Cupping by the Key: An Unexpected Source of Moisture in a Gym Floor

The Problem

I was called to repair a gym floor previously installed at an elementary school here in the Pacific Northwest. The entire gym floor showed signs of cupping; however, there was an area just outside one of the keys on the basketball court where the cupping was more significant and the floor, when stepped on, felt as though it were starting to lift from the concrete subfloor.

The Procedure

The new elementary school and the gym floor had been completed in the summer. The strip maple flooring was installed over a concrete slab overlaid with 6-mil poly and two layers of 1/2-inch plywood stapled together with rubber cushions spaced 1 foot on center. The wood floor was sanded and finished with four coats of waterborne gym finish. Several months later, in winter, there was a roof leak, but there was no evidence of cupping at that time. I was called in when the cupping appeared in the spring. Everyone at the school was convinced the roof leak was causing the moisture issue.

The Cause

As I drove to the school, I could see it was located at the base of a mountain. I noticed there was a lot of standing water in an adjacent field. It hadn’t rained in awhile, but in this area in spring, there is snowpack runoff from the mountains, and it was apparent that the water table in the area was very high.

In the gym, the cupped area seemed to fan out from the middle of the floor toward the interior wall of the gym. The maple flooring moisture readings ranged from 12-15%. I removed an area of maple most severely affected and moisture-tested the subfloor. The readings ranged from 25-30%. I realized that the high moisture levels were contained to one spot on the floor. As I started removing the second layer of plywood subfloor, I could see black holes where the staples penetrated the first layer of plywood. Removing the first layer of plywood, I noticed it was black on the underside, indicating excessive moisture.

Once the plywood subfloor was removed, I found the source of the moisture: a hole in the concrete and vapor retarder. Typically, when we install wood flooring in a gym, we plot out the game line grid and core through the flooring down into the slab and install the anchors for the volleyball posts. At least one of the holes had been cored in the wrong spot. When the groundwater increased in spring, it came up through the hole in the slab.

How to Fix the Floor

There were no visible signs of standing water or moisture droplets under the poly at the time the flooring was removed, so the school board decided to fill the hole with a fast-setting grout and overlay the area with new poly. The area was re-covered with new plywood, and the maple was spliced back in. The area of approximately 150 square feet was re-sanded and re-finished, and game lines were re-painted. There was still visible cupping throughout the floor, but it was important that there was no further loss of service of the gym. The intention was to monitor the cupping in the gym and re-address the floor in the summer.

In the Future

When it was discovered that the hole was cored in the wrong spot, the contractor should have filled the hole immediately and covered the affected area with new poly. Since there was still visible cupping throughout the floor I suspect that there might be more than one hole that was cored incorrectly.

The White Line Syndrome

The Problem

Recently there has been more discussion of white lines showing up over dark-stained floors. This is not a new problem for floor finishes, but it is becoming a more common one. As VOC regulations become more stringent around the country, more floor finishers are trying out waterborne coatings. They are also finding that there are more differences in the products

than just smell and application techniques. When a floor is stained with a darker-colored stain and then finished with waterborne coatings, white lines between flooring boards may appear anytime, especially during seasonal changes when the humidity drops in our homes.

 

 

 

The Reason

The reason for the white lines is that waterborne and other coatings tend to stretch, some more apparent than others. Research shows that the isocyanate crosslinked products we tested stretched across board edges, leaving white lines.

Which one is best? Plain Sawn or Rift & Quartered? Why should I buy Quarter-Sawn?

1)   Wood is a natural substance that is much stronger when the grain is continuous. Wood is a natural polymer, consisting of parallel strands of cellulose fibers held together by a lignin binder;

2)   Rift & Quartered Sawn Hardwood Floors (wood with straight grain) expands vertically, while the Plain Sawn expands horizontally. That being said, Quarter-Sawn wood is more stable than Plain-Sawn. Not only is it less prone to cupping, it also expands and contracts less.

3)   For the looks, Quarter-Sawn provides a “quieter” and straighter face grain than Plain-Sawn.

Knowing that Quarter-Sawn wood can out price Plain-Sawn by a factor of two (or more), these are the compelling reasons to buy it.

Hardwood Floor Care

Numerous times consumers are led to believe Murphy’s oil soap and wet swifter’s can be used on hardwood floors. Having seen the effects these products have on pre-finished as well as site finished floors I recommend you stay away from both.

Unfortunately wet swifter’s can cause damage to wood floors and Murphy’s oil soap and like products will leave a residue build up and dull any hard- wood floor surface appearance. Never use a product that claims to resurface the existing finish, these products are short term solutions that will end up costing you more when the time comes to refinish.

• Use cleaning products specifically formulated for polyurethane finished hardwood floors. I recommend Bona or Bruce wood floor cleaning products, which can be found at our pro-shop*, Home Depot or on line.
• Vacuum, sweep or dust-mop your floor once a week, or more if needed. The vacuum head must be soft brush or felt, and a wand attachment is preferable. A hardwood floor swivel head mop w/ terry cloth cover is highly recommended to eliminate finer particles of grit and dirt that can act like sandpaper on hardwood floors.
• Wipe up spills and tracked in dirt immediately. For spot cleaning use soft cloth with wood floor cleaning product.
• Mop floors w/ DAMP sponge mop and a non-abrasive, phosphate free cleaner. This mop should be designated to your wood floors only.
• Properly placed mats and throw rugs will help keep unwanted dirt and moisture off hardwood floors. Mats should be placed at entry doors. A throw rug or mat in front of kitchen sink will help prevent damage to your wood floor from moisture and excessive wear.
• Most carpet stores as well as Home Depot carry a rubber mesh fabric that can be cut to size and placed under throw rugs and mats. This prevents rugs from slipping and helps prevent moisture from building up under them.
• Put fabric faced glides under the legs of furniture to prevent scuffing and scratching.
• For large sofas, love seats, recliners etc. Home Depot sells Dura Grip a product that safely stops furniture from moving, as well as protects the floor. 

What You Should Know About Cupping

Cupping Defined

Cupping has occurred when the sides of flooring are higher than the center of the boards – the surface of the board has a concave shape.  Solid and engineered wood flooring can both cup.  We’ll discuss each of the construction separately, as the dynamics are different.  The driving force for cupping comes from a response of wood to a change in the moisture content (MC) in both cases.  Below the fiber saturation point (typical results 28%-30%) wood swells if the MC is increased and shrinks if the MC is decreased.  The mechanism that causes swelling can be understood by looking at the cellular structure of the wood.

Cupping in Solid Wood Flooring

Cupping occurs in solid wood flooring as a result of an elevated MC in the bottom of the flooring (Hardwood) compared with the MC of the face.  The general effect is easy to demonstrate by putting small drops of water on a small strip of paper and you will notice the paper start to curl away from the water.  It is not hard to see the effect in wood flooring, either.  For many species and aspect ratios, a piece of flooring placed on a towel that is kept too moist will show cupping within days.  With those demonstrations, the source of the water is easy to identify because water is liquid however wood can also take on water vapor from surrounding air to cause swelling in the same way.  Any air that has a relative humidity (RH) above zero has the water vapor mixed into it.  Given enough time the MC of the wood will come to an equilibrium value based on the RH of the air.  If the MC is increased the wood will swell, just as it does when liquid water is introduced.  The vapor driven process Is slower than when liquid water is present.  Problems caused by water vapor can take several weeks or even months to become evident.

Water emissions from a concrete or a wet sub-floor can cause a cupping problem in any climate.  Swelling is the largest in the tangential direction, which is across the face of a plain sawn flooring or across a rotary peeled face.  The amount of cupping will depend on the species of wood, cut, width and thickness of the restraining forces that may be present.

Cupping in Engineered Wood Flooring

Unlike solid wood flooring, some engineered floors can cup when the MC is lowered.  Although all engineered flooring is not designed in the same way, a common construction involves the face to be the desired species applied to a backer.  The backer  (often plywood yet sometimes HDF) provides structural integrity and stability in changing moisture conditions for the relatively thin face.  The MC of the face and backer should be the same at the time of manufacture.  The moisture level at the time of manufacture should also be the same as the MC the product will experience during use.  This type of construction involves two layers that react differently (CROSS PLY) when the MC changes after the flooring will perform best in the MC range near the value it had at the time of manufacture.

If the MC of the flooring is decreased, the face material attempts to shrink approximately 10 times faster than the plywood backer.  As the face pulls across the width of the piece against the backer, the flooring starts to curl up or cup.  This is similar to way that a bimetallic thermometer changes shape by bending when the temperature changes.  Dry cupping can be a yearly occurrence in cold or extreme temperatures areas with a long heating season.  There are some instances reported in wide engineered flooring where the curling is more pronounced along the edge of the plank above the groove.  Milling engineered flooring with more of the backer above the groove ( Lowering the groove) can lessen the problem of the flooring deforming when its dried.  Unlike the situation with solid flooring, extended acclimation times will NOT lessen the possibility that an engineered floor will cup at a LOW MC.

No Design is a cure all that will perform well in all environmental conditions.  Different products have strengths and weakness in different situations, and customers are best served when their vendor is knowledgeable about the properties of each product so that the chosen wood floor can perform exactly as hoped.

TEN THINGS TO NEVER DO TO A WOOD FLOOR

TEN THINGS TO NEVER DO TO A WOOD FLOOR

1.     Do not use abrasives or harsh chemicals to clean your floor. We do not recommend the use of ammonia, vinegar, dish washing detergent, powdered all-purpose cleaner, Endust, Pledge, oil soap, or any type of wax or silicone base products on your floor.

2.     Do not use hard casters on any furniture directly on your hardwood floor. Casters can cause isolated wear areas on your floor and increase the frequency of recoat and refinish projects.

3.     Do not use steam mops on your hardwood floor.  The use of water in all its forms, including steam, on wood floor should be limited to the degree possible. The high heat from some steam mops can also serve to thermally break down your floor’s protective layer over time. 

4.     Do not pour cleaning product directly on floor.  It may be tempting to create a puddle of cleaner and spread it out over your floor but this approach can have the same effect as using too much water when cleaning. Cleaning products that are applied too liberally in spots can absorb into wood, cause swelling and an uneven look over time. 

5.     Do not wait too long between refinish projects.   Any wood floor finish will eventually show wear patterns and surface scratches.  When you begin to see dull traffic areas, it is time to consider re-coating your floors. Re-coating is a one day process, and cost much less than sanding/refinishing.  You should plan on re-coating hardwood floors every 3 to 5 years depending on traffic patterns and wear.  Do not wait until you wear through the finish to the bare wood or sanding may be required.

6.     Do not wear high heel shoes on wood floors. High heel shoes can cause denting in hardwood. It is wise to leave high heels at the door and make sure that they are always in good shape. Damaged high heels can quickly become a chisel on your wood floor.

7.     Do not leave the legs of your furniture unprotected. Protective pads should be added under the legs of every piece of furniture that comes in contact with your hardwood floor.  The weight of furniture left unprotected can damage the finish and the wood over time.

8.     Do not drag furniture or heavy appliances over your hardwood floors. Heavy furniture should be lifted carefully off of the floor and placed back on the floor once in their new position. Felt pads, furniture straps, and additional assistance are all good ideas when moving heavy objects over wood floors.  

9.     Do not let your dog’s nails remain untrimmed. The sound of a dog nails as they skitter across a wood floor is not just annoying, but those nails will ruin your finish and eventually ruin your floors is they remain untrimmed. Both you and your pet will be happier if you trim their nails when required. 

10.  Do not allow pet stains remain untreated.  If Fido or Kitty have an accident on your wood floor, be sure and address the issue immediately.  The longer urine sits on a wood floor, the worse it is for the floor. If the accident happens on an area rug or throw rug, make sure to clean and dry both the rug and the wood floor underneath the accident.

 

What is a Janka rating and how important is it?

The Janka* hardness test is designed to measure the resistance of a sample of wood to denting and wear.   The test is one of the “Standard Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel Materials”.  Sound fascinating, huh?  Well, fortunately, it’s not a test you have to perform yourself.  As you have probably discovered, the results of the test on a huge variety of woods can be found all over the Internet, and, in fact, the ratings of most of the woods sold here at Prestige Flooring and Interiors can be found on the chart below.

For flooring, one of the most important criteria in choosing the right wood is its resistance to denting as a result of pressure from, say, a stiletto heel, the tip of a small chair leg, a pet’s toenail, etc.  The test, to some degree, replicates such impacts by embedding a .444-inch metal ball to one half the ball’s diameter into the surface of a clear (knot-free) wood specimen.  Two areas on the face of each specimen are tested, and the information is recorded and averaged.  A species’ Janka rating is a measurement of the amount of force (usually in pounds of force, or LbF) required to do this, and there is a standard deviation associated with each species.

What is important to understand is that there are many factors that will affect a floor’s durability beyond the Janka rating of its species.  Remember that Janka testing is done on solid, unfinished wood with a moisture content of around 12%.  Wood flooring, however, has been milled at about 6-9% moisture content and, by the time you walk on it, has been treated with several layers of protective finish.  With engineered wood flooring, you also have different materials underneath the hardwood layer, which will significantly affect the floor’s overall “hardness”.   Add to that variances in the quality of manufacturing and finish, and even grain direction, and you can see that a high Janka rating is by no means an assurance that your floors will stay clean and beautiful for all time.

In reality, all flooring is subject to damage and wear.  In the end, the primary factor in how well your wood floors hold up over the years is how you treat and care for them.  While it is advisable to start with a harder wood if you anticipate high traffic and rough use, keeping your floors clean, protecting them with rugs in high-traffic areas, and choosing woods and finishes more likely to mask scratches and dents can be just as, if not more important.