water after everyone leaves the shower will work their way into the subfloor, encouraging water damage and mold growth.

Identify the Source of the Problem

One initial way to start fixing the subfloor problem is to put insulation in the flooring, regardless of the structure of the subfloor or what’s located beneath it. Purchase rigid foam insulation, and place this between joists after you’ve fixed the damaged pieces of wood in the floor. Of course, this is only going to be a temporary solution if you don’t find and fix the source of water or moisture first.

Determine whether the moisture is coming from your toilet, seeping out of the shower or bathtub, or if one of the supply pipes in the floor or walls has a leak that’s feeding the dry rot. Once you’ve found the source: fix it! If your toilet seal is leaking, fixing the moisture problem could be as easy as replacing the seal.

However, if the rot seems to be coming from moisture seeping through concrete under your subfloor, you may have to alter the structure of your entire floor. If this is the case and the subfloor is attached to sleepers, you’ll have to replace all the sleeper boards with pressure-treated materials.

Do whatever you need to do to ensure the problem doesn’t repeat itself! You don’t want to be stuck pulling up your floor and changing the subfloor every few years – it will get expensive and isn’t a good habit structurally.

Replace the Damaged Subfloor Materials

Fortunately, with dry rot – unless the situation is really grim – you won’t have to replace the entire subfloor. Instead, you can repair the damaged pieces using new, pressure-treated materials, such as pressure-treated plywood, that are more resistant to water damage. If you can’t find the plywood, try marine plywood instead – though this is a more expensive alternative.

Once those are fixed, install the rigid foam insulation – not only will it help to preserve the subfloor, but it’ll also conserve energy. Many older homes don’t have insulation in the floors – especially bathroom floors – and you’ll definitely notice a difference in temperature when you head to the loo at night. The insulation comes in various widths, and you can cut the thicker material with a table saw if needed. The thinner varieties are easily shaped with a utility knife.

Insulation Installation

If your subfloor is made up of sleeper boards on a slab with joists suspended overtop, you’ll need to cut the insulation board to fit between the sleepers, and then use a shim to get the insulation flush against the sleeper heads. It’s alright if there’s space at the bottom, it’s not as crucial.

When the insulation is in place, run caulking around the space between the insulation and the floor joists, or use duct tape if you’re wary about caulking the floor. Reinstall the subfloor boards, as presumably you had to remove some to install the insulation, and then place your floor covering back down.

See Also: Floor Joists and Subflooring
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