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Brian Lewis 12-03-2010 08:33 AM

Hardwood flooring cupping
Our home ( bungalow ) is about 25 years old. Recently we had the parquet flooring removed and new high quality oak flooring installed over the original 25 year old plywood subfloor. The wood flooring was properly allowed to aclimatize to the house for 10 days before laying. The house is airconditioned in the summer and has a heated and finshed basement. The floor is now cupping in certain places and small hairline cracks are appearing on some boards near the end of the board. An inspector from the manufacturer came yesterday and found moisture and hummidity levels were perfectly normal and within acceptable ranges. He said the problem was due to my contractor not leaving any space between the flooring and the walls of the house - in other words the contractor laid the flooring tight up against each wall. Everything I have read about cupping says it has to be a moisture problem which is not the case here. So my question is can laying a hardwood floor tight against a wall result in cupping and small hairline cracks since according to the manufacuturer the wood has no place to expand to. Thanks for any help.

Joe1. 12-03-2010 02:56 PM

Hi Brian,

I just did a few repair sections on a floor, my first hardwood flooring project. I did a fair amount of research to make sure I did it right, and the floor should have about 1/4 inch space all around to expand. If you've ever seen oak floor change between seasons you'll understand why. In spots of my house I see large gaps practically disappear in the summer months.

So on the repairs I did I tried to leave about that much, 1/4 inch space. If one side had a little less, say 1/8 inch, usually the other side had a bit more space. I think I left enough, but we'll see what happens when summer comes.

So if he didn't leave a space, that could be the problem.


Daniel Holzman 12-03-2010 04:54 PM

The inspector is correct in stating that there should be a gap around the edge of the floor. Hardwood typically expands between 1 and 2 percent due to moisture changes, perpendicular to the direction of the grain. Different woods expand different amounts, for a full discussion of this issue either go to which is the national hardwood flooring association. Since the wood will expand when the relative humidity is high, it is essential to have a gap the wood can expand into.

HOWEVER, the effect of a lack of gap is to cause the entire floor to buckle. This is not what you described. Cupping of a board is almost always due to differential moisture conditions between the bottom of the board and the top. For example, if you spill water on the top of the wood, the top fibers expand, while the bottom fibers remain the same length. The net result is the the board cups downward (convex). If the bottom of the board is in contact with moist air, say a damp basement, the bottom of the boards expand, the top remains the same length, and the boards cup upward (concave).

You do not state which way the boards are cupping, however the appearance of hairline cracks suggests that the humidity in the house is lower now than when the boards were installed, causing shrinkage and cracking, which is normal. Cupping suggests there is a difference in relative humidity between the upstairs and downstairs.

The inspector is certainly correct in stating that there should be a gap around the edges, which if there is not is an indication of an improper installation, however it seems unlikely that the lack of a gap is related to the cupping.

rusty baker 12-03-2010 06:07 PM

What kind of moisture test did the inspector run and what were his readings? You have a right to know. You may have to hire your own inspector. The inspector working for the manufacturer has to keep them happy to keep his job.

Shamus 12-03-2010 06:10 PM

Daniel has given you the correct answer. Well done Sir!:thumbsup:

rusty baker 12-04-2010 11:54 AM

There are many of what we call "drive by inspectors". Sounds like that is what you got. Cupping is caused by moisture...period. That inspector works for the maker of the hardwood. He is there to protect them and/or the installer. Hire an independent inspector. The installer is responsble if he did not do a moisture test. Sue them if your inspector finds a moisture problem. I hope this wasn't LL, they do this all the time.

Floor Doc 12-04-2010 02:59 PM

Agree with Rusty .
Time has nothing to do with acclimation. Readings must be taken.

Cupping is caused by a rapid loss of moisture in the wood. Checks or cracks in the boards are also caused by loss of moisture.

Expansion space should be left at the walls .
Solid strip flooring does have a shrink/swell coefficient of 1% in the length and 4% in the width.
From what you describe sounds like the wood was installed wet .
Do like Rusty suggests and get a independent Inspector .
Good Luck.

Brian Lewis 12-05-2010 04:36 PM

Thanks to all who took the time to respond
Thanks to you all for the information and suggestions. Will definitely get an independent inspector. The manuafacturer is supposed to send me a written report in a couple of weeks. I will see what to do after that. We are not seeing any more cracks and the cupping is only on certain boards so I am begining to think that these paticular pieces were indeed not completely cured. Not sure what kind of instrument the inspector used to check the moisture level. It was digital and he took pictures of the readings with a digital camera. I'll post what happens when I have news.


Floor Doc 12-05-2010 05:10 PM

Sometimes the cupping will flatten out , sometimes it is permanent.
More subtle cupping can be caused by lack of proper acclimation (this is generally permanent cupping).

Post back if you feel you need a independent inspector . A few of us on this site know very good ones in every state.
Good Luck

Gary in WA 12-05-2010 11:38 PM

Check to make sure the problem boards are not directly over a downstairs bath fan that may have been jarred loose during the floor install. Or a caulking leak at the siding to allow moist outdoor air into that joist bay behind the rim..... Over a laundry room or furnace, HWTank? No signs of water damage in the ceiling below- discoloring, exposed tape on drywall, etc. Area near a bathroom on other side of wall with supply lines there? Or sponginess of floor near toilet in bath, same bay? Any water source nearby? Have next inspector moisture test the ceiling below. Just thinking out loud......


Jackofall1 12-06-2010 12:33 AM

Just a question, did the installing contractor lay roofing felt down ontop of the sub-floor? If not they should have, this certainly helps prevent humidity differencials.

Brian Lewis 12-06-2010 08:31 AM

Flooring underlay
Yes, the general contractor did indeed install an underlay before putting down the flooring. No other moisture problems re ceiling fans, etc. Wood flooring not in bathroom or kitchen - just living room, dinning room, hall and bedrooms (upper level only ). Basement fully finished ,not damp, and completely dry and heated in winter. Entire house airconditioned in summer. Thanks for your thoughts.

Floor Doc 12-06-2010 10:18 AM

Still say the wood was not acclimated to the site conditions .
Tar paper is a moisture retarder , not a barrier. It does help quiet the floor .
Aqua Bar B is the stuff to use , but it cost more money .
Don't expect any detailed report from the Rep . Lets wait and see what is in the report .
What species of wood is it ? , and what is the width and thickness ?

Brian Lewis 12-06-2010 11:36 AM

Type of wood
The wood is solid white oak and size of the planks are 3 1/2" wide and about 1 inch thick. Not sure if I am allowed to say who the manufacturer is but they are well known and guarantee their product for 35 years. We paid more for this type of wood than the standard Home Dept stuff thinking we would have no problems.


rusty baker 12-06-2010 02:19 PM

If it is Liquid Lumberdators, don't worry, everyone bashes them.

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