Why is Wood Flooring Kerfed on the Bottom Face?

Speculation, theories, and facts about why wood flooring and some door jamb stock is usually milled with grooves in the underside or back side. October 13, 2008

I was watching the Discovery Channel the other night and they had a show about how wood flooring was made. They explained how the bottom of the flooring planks had material removed in order to enhance stability. They didn't explain anything about how this action enhanced the stability.

This got me thinking about a pending project which has a couple of very wide solid lumber drawer faces. I usually shy away from those because most of my own work is flush inset and there's too much opportunity for cross grain expansion. This pending job calls for a 3/8 lip overlay slab drawer face. My instincts are to provide a stress relief in the back, similar to how flooring is done. We usually do something like this whenever a plank is wider than 5 inches or so.

The customer is going to ask me about the "why" of these kerfs and I want to make sure my answer is correct and easy to understand. This sketch is what I came up with. Is this accurate? Does it make sense?

Forum Responses
(Cabinetmaking Forum)
From contributor T:
Doesn't make sense to me, unless I am missing something in your description.

A. You are increasing the surface area of one side of the panel. This side will then transfer moisture better/faster than the other side. Not good (warping).

B. The panel will expand across the grain no matter how thin it is in some spots, or how much "straw" is removed from beneath those spots.

C. The problem of expansion of the drawer front is already solved with the lip.

Flooring and room molding is relieved on the back side because it is installed over uneven surfaces, and the relief allows the edges to contact the surface and be stable (not rock) when applied. The relief in the back of flooring does not stop expansion. Solid wood flooring is installed to allow the expansion; if it is not, it will buckle.

From the original questioner:
Thanks for the response. I always thought that the flooring kerfs were related to moisture stability. You could have saved me a lot of steps if we'd have just had this conversation a decade ago.

From contributor P:
If the drawer fronts are being applied to a drawer box, just use 4 screws near the corners to hold the DF to the box. I've done drawers with 12"H solid fronts like this with no problem. If you use properly dried lumber and alternate the direction of the growth rings when you glue up the boards, you should be fine. (This assumes you're using flat sawn boards. If you're using quartersawn, even better.)

From contributor M:
Not a value-added process. Contributor T is right - Discovery gave the wrong information. Think of it like kerfcore. When you make deeper cuts, the piece becomes flexible. The cuts you recommend will do very little to effect the rigidity of the piece.

From contributor D:
I once heard the backouts in flooring were to "let the roaches get out." Another woodworker said it was where the dust goes that gets in between the joints. Many say it is to prevent squeaks. I myself wish to offer "dynamic tension equalization" as a 21st century explanation.

From contributor L:
I heard a plausible explanation as to why there are grooves in the back of the flooring boards - weight. When they started to ship flooring by truck, it was charged by weight and distance. They couldn't change the distance but they could change the weight. By removing stock from the rear, they could remove 5-10% of the weight and the shipping charges would be reduced.

From contributor A:
Keep in mind a floor doesn't move. It is nailed or stapled severely to a stable floor. Then it is only finished 3 coats on one side. They certainly aren't doing it for stability reasons.

As for your current project, you are doing an overlay drawer front. There is no issue of expanding width enough to cause binding. When building inset, I can't imagine getting an order for a sold piece wide enough to cause concern, and in that case just make it out of plywood. I left a message for my flooring friend.

From contributor A:
25 year flooring guy returned my call this afternoon. He says it's an age-old question. The two major things to consider when installing flooring is that:
#1 subfloor is not flat.
#2 flooring is not face jointed or edgejointed.

If you look at most floor, it is not straight. The relief cuts make a 3/4" board almost as flexible as a 1/2"-5/8" board. He tells me that it can be quite a fight to get long pieces of flooring to accept each other due to the above reasons. The bigger question in my mind is why they do a similar relief cut on door jambs. Perhaps for the same reason. You want the board not to have a mind of its own. It should be dead straight from shim to shim.

From contributor T:
Floors move quite a bit. That's why they leave so much space around the edges: for expansion. They expand in the humidity and shrink in the dry season. Only one edge of a solid wood floor plank is nailed. The other is free to move, but the tongue is trapped by the groove in the next piece.

I believe the question was if cutting reliefs in the back of a board lessens the expansion caused by moisture content. I don't think it does, on a floor or a drawer front.

From contributor D:
A professionally made and laid floor will be kiln dried at 6-8%. This means that after install, it will only expand as a whole, as it approaches EMC of 8-9% or a bit more. This helps tighten up all the (lineal) joints. If it cycles back a bit with seasonal changes, it will only shrink a bit, with each board moving in its place, and not as an entire floor. On 2-1/4 strip, this movement is negligible, and wider plank floors will have appropriate disclaimers. In no normal case would or could a KD nailed floor pull away from the walls more than a fuzz or two.

From contributor I:

It's interesting that no real solid "this is why" answer has been posted. It won't either, not from any floor suppliers. They would love to see the uninformed skip this important detail, and laugh as they continue to do it like always. The old boys may have known a few things about wood that our high tech thinking zooms right by.

I believe these kerfs and hollow backs go a long way in preventing cupping. Cupped strip flooring looks awful, especially with a gloss finish, and throws off the tongue and groove alignment. I have seen the difference these small grooves can make in stabilizing a piece of milled stock. The grooves break up the compression stresses that occur on the underside of a piece. These forces push outward across the grain, giving the face a "trough" look. I don't fully understand why, but I don't need to. I have seen the effect these grooves have over plain S4S stuff and the difference is impressive. I consider millwork without it incomplete. The hollow back plough on trim does the same thing, as well as allowing clearance for the jamb/wall transition misalignment.

From contributor S:
When you do a painted panel, you can't just paint the front or it will cup. The same goes for plastic laminate - it will warp without a balance sheet. I do wood carving and when doing deep relief, you have to remove material behind your carving as well. If the kerfs on the back of hardwood flooring are to prevent it from warping, then why do we not have to do the same to any solid wood door panels? Is it because you alternate each board when gluing where flooring is one piece? I am leaning towards the thinking that it is because of floors not being flat. In the case of door jambs, why do they do the same to the back of fingerjoint pine jambs when the moisture content is minimal, and you use shims to square it up?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
The purpose of the cut-outs in flooring is for stress relief (to relieve casehardening), to keep the pieces from warping when they are machined. Old dry kilns did not have adequate stress relief capabilities, so by removing material, top and bottom, the stresses were balanced. It is not always done today and many people use different sizes. It is so traditional that flooring without the cut-outs would be rejected by installers.

The idea of relieving stress in panels is valid, as shown in the first posting, but most of today's lumber is already stress free (free of casehardening stress), so cuts are not necessary or helpful.

From contributor I:
As Dr. Gene has joined this, I would like to ask for theories on one more point. Some time ago I read an excellent article by Bob Flexner regarding cupped table tops. The culprit is called "compression set shrinkage." We have all seen old tables with curled table tops, and the curl is always up at the edges, regardless of strip width or grain orientation. This is caused by repeated exposure to water on the top surface. Even with a film finish, moisture causes the wood cells on the top side to swell and crush one another so that when dried back to ambient MC, they are smaller and occupy less space, thus shrinking the upper plane and cupping the panel. He presented a solution for fixing this but that is off subject.

My question is, even with modern dried wood, is not the exposed surface of flooring and trim also subject to this phenomenon? Wouldn't the stress relief on the back help to offset eventual cupping?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Contributor J has indeed stated the true facts. Here is why this happens. When wood is subjected to slow moisture gain, the cells expand slowly and the moisture travels into the wood, so the whole piece is slowly changing size. When water is added quickly, the cells at or near the surface that get wet will try and expand quickly, but will be unable to do so (being restrained by the cells further down that are not changing). So the top cells develop compression set; that is, they will be smaller than if they had been free to swell when the water was added. (Set is the term for a semi-permanent strain.) With flooring, oftentimes, the swelling will also result in a small ridge at the edge of the piece as the wood tries to swell.

Compression set is very difficult to remove (except by planing) as we would have to expose the wood to instant, very dry conditions in order to get instant, large attempted shrinkage. Sometimes, compression set on the outside is called reverse casehardening.

The reason that grooves work in manufacturing flooring is that you are removing approximately even amounts of wood (or even amounts of stress) from both faces and both faces have the same amount of stress (also called casehardening). The grooves will not help when water is added after manufacturing, as the bottom face does not have the stress that the top face has.

(This is why we have the rule that when finishing a table top or other large item, the finish should be the same, top and bottom. This will ensure that any size changes are the same top and bottom and any stress or set will be balanced and the piece will stay flat. This is also why we want a finish that is resistant to liquid water penetration. Wax is great indeed.)

From contributor N:
Two other, possibly valid reasons for the relief cuts on flooring:
1. In a glue-down application, it leaves a space for excess glue to go.
2. It makes it easier for the floor guy to determine which side is the face.

From contributor Y:
The kerfs on the back of flooring boards are there so that the piece sits flat on the subfloor. Otherwise the piece will rock, and have to be overly nailed. The kerfs also provide some air space for the floor to "breathe." Try leaving two boards together, even overnight, and you will detect cupping. Remember floors are only finished on the top!

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:

I think contributor N's #2 is the correct and only correct answer. We did some tests 20 years ago about the grooves helping the floor to breathe and found that the small openings provided no help at all.

I do not understand how two small channels will help a flooring piece lay flat unless they balance the stress by removing some of the wood, as stated earlier. If the channels helped the floor breathe, then the ends would change first and the center would never get the effect of any RH change.

In any case, the relief of stress was the original reason.

From contributor Y:
Gene, as someone who has had quite a bit of field experience in, shall we say... less than desirable conditions, I can offer an explanation for the relief cuts. The assistance in laying flat is more or less the correct explanation, but doesn't explain fully. Having a reduced surface area contacting the substrate allows the flooring to bridge over inconsistencies more easily, in the same way that a cabinet on leveler legs is easier to stabilize (on say a lumpy slab), as opposed to an integral kick or ladder base. Most flooring installations in a high production setting have rather uneven substrates to go over (swelled joints in the decking, etc).

In the same vein, the relief cut on the back of casings assists in bridging over the inconsistencies between the jamb openings and the wall surfaces, which would cause flat backed mouldings to flare off of the wall substrates on the outside edges.

Now as to whether this is the intended result of those relief cuts, or just the real world benefits, I'm not entirely sure.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
If you look at the size of the grooves, such as in the original posting, you can see that they reduce the contact by less than 25%. Many flooring products today have very small grooves. I do believe that if unevenness was the real reason initially, then we would have seen wider grooves. Likewise, if the goal was to make the wood more bendable, then the grooves would have been deeper. The initial purpose of the grooves was for stress (casehardening) relief.

From contributor V:
Okay, so what about door jambs? All of the door jambs I have dealt with over the years have the backs relieved in a similar fashion. Is it for the same reason? You would *never* want a jamb to be able to flex to follow a crooked trimmer. The relief cuts are usually 1/8" deep x 1" wide and so do not affect shipping weight enough to worry about. Anyone know why?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
If they were to help bending, then they would be deeper. As many softwoods are dried to 10-12% MC and are not stress relieved (as it is hard to do at this high MC), I suspect it is for stress relief. For stress relief, the groove would be shallow and wide.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
The major reason for the kerf on the back of wood flooring is to lighten the weight of the boards after manufacturing. This reduces the cost of shipping and makes the materials easier to handle. For stress relief I would cut across the width of boards and not with the length. This would be done at 8-12" on center. This makes the flooring more flexible, the cut should be at least a 1/4" "V" type cut that would allow the adhesive to enter the cross cut and hold planking over concrete.

Comment from contributor B:
I learned flooring from a guy 31 years ago who installed flooring all his life and he learned it from a guy that did it all his life. He said it made no difference. I manufacture and provide installation of hardwood and softwood flooring.

I was once asked to get new tools to provide 3/16 grooves on the backside of the flooring instead of the 1" groove or grooves we had always used because the customer was convinced it was better.

My observation is that the grooves do nothing for moisture stability as most homes these days do not suffer the environmental changes indoors as the old homes did. I have installed grooved and rabbited flooring side by side as that was the tool that happened to be in each individual machine at the time and I have seen no difference in performance of the flooring. I do find that grooved flooring can be harder to put together by a small margin if the floor is uneven. I also find grooved tools require more power to run.

Comment from contributor C:
The reason there are stress relieving grooves on back of floorboards go back to before high speed moulding. Before that time a moulder operator would inspect each board and choose the face/back so that the grooves were always to the outside of the tree. In other words the face of the board was always the side of the board facing the center/pith of the tree. This means that the grooves break the continuity of the annular rings and minimize cupping.

This is true for all flat/crown sawn boards but not for full quarter sawn boards which are stable. Roll forward some decades and with high speed moulders/automatic feeding there is no chance to select, so now the grooves are cosmetic at best. If you follow the logic above you will see that now putting stress relieving grooves randomly in reverse of boards increases the likelihood of cupping in 50% and decreases the chances of cupping in the other 50%.