Namba's World: Put a Cork In It!

Cork has been used for centuries, most notably for its sealing properties. I remember rebuilding the engine on my 1965 Chevrolet super sport and using cork for some of the gaskets; ah, a stroll down memory lane.

Cork is one of the only products that I know of that is used as an underlayment, a floor, and used as a sole for shoes, so, while pulling the cork out of the wine bottle, one can walk in their cork-soled shoes, on the cork floor, which is installed over the cork underlayment! Cork stoppers are still the predominant industry for cork but what happens to all the cork after the stoppers have been made (Photo 1)? Floor and wall coverings take on the second largest industry for cork. So, what exactly is cork and why is there a resurging trend as a floor covering? Cork comes from the bark of the cork oak, Quercus suber, which is part of the Beech family of trees. The majority of cork is grown in the Mediterranean of which, Portugal and Spain are the largest producers. Trees grow approximately 20 to 25 years before the first harvesting and then harvested once every 9-12 years in the spring or summer, as this is the time when the cork bark can be cut away easily from the trunk. A single cork oak tree can be harvested up to 150 to 200 years, making cork an ideal renewable resource, which is one reason to it's increased popularity, and as the cork oak gets older, the finer the quality of the oak (Photo 2).

The bark is carefully stripped off, processed, and made into cork stoppers; scraps are ground up and made into small granules, mixed with polyurethane binders and made into cork composite, then used in a variety of applications. Cork is very light, adding very little weight as an underlayment due to its cellular structure; it is made up of 14-sided polyhedron cells filled with nitrogen and oxygen. The cells are so small that there can be up to 200million of these cells in a cubic inch.

These cells interlock with each other and create a very impermeable product, thus, wine stoppers. The structure of cork also gives it elasticity, so if something is placed on flooring and leaves an indentation, the cork has the ability to recover to its original state. An example is a wine stopper that has been removed from the bottle; it expands to its original form once pulled. Cork is ideal for sound and vibrations, and has excellent insulating properties.

Suberin is a complex fatty substance that is the basis of cork, making it impermeable to gas, liquid, and unaffected by water. It is also fire and insect resistant.

Cork is used for sound control in many high-rise buildings and condominiums in order to meet building codes for sound transmissions through floors and walls. There are two ratings that are used for sound ratings. Sound Transmission Class (STC Ratings), ASTM C634, is a rating, which quantifies the transmission of airborne sounds through building elements such as wall or floor systems. Radio, television, and voices, translate to the STC rating.

Impact Isolation Class (IIC Ratings), ASTM C634 quantifies the transmission of impact sound energy through a floor. Foot traffic, items dropping onto the floor, furniture moving, translates to the IIC rating.

If and when you ever install an acoustic barrier, make sure to follow the manufacturer's recommended procedures, as many building codes and condominium associations have strict tolerances to noise levels.

Here is a basic example of installing an acoustical cork underlayment with a glue down engineered wood floor. Photo 3 shows installation of a three-inch, 6mm cork isolation barrier. A manual 1/2-inch crown staple gun is used to hold the cork up against the wall and any vertical partitions. Staple towards the top of the cork, as you do not want the staples to be set permanently because they will transmit sound through the walls.

Staple just enough to hold the barrier in place and then dry lay the 6mm cork onto the substrate, cut net to isolation barrier and if using panels instead of sheet goods, offset seams (Photo 4).

Spread manufacturer's recommended adhesive with recommended trowel notch onto the substrate, or, some manufacturers recommend using the same adhesive under the cork that you will be using to install your flooring (Photo 5).

Roll with recommended roller (Photo 6).

Install wood directly to cork with wood manufacturers recommended installation procedures. Cut away the isolation barrier and remove staples (Photo 7). Leave a 1/8-inch gap between the base molding and the flooring (Photo 8); the base cannot have direct contact with the flooring, as it will transmit sound directly through the wall. Apply a bead of caulk between the base and the flooring that is recommended by the cork manufacturer.

Cork underlayments come in sheets or panels in varying thickness and can be used under several different flooring products, follow each manufacturers installation guidelines and make sure to allow for height of underlayment under flooring, doors, appliances, etc.

The popularity of cork installations has become much easier with the click or tap-in plank systems (Photo 9).

Suzy Namba is shown installing a tap- in cork floor in Photo 10. Cork is being used in resilient flooring also; shown here are samples of a tile and a stair tread using a combination of rubber and cork. The cork helps to provide slip resistance and reduced noise.

At a recent sand and finish course at the WFCA, we put together a module for a pre-finished cork tile installation. First, set lines were established to mark the center of the panel and then the installation proceeded, make sure to mix up several cartons of cork tile as there is color variation due to it being a natural product, you may have to cull some tiles to get a good blend (Photo 11).

Proper acclimation of cork is critical to a successful installation. The tile manufacturer for this install requires a high solids SBR water based adhesive. One thing that I noticed with several manufacturers of cork is that many recommend different adhesive for their products, high solids SBR water based, water based contact adhesive, PVA adhesive, so once again, follow each manufacturers recommendations. After the installation, the floor was rolled with the proper roller in both directions; this was followed by rolling a second and third time a few hours after the installation to insure proper adhesion. A rubber mallet and seam roller come in handy for tapping or rolling down seams and corners. After the adhesive cured we decided to try a couple of refinishing techniques; keep in mind, this was done on a practice module and here are some of the results we found (Photo 12).

Todd Schutte from a sanding equipment and finish manufacturer using a drum sander with the drum set at the lightest pressure with a 100-grit abrasive. There is 3/4-inch plywood on 19.2-inch centers, the weight of the machine along with the weight of the operator caused some deflection in the flooring causing a slightly uneven cut with the drum sander, the plywood was also running parallel with the joists.

As shown in Photo 13, a 97-pound floor buffer attached to a dust containment system with a double sided 60 grit abrasive was used, but tended to load up the abrasive, a 36 grit abrasive worked better (Photo 14).

For edge sanding, do not use a hand scraper in the corners as it tends to dig into the cork; instead, hand sand or use a delta sander (Photo 15). Using a 150 double-sided abrasive flattened out the uneven cut from the drum sander, this was followed up with a 180 double-sided abrasive. A Tampico brush was used on the buffer to remove any loose particulate followed by a thorough vacuuming and tack (Photo 16).

We decided to stain a small area of the cork to see how it would take; the stain took very well and had an even appearance. For the finish coats no sealer is used, as it is not recommended for this floor; three coats of a waterborne finish were applied, with a maroon pad and delta strips used between the first and second coat (Photo 17).

Photo 18 shows the finished floor. Here's what we found with our test module.

Make sure to determine the thickness of the installed cork flooring before sanding to determine how much can actually be sanded or screened.

A drum sander can be used but, the machine must be adjusted properly, the substrate must be flat with no deflection over a wood subfloor, and the operator must be very skilled as the drum sander can be very aggressive.

A buffer works well and is much more forgiving than a drum sander. A 3-disc type sander would work well also.

Start by using higher grit to determine what works best; don't start with too aggressive an abrasive.

If you know the manufacturer of the cork product, contact them for the recommended abrasive.

Double-sided sanding discs work very well but you will tend to use more discs. A screen will not load up as quickly as a disc but it will not sand as flat as a double-sided disc.

Hand sand or use a delta sander in corners, no hand scrapers.

To seal the joints on a prefinished floor, a 180-220 screen or disc can be used followed by two coats of finish.

For finish coats, apply 3 coats and make sure to follow cork manufacturers recommendations in regards to using a sealer and finish for a sand/screen re-finish.