Flooring grooves and end splits

Q: We purchase maple panels from various suppliers. These panels are used in the construction of doors for kitchen cabinets. During the first months of the year we see a lot of splits in the glue joints between the staves of the panels from some manufacturers. We suspect that the glue has not been applied correctly, but the supplier says that we are damaging the panels with our dry assembly facility. Can we have your opinion?

A: I've been in this business since 1961, and it seems for some people, end splits in panels are an annual problem, while others have no idea what we're talking about.

What is happening is that at the time of assembly, the individual staves are a bit too wet for the air in the plant. As a result, the ends (which dry faster than the rest of the piece) shrink between the time they are sawn and the time they are glued. Hence, when the staves are put together and glued, there's a small gap at the ends which, for most commonly used wood adhesives, means a weak joint.

The manufacturer may inspect the panels and see nothing wrong. Now, when you put them in a drier condition, the wood will shrink a little and put pressure on the joint. If the joint is weak enough, it will fail. So, unless your facility is exceptionally dry, which I think means under 30 percent RH roughly (which is a value that your customers will have in their home), I do believe that your operation can be blamed.

Incidentally, your supplier of the panels has two options that will eliminate the problem 100 percent:

1. Get the moisture in the lumber very close to the moisture in the air so that no drying occurs between stave sawing and gluing. This requires measuring both the wood MC and the air's EMC (which is related to relative humidity).

2. Once the staves are sawn, make sure that they're glued within 15 minutes. No appreciable shrinkage can take place in that short time. Now, it'll take a lot of stress to break a strong glue joint, and small changes in RH or moisture content will not create enough stress.


Q: When we look at a piece of solid wood strip flooring, we always see two grooves on the bottom side. These grooves seem to vary in size (width and depth). Can you tell me why these grooves are there?

A: I believe that the grooves were originally added to provide for stress relief (also called casehardening relief). Why? In the old days, lumber would be air-dried for six months or so and then put into a 100-foot long progressive kiln, with the lumber on a cart that was on tracks and every day the lumber was pushed a little further into the kiln. In 10 days or so, the lumber finally was pushed out of the kiln.

The entering end of the kiln was cool and humid while the exit end was hot and dry. The exit end did not have enough steam to do an adequate job of stress relief (also called conditioning). However, in most cases, with such long air drying, there was little or no stress anyway. So, overall, this loss of steaming at the end wasn't a big deal.

However, once in a while, when the piece was sent through the flooring machine and milled into flooring, the piece would curl toward the top face due to a small amount of drying stress or casehardening. To offset this occasional tendency, grooves were milled on the back side to balance the stress.

Obviously, with today's modern kilns (if operated properly) there's no need for such grooves, as all the stress should be removed in the conditioning process at the end of the drying cycle. However, as you state, the consumer still expects to see these grooves.

Sometimes I hear that the grooves are there so air can flow underneath the flooring and avoid moisture build-up. The idea of air flow doesn't really make sense to me. Consider that these very small holes often would be 30 feet long. Without a fan moving air through such long, narrow holes, there wouldn't be any air movement at all. In other words, the air in the grooves is stagnant, for all practical purposes.

Another reason that I've heard is that these grooves reduce the weight of the flooring. The idea of reducing the weight by a percent or two is indeed a positive effect of making the grooves. In addition, the shavings provide some boiler fuel. But I doubt that these benefits were important or necessary in the old days.

Finally, I've heard that the grooves make it easier to hold the flooring flat. I don't see how small grooves weaken the wood enough overall to make a significant difference.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.