Reducing warp during kiln drying

Weights? Restraints? What are the best methods for preventing warp in boards near the top of the drying stack? February 13, 2001

What methods are you using to reduce the warp in the top few tiers of lumber in a dry kiln? Are you using weights or restraints?

Forum Responses
In general, the slower you dry, the more warp you get. By drying quickly, you develop a strong stiff outside shell which resists warp caused by the interior wood.

Top weights do help. Restraints are awkward with marginal results. Check the temperature difference between the top and bottom layers. Typically there is a large difference and this will cause more warp.

The only commercial use of weights I have seen was in a mill drying 2 x 4s of southern pine. Weights do work, but their use is cumbersome in a commercial setting. Substantial weight (100# per sq ft) is needed, incidentally.

Gene Wengert, forum technical advisor

This is what we have found about the top layers of wood on the pile warping: We dry about 3000 BF of big leaf maple and alder every 20 or so days. When stacked in our Nyle 250, there are two stacks, end to end, each eight feet long.

When the piles are in place, we sticker the top and put a sheet of 3/4 plywood on each pile. My partner and I used to struggle getting 12 16x16x8 pier blocks on each pile. After all the work, we found we still had 3 or 4 boards on the top three or so layers warped. One time we tried just using the plywood without the blocks and we got the same number of warped boards without the weight.

From the original questioner:

I have worked at softwood mills that used a plastic strapping on the lumber that shrank with the load maintaining tension. They had great results but their bundles were perfectly square with all even lengths. Now I am in a hardwood mill (predominantly hard maple) and I fear that strapping loads could cause more harm than good, since the bundles aren't square or as stable as the 2" softwood lumber. Adding small bunks to the top of the load and then the strap might be a solution, but I will have to check the labor time required and the loss of kiln capacity.

In New Zealand they use concrete weights put on with forklifts, with good results. This is for construction and appearance grade lumber. Because they are set up to do it, it's not that difficult. Temperature difference and humidity control is critical.

If you use weights, they must be kept on for about 3 days after the wood leaves the kiln while the MC adjusts (equalizes). This is what many people overlook.

The use of straps to hold the lumber is a neat idea but I did one study that showed no effect as the weight applied was too small. Warping forces are very large.

Gene Wengert, forum technical advisor

What about metal banding before it goes in the kiln?

The wood shrinks, so the bands loosen almost immediately.

They also have the same problem--they do not provide enough restraint force. The straps--metal or fabric--will dig into the wood before they provide enough force. As I said, warping forces are huge.

Gene Wengert, forum technical advisor