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What Causes Cupping in Wood Floors?
By Howard Brickman

The official topic of this blog will be an excruciating discussion of the term “cupping” for your consideration. But first…

My apologies for taking so long to get another submission ready, but for me writing is very hard work and extremely time consuming, and I marvel at those gifted individuals who can churn out wonderful written content on a regular schedule. I must admit that I’m not certain that I would be able to increase my output even if I were offered bushel baskets of legal tender to do it on a full-time basis. Time is finite and moves at an increasingly rapid pace. I remember watching the classroom clock as a youngster on Friday afternoons thinking that 4 o’clock would never arrive, and now Friday afternoons pass by faster than the pickets on a fence. But I digress…

If I were going to give an official definition for cupping it would be, “boards that are concave on the face.” There is a common misconception in the wood flooring bidness that all cupping is moisture-related and that pressure that develops due to swelling is the primary cause. Let’s explore some thought experiments.

Experiment 1: We place 10 S4S red oak boards ¾” x 4” x 72” edge-to-edge, which approximates a panel ¾” x 40” x 72”. Then we place pipe clamps at 3” intervals across the 40” dimension and tighten the clamps until a pressure of 200 pounds per square inch is reached. What do you think is going to happen to the shape of the individual boards?

Experiment 2: We place 10 pieces of red oak flooring ¾” x 4” x 72” edge-to-edge, which approximates a panel ¾” x 40” x 72”. Then we place pipe clamps at 3” intervals across the 40” dimension and tighten the clamps until a pressure of 200 pounds per square inch is reached. What do you think is going to happen to the shape of the individual boards?

Experiment 3: We nail 10 pieces of red oak flooring ¾” x 4” x 72” at a MC of 6-8% to a ¾”-thick plywood panel 48” x 72” at a MC of 6-8%. Then we place 1½” deck screws at 3” intervals into the first and the last boards so that they will be prevented from moving. We predrill the oak so that there will be no splitting. Then we place a bath towel on the face of the boards and saturate it with enough water to completely wet the towel but not have water puddling onto the surface of the flooring. Then we put a piece of 6-mil polyethylene over the towel to keep the water from evaporating. What do you think is going to happen to the shape of the individual boards?
 
Experiment 4: We nail 10 pieces of red oak flooring ¾” x 4” x 72” at a MC of 6-8% to a ¾”-thick plywood panel 48” x 72” at a MC of 14%-16%. Then we place 1½” deck screws at 3” intervals into the first and the last boards so that they will be prevented from moving. We predrill the oak so that there will be no splitting. Then we put a piece of 6-mil polyethylene covering the underside of the plywood to keep the water from evaporating. What do you think is going to happen to the shape of the individual boards?

Experiment 5: We nail 10 pieces of red oak flooring ¾” x 4” x 72” at a MC of 14-16% to a ¾”-thick plywood panel 48” x 72” at a MC of 6-8%. Then we place 1½” deck screws at 3” intervals into the first and the last boards so that they will be prevented from moving. We predrill the oak so that there will be no splitting. Then we put a piece of 6-mil polyethylene covering the underside of the plywood to keep the water from evaporating. What do you think is going to happen to the shape of the individual boards?

Experiment 6: We nail 10 pieces of red oak flooring ¾” x 4” x 72” at a MC of 14-16% to a ¾”-thick plywood panel 48” x 72” at a MC of 14%-16%. Then we place 1½” deck screws at 3” intervals into the first and the last boards so that they will be prevented from moving. We predrill the oak so that there will be no splitting. Then we put a piece of 6-mil polyethylene covering the underside of the plywood to keep the water from evaporating. What do you think is going to happen to the shape of the individual boards?

The great thing about these thought experiments is that we could actually do them. If you had a university or commercial testing company do these for you, it would cost many thousands of dollars. If someone wants to send me $12,479.00, I will do the experiments and send you a report on the results with cool pictures. In my next episode we will start to discuss the individual experiments. I want to thank Don Sgroi for the very thought provoking e-mail, which is the inspiration for what will I think be a very interesting series of blogs.
What a greatly needed and apreciated testing inititave with consideration to almost every job site condition one would encounter. Are you also measuring and controlling the RH in all tests. Thanks for all that you do to enlighten us so that we can adress these issues with truth and honest dialogue.
Comment By Riley Gazzaway At 12/13/2011 7:54 AM
When is your next episode? I'm waiting on the edge of my seat.
Comment By Sherrie VandePutte At 12/13/2011 7:38 PM
Howard I appreciate you starting this discussion on my behalf so the least I can do is see if I can answer each one. For all you other readers of Howards, if he can take his time to pose the questions posts the least we can do is take a stab at answering them. The worst thing you can be is wrong but it is a good learning experience. Many thanks to Howard for sharing his knowledge and experience with us.
Ex1- edge crush to two outer boards from clamps.
Ex2- boards will buckle away from clamps due to normal undercut design of flooring.
Ex3- Crowning due to surface moisture.
Ex4- Cupping due to excessive moisture in plywood.
Ex5- Moisture will absorb into the air and ¾ ply causing gaps in the flooring.
Ex6- As flooring dries from the surface the floor will exhibit cupping and gapping.
Don Sgroi
Comment By Don Sgroi At 12/14/2011 1:08 PM
I'm anxiously waiting for the results on your next blog. Thanks
Comment By Willie James At 12/15/2011 8:47 PM
Hi Howard,

Thank you for this post. So I will take my shot at this:

1. Boards will exhibit edge crush and some lifting in the middle. I am assuming you refer to four sides smooth lumber pieces which are square on edges.
2. Buckling.
3. Crowning.
4. Cupping.
5.I would say it depends on the ratio of water evaporating from the floor: Up towards the air to down towards the plywood.
If the ratio is the same then just gaps.
If water is released to the air at a greater rate than the bottom we will see cupping and gaps.
If the RH above is high enough that water is only being absorbed into the plywood then crowning will occur.
6. If the RH level permits it could be cupping and gaps OR no movement at all.


And now for the fun part:
I have a question about that number you threw there at the end. Do you charge by the square foot? Can you break it down for me for labor and material?
Can you match a lower bid I got?

Hope you are all smiling by now!

Merry Christmas!

Avi Hadad, Avi's Hardwood Floors
Comment By Avi Hadad At 12/16/2011 3:17 PM
A pipe burst and the water was mopped up quickly and the insurance man sent a drying team out the next day. They put the drying mats down with heated fans and dehu's. This was kept up for 5days. The room seemed to be 100degrees for the entire time. 3weeks later the floor is crowned like crazy and you see gaps in the floor as well as in all of the other trim like doorcase and crown. I could not get a moisture reading from the hardwood with my meter(its lowest # is 6%). The customer and his ins guy wants it simply sanded to remedy the crown, but i am scared of it re-balancing and doing something I cannot predict. Can I please have yalls advice?
Comment By jeremy At 12/23/2011 9:11 AM
Jeremy, I've posted your question in our online discussion forum at: http://bit.ly/tVQmaR
Comment By Kim Wahlgren At 12/28/2011 11:03 AM
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