Will weights keep lumber flat?
22WPELan gene-wengert DB

Q. With the price of lumber increasing and the importance of high quality, we are considering concrete weights on our stakes of lumber to help keep the lumber flat. Good idea?

A. Simple question indeed, but a complicated answer. Three main points:

First, weights will control (somewhat) some of the warping during drying the top three to five layers, but the weight will not control cupping (warp across the width). Weights are most effective on 3/4 to 5/4 lumber.

Second, the weights must be kept on the stack of lumber whenever there is a moisture change going on. This means the weights need to go on right after the lumber is stacked and they cannot come off until the lumbers reaches its final, low moisture content. This end point requirement often means, especially for softwood lumber that is often kiln dried to 10% MC to achieve better machining, the weights must be left on during initial storage, before unstacking.

Third, appreciate that if a piece of lumber wants to warp and there is a moisture change, during manufacturing after drying or even in a customer’s home or office, the lumber or lumber pieces will try to warp. (See Wood Doctor September 2021 at WoodworkingNetwork.com for more info.). If weights hold the lumber flat, there is still a lot of internal stress in the lumber that can show up when cutting the lumber into smaller pieces.

In air drying, weights can help, but keeping rain water and direct sunlight off the lumber’s top layers is much more effective than weights. Concrete does deteriorate in the kiln, so river rock rather than limestone concrete should be considered. Also, the concrete has internal, metal reinforcing to help to prevent pieces from falling off. I have seen weights that cover the top of a pile that are about 6-inches high. I have seen research suggesting 50 psi uniformly on the top (A 4-foot x 12-foot stack would have 2,400 pounds total.)

I have also seen where companies feel that the overall cost is not worth the benefit. I have seen them used rarely with softwoods, but maybe only once in 50 years with hardwoods. In fact, if you have money to invest, I always suggest perfecting sticker and 4x4 alignment before using weights. If sticker and bolster alignment is poor, the weights tend to make warp worse.

Special note: With a low density wood, the extra weight can cause stickers to indent the lumber in the lower layers.
Overall, I do not feel that weights in a commercial drying operation are worthwhile.

Q. Thank you for all your generosity in sharing content through the free NHLA webinars (NHLA.com). I have found them to be an excellent resource for new learning. I was recently watching your webinar on rough mill yield improvements. You noted that your cost of cutting 1,000 square feet would be about $336. Do you care to share what is included in that number? Additionally, what have you found to be your best resource for determining prices for finished goods? 

A. Thanks for your kind words.

The cost estimates you asked about were developed from an actual manufacturer of a wood product and are pre-pandemic. Obviously, I cannot share their product or their name. The numbers I use do not include profit, as they did not want to share that. Plus, it is hard to assign profit to each operation in a plant. One way to estimate profit would be to figure estimated costs and then compare them to what the open market price is for buying the same rough mill product. 

The $336 number was the cost of cutting (labor and machines), including benefits and maintenance. They then added an overhead charge.

As you already know, a rough mill and manufacturing operation does not make any profit until the goods are sold and payment is received.  

It makes sense to me that product value and profit are related to the volume of a product you make. As a start-up, you might have to provide some product at a non-profit-cost until a market is established. Then, when the customer sees the quality you have, and maybe you point out the benefits of such quality to them, as they might not fully appreciate that, you can then set a reasonable price.

The Wood Component Manufacturers Association (wcma.com) or the Wood Products Manufacturers Association (wpma.org) might be a good contacts for some of your questions.

Q. We are working with white pine much of the time. Recently, we had a load of lumber that seemed to clog the sandpaper when we began to sand pieces in our furniture. Any ideas?

A. Sandpaper is made using closed coat and open coat styles. The closed coat has the sanding mineral or particles closer together than the open coat. So, closed coat seems to clog (technical term is “load”) faster. This can be an issue at times for sure.

In your case, I suspect another factor related to the lumber you bought. Pine and some other softwood (needle, not leaf trees) species have runny, sticky sap that sometimes can even ooze out of the wood after it’s put into use. To avoid this, the lumber is dried at around 180F at the end of the drying cycle, which evaporates that portion of the sap that will be runny at room temperature. The sap left in the wood after this high temperature treatment (technical term: after “setting the pitch”) is hard at room temperature (up to 140F about) and will not move. 

If the heat-treated wood is subsequently heated after manufacturing a product, (sanding with dull sandpaper can create enough heat at times) then some sap will soften and ooze out. With sandpaper, when the oozing sap cools and hardens on the sandpaper, this loads the sandpaper quickly.

Specifically, I believe your lumber did not get the normal, typical high temperature “setting the pitch” treatment. Always ask to make sure your lumber has been treated. Your sandpaper characteristics or techniques may have contributed, but not caused the issue. 


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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.