Planning a bathroom renovation?
Homeowners fixate on tile patterns, vanity finishes and designer fixtures. But nobody discusses…
Your bathroom is only as good as what’s underneath it.
Subfloor and drainage. These are by far the two most crucial aspects of any bathroom installation. Mess these up and you’ll be tearing it out in a few years. Do it right and it will last a lifetime.
Here’s how to nail both.
Here’s What’s Inside:
- Why The Subfloor And Drainage Matter So Much
- Choosing The Right Subfloor Material
- Drainage Essentials You Can’t Skip
- Linear Drains vs. Traditional Drains
- Top Mistakes That Wreck Bathroom Floors
- How To Inspect Before You Start
Why The Subfloor And Drainage Matter So Much
Bathrooms are the wettest rooms in any home.
Every time you take a shower, flush or splash water someplace. If your subfloor isn’t sealed properly…that water is going somewhere it shouldn’t be. Like into joists, walls and the ceiling below you.
That is why adding a bathroom in a home addition always requires special consideration for the flooring structure. A bedroom won’t experience daily contact with water. A bathroom will. Especially the larger the bathroom is.
Water damage is a HUGE issue in houses. Did you know that 14,000 Americans deal with water damage each day? Many of those issues begin in the bathroom because the foundation beneath was never made to deal with moisture.
The good news?
A stable subfloor and proper drainage will keep your bathroom safe for years to come. That’s one reason why hiring a trusted bathroom remodeling team is so important — particularly for additions to new homes that require a floor plan built from scratch.
Choosing The Right Subfloor Material
Not every material belongs under a bathroom.
Homeowners biggest mistake is not treating the bathroom subfloor differently from the rest of the house. Bathrooms require water resistant materials that will not rot, warp or promote mould.
Here are the best options:
- Plywood: Exterior grade plywood is good for most bathrooms. It stands up to moisture better than regular plywood.
- OSB: It’s less expensive, but waaaaay more susceptible to water. Only use if you have a really good waterproof layer on top.
- Cement Board: A must-have underlayment for tile. It won’t break down when wet.
- Foam Tile Backer: Relatively new product. Light weight, 100% waterproof and extremely easy to install.
Typically plywood and cement board (or foam backer) underneath finished tile is the correct duo.
FYI: Occasionally bathrooms in home additions will be on slab foundations. If this happens, the slab is considered the base, but you’ll still want to apply waterproofing prior to flooring.
Drainage Essentials You Can’t Skip
Drainage is where most bathroom renovations get it wrong.
Drains aren’t just holes. They’re systems. And if any element of that system is messy, water will pool. Or leak. Or back up where it doesn’t belong.
Here’s what proper bathroom drainage needs:
- Proper slope: Shower floors must slope 1/4 inch per foot of floor toward drain.
- Sealed drain flange: This stops water leaking around the drain pipe itself.
- Waterproof membrane: This is either a liquid membrane or sheet membrane which is installed under the tile to prevent moisture from penetrating through to the subfloor.
- P-trap: Keeps sewer gases out of the bathroom.
- Vent pipe: Lets the drain breathe so water flows out properly.
If you are planning any home additions where you will be adding plumbing lines, NOW is the time. Retrofitting drainage will make you cry and almost always ends up costing more.
Linear Drains vs. Traditional Drains
A common question for modern bathrooms…
Linear drains are…linear. Long and skinny — they rest along one edge of the shower. You can have a single slope floor with them (much easier to tile) and they have a much more modern feel. They work really well with curbless showers too, which are super popular.
Centre drains are less expensive and easier to install. The floor slopes towards the center from all sides. They work well for most run of the mill bathrooms, and most contractors know how to install them.
For high-end home additions, linear drains are usually worth the extra money.
Top Mistakes That Wreck Bathroom Floors
Here are the biggest blunders homeowners and DIY-ers make:
- Skipping the waterproof membrane. Tile and grout isn’t waterproof. Water will find a way through.
- Incorrect subfloor thickness. Anything less than 3/4 inch plywood will flex and lead to cracked tile.
- Slopes down towards the drain poorly. It can puddle up and cause mould just from a tiny flat spot.
- Installing regular drywall behind the shower. Use cement board or something equivalent that is waterproof.
- Forgetting the vent line. No vent equals slow drains and gurgling sounds.
Each one of these errors can cost you thousands of dollars in repairs eventually. And don’t think insurance will cover it if it was caused by a slow leak you never knew you had.
How To Inspect Before You Start
Before any bathroom project kicks off, the existing structure needs an honest look.
If you can access the subfloor from below (basement or crawlspace), check for:
- Soft, spongy or stained spots
- Mould or musty smells
- Sagging floor joists
- Old plumbing leaks
Especially if the bathroom you’re adding is on the 2nd floor of your home or above another finished room, you’ll want to pay close attention to this step. One little drip above can lead to one big flood below.
Moisture meter = inexpensive device that detects unseen pockets of moisture within subflooring. New studies show that 25% of bathroom leakage is due to deteriorated shower pans & leaking tub drains — problems that have been buried unseen for decades.
The bottom line?
Inspect first. Repair later. Building on a bad subfloor is pouring money into a hole.
Bringing It All Together
Bathroom renovations get all the attention for the parts you can see.
However it’s what you don’t see — the subfloor and drainage — that will make or break your bathroom. Tile, paint and fixtures can be changed in the future. You can’t change a rotting subfloor.
To quickly recap:
- Pick the right subfloor (plywood plus cement board is a great combo)
- Apply a proper waterproof membrane over the top
- Slope the floor 1/4 inch per foot toward the drain
- Use the right drain style for the space
- Inspect before building over any old flooring
Whether it’s minor improvements, complete makeovers or even major home additions. When done right, a bathroom can last 25 years or more with little issues. If done incorrectly it can fail within two.
Do it right the first time. The rest of your bathroom is only as good as your subfloor.