My hardwood is in a convex shape (cupping vs crowing)

Q: My hardwood is in a convex shape; meaning, the two sides of the wood are up, but the middle part is down. A “C” shape. What is going on and how can I fix it?

A: This is called “cupping”. When the centre is raised it is called “crowning”.

Both are caused by excess moisture in the wood. It can be caused by extremely high humidity, dampness under the floor in a crawl space or a water leak from somewhere in or about the house. It could be from the roof or a window with large amounts of water seeping inside the wall and under the floor. Or could be from a leaking dishwasher, bathtub etc.

Find the source of moisture and eliminate it. A dehumidifier may help. When the moisture in the flooring is within normal range (you need a moisture meter) and it hasn’t flattened out on it’s own, it would have to be sanded flat.

Related Q: Our sink clogged and our dishwasher had leakage in the kitchen, which has tile flooring. On the other side, in the living room, our hardwood floor started cupping – the next day. Even though we had cleaned up all the water. Does cupping happen this soon after water leakage or could it be from previous flooding?

A: Water has likely gotten under the floor. Cupping occurs when the bottom side of the wood is wet and the edges curl upward. Crowning, when the center of the board raises is from excess water on the surface. This seems clear. The dishwasher leaked and the water ran onto and under your wood floor. Unless it actually heaves I would let it fully dry out and see what it does. You can’t do a thing with it until then unless you intend to replace the floor, in which case removing it as soon as possible will help the drying of the sub floor along.

Crawl space or leak in kitchen to blame for cupping?

Q: I have a problem with my red oak hardwood floor cupping. It started slowly at the island in the kitchen and has spread slowly. The floor is less than 4 months old.

With background: I had a hardwood floor installed in a foreclosed home that I bought. The home initially had water damage and hence we had to remove the entire subfloor and install new boards, then tar paper (felt paper) was laid, and then the hardwood was laid. The hardwood sat in the home for over a week before installation. The man that installed the floor has done this type of flooring for about 20 years.

The house is over a crawl space that does have some moisture issues. There is proper ventilation, but we have not gotten to laying a tarp on the dirt. I just recently notice a leak under the kitchen sink due to an improper installation by a plumber. I am not sure where to place the blame.

I would like to know if moisture can get from a crawl space, through the tar paper, to the floor and cause the damage, or if it would be more likely the plumbing that has caused the problem? Also, is it possible the floor could return normal? We live in Michigan.

A: Cupping indicates excessive moisture moving from the bottom of the floor to the top. Of course a wet crawl space is something you want to attend to. The felt which was installed first serves as a moisture retardant. I wouldn’t view it as a vapour barrier since it has dozens of holes punched through it when nailing down the floor. Laying out a tarp on the dirt should help. However, the leak at the island sounds like the most likely source of the problem.

I have seen floors flatten out in time. You will need to be patient and give it that time to thoroughly stabilize and see what it does.

Sanding a floor before this moisture stabilization occurs would likely lead to the floor crowning, which is reverse cupping.

Newly installed oak wood floor is cupping

Q: We had 1/2 white oak installed in our dining and kitchen area to match the 40 year old oak floors. The contractor did not acclimate the new wood at our site. It is now cupping, but the old floors are not. Do you feel like he should replace the flooring?

A: I think you need to determine why the floors are cupping, which is a clear indicator that the moisture in the wood has risen sharply since being installed in your house. Why would that be? Do you have a leak some place? Is this over a crawl space that is very wet? It couldn’t be so humid in your home at this time of year. Some measurements need to be taken to find out where all the moisture is coming from.

Similar Q: We had new oak floors installed in summer 2016. After about 7-10 days we noticed planks lifting in one area. After about 4 weeks it was up to 15 planks lifting. In addition, the entire floor is cupping. The installer believes it is all due to too much moisture in house. However, the new hardwood floor replaced an old hardwood floor. Never had any issues with the old wood floor. In addition, the other rooms in the house on same floor with new hardwood floor are also hardwood, we’ve never had any cupping issue. I think it is a installation problem. What do you think?

A: If the floor is heaving and cupping this is a sure indicator of a moisture imbalance. It sounds like the floor is under pressure. Somebody should come in and check with a moisture meter. 7-9% is normal. Any reading significantly above this will create a problem.

Follow-up Q: Thanks. We tested moisture in different parts of the floor. Again every plank appears to be cupped. The floor is about 700 sq feet. The monitor reads moisture levels all over the map. Anywhere from bone dry to 15%. I asked that the manufacture of the floor be called in to inspect. Anything else I can do?

A: It is always a good idea for the sub floor and floor to be installed are tested with a meter for moisture content. The sub floor should not be more than 4% higher than the floor to be installed. I don’t know if this was done. I don’t know how high the humidity is in the house. But generally cupping where the edges of the boards are curled as opposed to crowning where the center of the board is raised is from the moisture coming from the underside of the floor. This isn’t a new house is it?

Follow-up Q: Not a new house.

There are rooms (den and office) under the floor with new hardwood floors. We have custom wood furniture in that first floor room and custom wood crown molding. Both are about 5 years old. No warping or sign of moisture problem. The new hard wood floors replace older hardwood floors. Older floors were perfectly straight even in summer months.

A: It is bewildering because if the new flooring was poorly stored and not protected before delivery to your house and it had unbalanced moisture content it likely would have shrunk and produced gaps after being installed in your place. I would have the manufacturer send a rep and perhaps if you can contact an inspector from the National Wood Flooring Association they can find the problem.

Bowed flooring in front of refrigerator

Q: We have lived in our home for one year. When we moved in we had our red oak hardwood floors refinished. We also had our crawlspace treated so that it is clean and dry. Now, over the past 3 months of summer, an irregularly shaped area of flooring has become bowed, starting with a couple of feet and spreading to become about 20 sq ft. The area is in front of the refrigerator and extends into our pantry, which is beside the refrigerator, separated by a small drywall. We have had the appliance repairman out (fridge works fine), the crawl space guy out (no evidence of moisture anywhere), the plumber (no evidence of leak), the foundation expert (nothing). All meter readings say floors read at 8% and crawl space humidity is very low and dry. Floor guy ripped up boards in pantry. We found evidence of staining on subfloor! However the color of the floorboards is yellow, not black or dark as would be expected from water. We are assuming (?) there has been some previous spill, oil-related (?) that seeped into subfloor. Either way we are having floors replaced in the area, but I’m asking: do you agree with this assessment, and is there anything we should do to prep the subfloor before laying new floor over it? It appears dry now.

A: Cupping is caused by excessive moisture imbalance on the underside of the floor. As soon as I saw the sentence ‘in front of the refrigerator’ I thought ‘oh no. his ice maker is leaking’. Or the pan under the appliance that catches defrost water has over flowed.

The only advice I can offer at this point is that ‘appear dry’ isn’t enough. The sub floor and new floor need to be checked with a moisture meter and not be more than 4% difference between the 2, with 7-9% generally considered normal for hardwood such as oak. Have the new flooring in the room for a week before installing.

Will floor cupping go away on it’s own?

Q: We have cupping from a recent leak. We had our red oak floors refinished a few months ago. Unfortunately, we recently had a water heater leak on to our floors and several boards now have cupping. The floors had water on them for an hour or two and we dried them with towels and fans. Below the floor is a conditioned crawl (temp is about 64 degrees and humidity around 55%). It has only been about a week since this happened. Will hardwood floor cupping go away on it’s own? Is there anything we can do to help the process along?

A: It is possible the cupping will go away on it’s own. You just need to give it time and wait to see how it reacts. I have seen floors flatten out totally.

I’m facing a similar situation. Water was dripping into the basement from the tub which is next to a bedroom with oak strip I have just completed staining and finishing. Now I will have to wait and see it anything happens to my own floor. Fans, dehumidifiers are about the best way to reduce the moisture.

Related Q: I just installed 3/4 x 5 oak. I acclimated it for 2 weeks. It is cupping slightly. Will this lay back down on it’s own in time?

A: It might lie down but you have to find out why it is cupping. Clearly there is a moisture source and because it is cupping, not crowning that source is likely from beneath. I don’t mean to say you don’t have a leak in your roof, but the moisture, if that is the case is running down the walls and under the floor. Is your floor over a crawl space?

Similar Q: Our small capacity hw heater broke and spilled water, which found it’s way down under a plywood laminate floor in our small hall. It had just been glued to old concrete. Can I remove the wood base around hall to dry it at that point and cut openings in the floor to be later infilled?

A: Given that the floor is glued down, hopefully a water proof polyurethane adhesive was used. Given that it is glued, there really isn’t any room under the floor to allow for much water to sit. I would have a dehumidifier running and wait and see if any movement starts happening in the floor. No sense getting radical if you don’t have to. You could remove the base but I wouldn’t start boring holes in the new floor just yet.

Isn’t this some version of Murphy’s law at work? You just install the floor and this happens?

Follow-up: Thank you very much. I believe you are right. Floor remains very level. I just removed a wood baseboard at the point where the water entered and wood was already dry to touch at that spot. I’ll wait and see..

Can cupping be caused by too small an expansion gap?

Q: I have a wood floor that has been cupping since installation. The installer argues that we have moisture issues and we do show high moisture ratings. However, when we take up the baseboards the wood floors have been installed up to or within 1/8 inch of the drywall. I know wood needs space to expand but I am unsure as to what size expansion gap for hardwood flooring is appropriate. Most significantly, can wood expand in width and in length? If it can expand in both directions then would it expand uniformal across rooms? (This is a first floor install over a crawl space.)

A: Wood expands across it’s width, the wider the board, the greater the expansion can be. There are formulas for calculating this, but I don’t have a chart I’m afraid. If this is 2 1/4 wide in a small room, only a 1/2′ gap may be required. Larger room, wider boards, 3/4″. You really need to correct the moisture issue though. Boards cut tight to the side walls will be part of the cause of a floor buckling. Cupping is all about too much moisture.

Cupping caused by off/on air conditioner?

Q: We installed a hardwood floor with professional installers (sanded and finished) over a large span of time, the first of May. The wood was in the house for five weeks before being installed. The customer called the second week of August to say the floor was cupping. Upon inspection the centre of the area is cupping but perimeter is not. The top surface had a reading of 12%, while the underside read 8%. I surmise they used water to clean floor, but customer denies they used water. Is there any other explanation?

A: This is not an easy job, is it? Maybe when floor guys get their heads together a problem can be figured out. Floormasters is another good source. I think, given the amount of time the wood was in the house and the amount of time before the floor cupped (3 months) really means this problem is something you caused. I have seen jobs near large bodies of water for example, where the floor was not acclimated. It was taken in, installed, sanded and finished. Within a few weeks it cupped. This job is 3 months. I am not so sure cupping indicates excessive water use in cleaning. You would more likely find crowning, where the surface is wet and the underside of the board is more dry, so it crowns with the centre of the board raising, and the edges lower. Is it possible there is a leak from above, running down a wall and under the floor? Yes, I know the underside is normal reading, but how long have they waited to call you since the cupping started? There can be only one explanation for cupping and crowning:excessive moisture. Finding the source is the tough part. Did the installers check the sub floor and the hardwood before installing to make sure it was within limits? That is 4%? If the sub floor in the centre of the room had gotten wet for some reason and they put the floor down on top of that, the result would be what you see. There has to be a reason why the middle is cupping and the edges aren’t. Still, if the sub floor was wet it would not have taken 3 months for the cupping to happen. Just thinking out loud.

Follow-up Q: If the customer turned off the air conditioning during the day and turned it on when they returned, would that cause cupping wood floors? The outer perimeter, a water closet and small hall, have no cupping. Could it be caused by not running the air conditioner and the moisture builds up and falls to floor when air conditioner is then turned on? I also read that a new home contains about 500 gallons of water from all the new materials to construct the home.

The wood sat in the home for at least 4 weeks prior to installation, the last two weeks with heat on since it was cold.

A: I think it could easily be cupped because of high humidity in the home. If it was installed at say 40% relative humidity, and that is what it was acclimated to and then the count goes up to say 84% that would be more than double the amount of airborne moisture and the floor certainly absorbs and releases this. It would be interesting to leave a hygrometer on the floor in the cupped area and see what the readings are, especially during the day when the air is turned off. I would put this down to an environmental issue.

Oak wood floor is lifting

Q: I have recently laid a oak floor in my living room and dining room. The floor has started to lift. Is hardwood floor lifting up common and does it settle down after a while?

A: No, this is not common. Did you do moisture readings of the floor and sub floor first to make sure it was safe to install the floor? Did you acclimate the floor to your home by bringing it in for 5 days or more prior to installing? Did you leave an expansion gap along end walls?

Cupping from humidity

Q: We put in a hard wood floor this February, 2008. Our room is 24 x 27. It has been really humid this past week and we have two places that are cupping. How can we fix this without tearing out the floor and starting over? We do not have air in our house.

A: Buy a dehumidifier with a digital read out. One of these units that will handle about 3000 sq. ft. and you can set it to the RH you want the house to remain at. $300 Or so, it’s good insurance for your investment.

Seams rising, creating a wavy effect

Q: I had hardwood floors installed last spring. We noticed within a few weeks that the seams were rising, creating a wavy effect. Our flooring contractor advise us that this fall when we begin to use the heat it will correct itself. It has not. What do you think?

A: Wavy hardwood floors? It sounds like your boards are cupping, leaving raised edges. This only happens in abnormally high moisture situations. The installer should have checked that the sub floor was within 4% moisture content of the flooring, and ideally the floor itself should have had a reading of 7-9%. It is particularly important also to leave an expansion gap along the side walls in case the floor expands from too high humidity or an actual water leak which would cause the floor to swell and press against the walls. I think your contractor should first come and put a moisture meter on the floor to see how it reads now. If it is within normal range but hasn’t flattened out it may need to be sanded flat. However, sanding such a floor while it still has high readings will likely cause the floor to crown, and then the middle will bow and be higher than the edges.

After having the dehumidifier running non-stop in the crawl space

Q: You were kind enough to give your opinion on our floors last fall. Now we’ve given it several months, with a dehumidifier running non-stop in the crawl space. The floors have gone down some, but there is still a noticeable cup. Apparently the wood floor installer said that it is relatively dry beneath. Although we’ve had a weird winter, it has been very dry due to heat for at least 2 months.

We’re thinking have them come back in now and resand. We tend to have very rainy wet springs/early summers (although it could also be a drought!) Does this sound reasonable to you?

A: Put all the meters on the floor and check for moisture. If it is ready to go, then give it a go. I wish I could say something more.

Severe cupping

Q: We are building a new home. It is pier and beam construction. Our cherry floors were installed in Nov. and then covered with cardboard to protect them until finishing them, last month. Uncovering revealed severe cupping and the wood was above 10% moisture and could not be finished. The flooring company suggested running heaters, which was done for 2 weeks with little result. When the contractor got the central heat running the floors began to dry out but then actually increased in moisture again. There is no water in the house or under the house. We had a lot of rain and cold in Dec. and Jan. and they are telling us this is a normal thing and the floors will be fine once dry and finished. What do you think?

A: When the floors were first installed, did anyone check the sub floor first to make sure it was within 4% of the cherry floor? If the plywood was wet, installing a floor over top would cup the floor and delay considerably the length of time it takes for the moisture to transfer through the cherry. Have you checked the RH in the home? A hygrometer and moisture meter are important tools at this point. If you have no moisture issues under the house, and no leaks from the roof or windows (this would probably cause a more localized issue) then I have to wonder about the sub floor itself.

Cupping and dehumidifier not changing things

Q: Our solid oak floors (installed in March 2006) are cupping. We were told to run a dehumidifier which we are doing, but we can’t see or feel any difference.

A: I would purchase a hygrometer from a local electronics store which can tell you what the relative humidity is in the room, or even at floor level. Moisture is the only cause of your problem. Identifying the source can be more difficult. A crawl space under the floor? A leak in the roof or a water pipe? Excess humidity? Wet mopping a floor during cleaning?

Likelihood of cupping?

Q: If we install 3/4″ x 8″ t&g solid wood, wormy red oak (finished in place) floors in the non-conventional, non-recommended fashion of fully adhering the boards over a structural concrete slab, using Sika’s T55 adhesive… and if we have no moisture drive problems from the slab and also have a well functioning humidity control system in place in the home, how likely is it that we will still experience cupping of the wood floors?

A: Cupping occurs when wood absorbs moisture faster than it can release it. The subsequent side pressure against the other boards or structure assists in pressing on the board edges and cupping results. You must be very confident in the concrete slab, your humidity control and the milling of the boards to even think of doing this. Unless the boards are all arrow straight, how are you going to pull them tight and keep them that way during the installation? This might work, but I wouldn’t bet the farm on it. I think I would install appropriate sub flooring and nail it down, and mix another width in the mix. I would also glue the end joints of the wider boards.

Cupping in kitchen and foyer

Q: We bought a home that is 12 yr. old this past February. The dining room, living room, and family room already had red oak hardwood flooring (solid). The kitchen and foyer had tile. Before we moved in we had the tile taken out and hardwood installed to match existing (hardwood sat in the house for awhile before installation). We also put it in the upstairs hallway. The newly installed hardwood is now cupping while the existing is flat. This makes me think it is not a humidity issue or else it would all be cupping – right? I live in VA which is pretty humid and would appreciate any insight you could give.

A: you mentioned that the new floor was in the house for some time before installation. So, the new wood was acclimated to the house. That is one thing we can remove from the equation. What is under the area where the new floor was installed? A crawl space perhaps?

The 2 areas for the new installation would happen to be 2 areas in the house that might be subject to the most moisture: a kitchen with lots of cooking and steam and spills, and a foyer, with wet feet. So, yes, I could see that those areas might do something funky while the older floors in the living room and dining room might not.

Wavy appearance / waves on hardwood floor

Q: My hickory 4-inch-board floors were installed in December 2005 in a brand new home. The boards are not laying completely flat. They have a wavy-like appearance and you can feel the waves with your hands. I noticed the waves on hardwood floor probably a month or so after installation. I have been waiting, thinking they might flatten out in time. Also, I hear loud “cracking sounds” coming from the flooring from time to time. Could you tell me what caused this and if there is anything that can be done about it?

A: It sounds to me that you are describing “cupping” where the edges of the boards curl or raise. This is an indication of a moisture issue. The boards are expanding from excess moisture and pressure. Is this floor installed over a crawl space? I would do 2 things at this point. Go to your nearest electronics store and purchase a little device called a hygrometer, which gives you the temperature and relative humidity of the room. Place it on the floor for an hour or so to get a reading. Run a dehumidifier. If you had a moisture meter, I bet the reading of the floor would be over 9%.

Follow-up Q: The floor was installed over a poured concrete basement. The floor was installed in December, though, and it was not very humid then. Will it ever flatten out or will I just have to live with it? I doubt that the installer would do anything about it or maybe he couldn’t. I had been running the heat and air, however I live in Atlanta, Georgia area and it does get hot and humid here. I will buy a hygrometer and see what it registers. Will the dehumidifier make the boards lie flat again? I have a dehumidifier in the basement, but not the main floor of house where wood is.

A: One never knows for sure if a floor will flatten out. But the sooner you get on it and remove or expedite the transfer of moisture, the better are the odds.

Historic floods and hardwood floors

Q: We have a new wood floor installed in a new addition, over a newly built crawl space. The floor was installed during a very wet period of the summer (historic floods took place near that time). The floor was flat after finishing. We covered it with drop cloths for a few weeks while trim work was done. When we removed the drop cloths, the floors had cupped. There had also been a problem with drainage that the builder had fixed, but not before it flooded the new crawl space. Now we are stumped – the job is done, we are about to have it inspected, and the builder is saying to wait until winter for the floor to flatten out with the heat. What if it doesn’t? What if it still needs to be resanded and refinished? Whose responsibility is this? We are reluctant to move into the space before this issue is resolved, but how long can/should we wait?

A: It seems obvious that the water source is not in question. I don’t think covering the floors with drop cloths for a few weeks was a very good idea since it would not only slow down the curing of the finish, but may impede the normal transfer of moisture that does occur with wood.

It may well have cupped in any case, given a substantial amount of water under the crawl space. I think efforts should have been made to ventilate and dry that area as soon as possible.

At this point, it really is important to allow sufficient time for the wood to adjust and it is possible that it will flatten out once the heating season begins. The problem is, if you sand the floors too soon, they may bend the other way or crown. I think you need to have an agreement with the contractor that if they don’t flatten out during the winter, he will have them re finished. I know moving into the area and then taking everything out is aggravating. But at this juncture, time, ventilation and warmth are the only friends you have.


Q: We are in the final stages of having red oak flooring installed in our home. We chose to have a water based varnish applied vs. the oil to allow more of the true wood colours to show. Prior to application of the water based varnish the floor was smooth and level. Subsequent applications of varnish have resulted in slight cupping in the flooring. The installer has indicated that this is a result of humidity getting into the flooring material. The thing I don’t understand is why didn’t this show prior to the varnish being applied? The project was started in early July with the flooring material having been nailed down in place for two to three weeks prior to the first application of varnish. Prior to that time the installer left the bundles of lumber in our house for 10-14 day so it could become acclimated. Had we been advised that the water based product could result in cupping we probably would have made another choice. What if any alternatives are available to us? The installer has indicated that it (the flooring) will probably “lay down” more later. Could this have resulted from improper application of the varnish by the installer? We have a beautiful but wavy floor.

We moved into this home in Sept-81. The oak plank flooring is installed over ¾ plywood sub floor with a crawl space. The crawl space has heavy plastic over the earth underneath. Prior to the red oak plank flooring we had Ash parquet flooring that was installed in ’81 which gave us no problems.

A: I have known water borne finishes to cause cupping. It is important when using these finishes to vent the area and cause the small amount of water to evaporate away from the floor as soon as possible. This also means allowing time for each coat to properly dry before applying another, and following the recommended spread rate. I would check the humidity in the area, at floor level with a hygrometer, and if excessively high, get a dehumidifier or fan blowing. Heating season is almost upon us, and hopefully, with the furnace running, the floor will flatten out.

Large bubble

Q: There is a large bubble in a click-type engineered floor in our basement. It was installed 6 months ago. The dealer says it is due to excess moisture and will charge for a T cap, removal of quarter round and repair to board. Is this reasonable?

A: Well, I guess one has to ask if any contractor is responsible for the maintenance of the environment year round in a house. If there is any sort of installation in a basement, I always enquire about moisture and humidity. Sounds like you need to have a dehumidifier running during the summer, just like me. I can’t give a better answer because I only have your brief email to respond to.