DIY permanent bathroom floor made with cheap cork underlayment

There’s no home-improvement motivation like sewer water dripping through the ceiling onto your couch!  I had noticed the cracked tiles around the base of the toilet a couple years ago, but chose to ignore them.  Our “master bath” (shower+toilet) had clearly been a cheap finish to a big addition and I wanted to believe the cracked tile dated to the 1998 toilet installation, rather than suggesting an underlying problem.

Of course, I was wrong.  A quick examination of the toilet bolts showed that they were rusted through on one side and nearly corroded-through on the other.  The subsequent rocking of the toilet had allowed flushed water to leak past the wax seal and slowly saturate the particle board underlying the tiles, as well as the 19mm plywood sub-floor.  The moisture and rot further destabilized the toilet and the surrounding tile began to crack with the rocking.  Luckily, the ceiling rafters were unscathed (dry, no rot), so I only had to demo about 1 m^2 of the subfloor and 2 m^2 of the drywall ceiling in the playroom below (after cleaning the poop water out of the couch).

Looking at all the carefully labeled bathroom tiles and starting to imagine chipping all the old grout off of them, I realized that I’ve always wanted to try a cork floor!  So, out came all the rest of the tiles and particle board, and into the trash went all those labeled tiles, along with the moldy drywall and sewage-soaked wood.

With the rafters drying out and new drywall on the ceiling, the main problem became how to install a cork floor.  The Internets made it clear that not only is there some controversy about using cork below grade, and in basements, bathrooms, and kitchens, but also manufactured cork comes in many forms: cork tiles with adhesive backing; cork tiles that click together; cork tiles that you glue down, often with a water-borne contact cement (like the oft-recommended Waktol D3540, Roberts 7250, and Dritac 6200); and tongue & groove cork planks.  Most of these products cost 3-4$/ft^2 (with tiles usually being 1 square foot each), plus $10-50/gallon for glue.  One basement job estimate was about 2x that cost installed, ~8$/ft^2 for snap-in cork floor delivered and installed by Home Depot.  Another estimate in a floor-contractor forum was 10-15$/ft^2 installed.  I only needed to cover about 20 ft^2, so the DIY materials cost of a few bucks/square-foot didn’t seem too expensive, but many of the manufactured cork products (e.g. GreenClaimedTorly or Forna) come pre-coated with a sealant on the upper surface or require sealing of that surface and/or seams after installation (most commonly polyurethane, sometimes with an embedded more durable material like ceramic beads).  I’ve grown to hate polyurethane from having it fail on our oak floors (e.g. with water damage from a houseplant or wear in a high-traffic area) and then be near-impossible to spot-repair.

Instead, I wanted a bare or uncoated cork so I could seal it with an eco-friendly, low-VOC product made by Osmona.  I had loved re-finishing oak floors in our previous house with Osmo Polyx-Oil (hardwax-oil”) and was delighted to notice that it was also recommended for cork floors.

As a boat-builder familiar with water infiltration and damage, I was also worried about the seams between cork tiles or planks in the floor of a bathroom (or kitchen).  Would water from a dripping shower or cleaned human or overflowing toilet work its way through the seams?  Most folks seemed to recommend only the pre-sealed tiles in bathrooms, and then sealing the seams with 3-4 coats of a sealant.  That sounded like a lot of work and waiting, as well as the normal nightmare of having to re-finish the polyurethane every 3-7 years or replace the whole thing when only a spot repair was really necessary.

Thus, to pioneer a super-cheap yet functional solution I went looking for DIY bathroom or kitchen floors made with bare cork (ideally cork underlayment which costs only ~$0.80/ft^2 — 3-6x cheaper than tiles!), but found little guidance about how to install a permanent, waterproof cork floor.  This temporary cork floor from a renter in New York was heading in the right direction, but they just laid underlayment down without any gluing or sealing.  This general guide to installing bare cork from familyhandyman was helpful and taught me that the traditional finish for a cork floor was a paste wax.

Without much guidance available, I decided to experiment.  I bought some 6mm (1/4″) thick underlayment for $0.78/ft^2 from Amazon as well as range of adhesives and the Osmo-made sealant I hoped to use.  Using 20cm squares, I tried gluing down single layers on a piece of clean 1/2″ plywood, as well as laminating triple layers of cork underlayment. I also coated experimental squares of cork with the Osmo Polyx-Oil.

The sealant test surprised me a bit.  The epoxy was just too hard, making the cork feel stiff and reducing the likelihood that I’d ever get it off (or even get through it to the screws holding down the underlying plywood) in a worst case scenario.  The 3M spray glue and the DAP contact cement wasn’t holding down the edges well, plus I couldn’t imagine coating 2 surfaces, keeping them from sticking to themselves while getting tacky, and then successfully lining them up on the first (and only) try.   I was going to go with the official cork underlayment adhesive (easy to spread with the right tool and forgiving if the cork needed to be repositioned once in contact with the adhesive), but it was stinky and didn’t cure completely, even after a few days (especially in the 3-layer lamination test).

In the end, I went with interior/exterior Titebond II wood glue.  Though I could have used an extra bottle of it, I ended up using some plain old interior Titebond glue and it seems to have mostly held the cork down.  In places where I detected it was lifting a bit as I cleaned it and then sealed it, I was able to inject some 2P-10 superglue (cyanoacrylite) using a syringe attached to a ball inflator needle, getting the loose portions to stick down by applying pressure for a minute or two.  After replacing the molding, I caulked the top of it (as well as the base of the shower) and let it dry for ~24 hours.

I put the Osmo Polyx-Oil put down in two coats.  First coat went on thick (used about 1/2 of the 750ml can) and dried and soaked in for 36 hours.  Then I installed the toilet flange, wax seal, and toilet.  The second coat of Polyx-Oil took less (about 150ml).

I may add a third or fourth coat if the water doesn’t bead up enough, or I decide to try to fill up the voids in the cork over time…

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So far (a few days into use of the bathroom), all family members seem satisfied.  There’s general agreement that the cork has a warm feeling — both to the touch with bare feet and in terms of the colors in the bathroom.  My only gripe so far is that the cork and sealant are giving off a noticable waxy/woody smell.  With luck it will go away, but I’m worried the solvents in the Polyx-Oil are interacting with the cork organics and/or the Titebond adhesive under the cork.  Luckily, the cork seems to be staying adhered to the underlying plywood fine…  I’ll provide updates as we make further observations and use the bathroom more.

Budget

  • Final DIY materials/tools cost: $65 total, including experiments; $47 for cork flooring (underlayment, adhesive, and sealant) which for 20 sq.ft. area comes to ~$2.40/ft^2
  • Spreadsheet with source and links

Test results: adhesives

  • Titebond wood glue
  • 3M spray glue
  • Dap water-borne contact cement
  • Roberts 1407 (wanted Roberts 7250 as recommended on cork underlayment label, but could only find locally in 4-gallon containers)
  • System Three epoxy

Test results: sealants

  • Osmo Polyx-Oil (interior)
  • Osmo Deck oil (exterior)
  • Fill voids in cork with Osmo UK wood filler or another wood filler?

 

 

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