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…


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.


  • 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?



Science clarifies risks of “the really big one” for Seattle residents

All the recent hoopla regarding “The Really Big One” (2015 New Yorker article describing the terrifying possibility and risks associated with a magnitude 9 earthquake in the Pacific Northwest) motivated me to do two things: take some reasonable actions to be more prepared; calm my relatives down a bit by elucidating the risks of living with the Cascadia Subduction Zone based on my reading the relevant primary scientific literature.  The key to the latter task is clarifying two key parts of Schultz’s story:

“…the odds of the big Cascadia earthquake happening in the next fifty years are roughly one in three. The odds of the very big one are roughly one in ten.”

and (adding bold emphasis and changing written out numbers to actual numbers)

“Thanks to that [Goldfinger’s] work, we now know that the Pacific Northwest has experienced 41 subduction-zone earthquakes in the past 10,000 years. If you divide 10,000 by 41, you get 243, which is Cascadia’s recurrence interval: the average amount of time that elapses between earthquakes. That timespan is dangerous both because it is too long—long enough for us to unwittingly build an entire civilization on top of our continent’s worst fault line—and because it is not long enough. Counting from the earthquake of 1700, we are now 315 years into a 243 year cycle.


It is possible to quibble with that number. Recurrence intervals are averages, and averages are tricky…”

Ok, so let’s quibble by looking at Goldfinger’s work, specifically his analysis of layers of submarine mud that avalanche deeper into the sea during big earthquakes (a.k.a. turbidites).  In “Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone” (Goldfinger et al., 2012), you can find in the abstract the results that underlie Schultz’s prose:

The combined stratigraphic correlations, hemipelagic analysis, and 14C framework suggest that the Cascadia margin has three rupture modes: (1) 19–20 full-length or nearly full length ruptures; (2) 3 or 4 ruptures comprising the southern 50–70 percent of the margin; and (3) 18–20 smaller southern-margin ruptures during the past 10 k.y., with the possibility of additional southern-margin events that are presently uncorrelated. The shorter rupture extents and thinner turbidites of the southern margin correspond well with spatial extents interpreted from the limited onshore paleoseismic record, supporting margin segmentation of southern Cascadia. The sequence of 41 events defines an average recurrence period for the southern Cascadia margin of ~240 years during the past 10 k.y.


19+3+18=40 events, and 20+4+20=44 events

so it seems Schultz tries to be sort-of conservative and chooses to divide by 41:

10,000 years / 41 events = 243.9 years between events

= 244 year average recurrence time

We could get a range of recurrence times instead by using the range of number of events — 40-44:

10,000/40 = 250 years

10,000/44 = 227 years

And if we average those estimates, we’d get (250+227)/2 = 238 years

So, rounded down (for whatever reason) the bold math above yields (approximately) Schultz’s 243 year average recurrence time for a “big Cascadia earthquake” by which she means either a full-rupture earthquake (involving the whole boundary between the North American and Juan de Fuca tectonic plates, from British Columbia down to northern California) or a shorter/smaller ruptures at the southern half or 2/3 of the margin (CA and OR).  But we could break these averages down for big full plate ruptures (let’s assume 20 rather than 19), 50-70% southern ruptures, and smaller partial southern ruptures.  In the grey literature (not peer-reviewed) document “CHARACTERIZING THE CASCADIA SUBDUCTION ZONE FOR SEISMIC HAZARD ASSESSMENTS” Wong et al. (2014) make a similarly motivated division of earthquakes, assuming they would fall into groups of magnitude 9, 8-8.8, and <8.  Adopting a similar terminology (and implicit assumptions), we can go back to the Goldfinger abstract and calculate

10,000 years / 20 full-length events = 500 years between magnitude 9 events

10,000 years / 50-70% length events = 2,500 years between 8-8.8 events

10,000 years / 20 full-length events = 500 years between magnitude <8 events

In the spirit of quibbling, what magnitude earthquakes are we talking about in the New Yorker article?  Schultz implies we are talking about the really big (~9.0) and big ones (>8), but not the <8 events:

If, on that occasion, only the southern part of the Cascadia subduction zone gives way—your first two fingers, say—the magnitude of the resulting quake will be somewhere between 8.0 and 8.6. Thats the big one. If the entire zone gives way at once, an event that seismologists call a full-margin rupture, the magnitude will be somewhere between 8.7 and 9.2. That’s the very big one.

So maybe it would have been most appropriate for her to sum the number of these larger events from the turbidite record, but leave out what Goldfinger termed the “smaller southern-margin ruptures” —

10,000/(20+4) = 10,000/24

= 417 years between earthquakes greater than magnitude 8.0

If so, then we can transform one of her scariest sentences into something substantially less terrifying: “Counting from the earthquake of 1700, we are now 315 years into a 243 417 year cycle.”  So, on average we shouldn’t expect a big or really big earthquake for another 100 years or so.

If you want to get more geographically explicit, consider this nice figure from “Tsunami impact to Washington and northern Oregon from segment ruptures on the southern Cascadia subduction zone” (Priest et al., 2014), modified from Goldfinger et al. (2012) —

Yellow text show recurrence times for different inferred segments of the full-rupture patch of the Cascadia Subduction Zone.

Yellow text show (simple average) recurrence times for different inferred segments of the full-rupture patch of the Cascadia Subduction Zone.

Of course, all this averaging assumes that earthquakes are random (time-independent) rather than cyclical or periodic (time-dependent), but Goldfinger et al. (2012) point out that — conveniently — it doesn’t matter if you use simple averages or complicated earthquake modes, you get about the same computed likelihoods:

Time-independent probabilities for segmented ruptures range from 7–12 percent in 50 years for full or nearly full margin ruptures to ~21 percent in 50 years for a southern-margin rupture. Time-dependent probabilities are similar for northern margin events at ~7–12 percent and 37–42 percent in 50 years for the southern margin. Failure analysis suggests that by the year 2060, Cascadia will have exceeded ~27 percent of Holocene recurrence intervals for the northern margin and 85 percent of recurrence intervals for the southern margin.

These statistics are what underlie Schultz’s memorable 50-year odds: 1 in 3 (~30%) for a big one; 1 in 10 (~10%) for a really big one.  To the extent that either probability is worrisome, it’s got to be the 30% chance of a big one down south in the next 50 years.  Interestingly, in her follow-up article “How to Stay Safe When the Big One Comes” Schultz clarifies that the 30% probablity is indeed for a magnitude 8-8.6 event:

The odds I cite in the [original] story are correct: there is a thirty-per-cent chance of the M8.08.6 Cascadia earthquake and a ten-per-cent chance of the M8.79.2 earthquake in the next fifty years.

But from Seattle’s perspective, what will be our experience of a magnitude 8-8.6 earthquake, particularly one with an epicenter in Oregon?  It’s not clear to me if the shake maps Schultz provides in her follow-up are for a full- or partial- rupture.  The symmetry of the contours suggests they are for a full-margin rupture.  We need clarification, or another model run (for the full Northwest region) of this most likely (30%) type of earthquake!

The most helpful things I’ve found as I continue to “feel” the risk and decide whether and how to proceed are this timeline from this PDF

Seem likely the next one will be big, not really, big, and maybe pretty soon?

Seem likely the next one will be big, not really, big, and maybe pretty soon?

— and this figure depicting the height of a worst case tsunami as it moves up the OR/WA coast (from a magnitude 8.7 earthquake, aka simulation C588 centered in southern Oregon) —

As the tsunami propagates northward, the maximum wave height deceases from ~10 meters near the epicenter to ~2 meters in northern coastal Washington.  There is no tsunami risk in Seattle from a big or even a really big subduction earthquake.

As the tsunami propagates northward, the maximum wave height deceases from ~10 meters near the epicenter to ~2 meters in northern coastal Washington. There is no tsunami risk in Seattle from a big or even a really big subduction earthquake.

Despite all this scientific quibbling, I applaud Schultz on getting us all to be more prepared.  Here in northeast Seattle, I plan to refresh our emergency plans and kits, and look into a seismic retrofit for our 1926 house.


U Mich makes algal crude in minutes

A Wired article (thanks Mike!) that got me thinking about how much of crude oil’s energy is geologic, rather than photosynthetic.  Upon deposition of biogenic sediments, there is tectonic transport, geothermal heating, and compression in subduction zones or beneath additional deposits.  How to account for these energetic contributions?

Also, the researchers’ general approach seems sort of sloppy.  A good terrestrial farmer would harvest a crop (of plants), mechanically process it into products, and compost/recycle the “waste.”  Here the crop of algae appears to be simply cooked en mass (including with water) into crude with little analysis of how distillation or cracking would generate products from the resultant soup.  Given that we have the option of processing the crop before cooking it (which was long-missed for fossil fuels), is it more efficient to process before attempting thermo-baro-chemical transformations, or to crack apart the goo once cooked? 

Compact fluorescents:75% power savings

Brightened up the playroom and kitchen a bit today by replacing ~500W of incandescent bulbs in recessed ceiling fixtures with 7x23W (=~175W) CFs from Home Depot (about $4 each).  The “green” n:Vision bulbs have a pleasant, warm glow to them (the “blue” and “red” really would work only in the garage) and Annie agreed the situation was improved.

That’s a power reduction of about 70% for those two rooms.  For an optional source of additional brightness, I retrofitted a broken halogen torchiere lamp to take 2x25W CFs (equivalent light output to 150W incandescent).  Since the kitchen already had about ~150W of CFs lighting, that brings the total lighting demand for those most-used rooms in our home to 325-375W.