Experiments

Introduction

Have you ever purchased wood from the hardware store only to have it warp or twist to a point that you couldn’t use it? Unfortunately, I had that very thing happen with a load of 4×4 pressure-treated lumber. In this post we will look at a DIY way to salvage the wood.

The Problem

A few months ago, I purchased several 4×4 pressure-treated posts that were 12 feet in length. They were for a greenhouse project that I hoped to have done soon after. Unfortunately, life happens, and the project was delayed by several months. During that time, the posts dried, and warped to a point where they were no longer usable for the project.

The sensible thing to do would be to throw these away and get some new ones, especially since the posts I bought were the last ones at the lumberyard, so they weren’t exactly ideal to begin with. But I am stubborn and I’ve always wanted to be able to salvage wood like this. Plus, legal disposal methods in my area can be quite costly, so salvage is a better option.

When wood comes out of the mill it is fairly straight, but can also have high moisture content. Typical framing lumber is kiln dried to help reduce the effects of warping, bowing, or cupping. Pressure-treated lumber is a different beast. After the wood is cut to size, it is pressure-treated with a preservative. Depending on the product and treatment process, the lumber may be sold with a relatively high moisture content. Some products are kiln-dried after treatment, while others are sold without that additional drying step. As the lumber dries after purchase, uneven shrinkage can contribute to warping and twisting.

The Solution

The good thing about wood is that you can relieve the stress in the wood by increasing the moisture content. This is a common woodworking practice. For example, to take the cup out of a plank of wood, you would wet the cupped side, lay it cupped-side down on a flat surface and apply weights to the top to help flatten the board.

For the 4×4 posts, it was going to take more than just wetting the surface to get the posts straight again. This would take a soaking, plus a coordinated drying method. So the plan became to soak the posts to relieve the stresses inside, and then secure them during the drying process to reduce the tendency to warp.

The Tub

My posts are 12 feet long, and I had 11 that I considered to be unusable, so that would require a fairly large tub. I wanted to build it with materials that I already had, so I chose dimensions to accommodate the materials I had on hand.

My tub measured 20 inches wide, 14 inches deep, and 150 inches long. I made it 12.5 feet long so that water would be able to touch all sides of the wood. I didn’t want the posts pressed against the walls at either end. The materials that I used were old 2×6 boards, and some 5/8″ OSB. Because of the length, I had to make a joint to connect two sections together to get the desired length.

Design

The tub is supported every 30 inches by a 2×6 placed on its edge. This gives me support and clearance to get my pallet forks under it to move it. Also, a 12 foot long 2×6 runs down both sides of the tub to give it more rigidity, especially while moving it.

Keep in mind that water is very heavy. A tub this size is capable of holding 182 gallons of water at 8.3 lbs per gallon. That comes out to over 1500 lbs. The tub needs to be constructed to support that weight. It should never be moved while full of water unless built to do so.

To make this an actual tub that can hold water, I lined it with 6 mil black plastic. I used two layers because of the rough surface of the OSB. Before I lined it with plastic, I used some duct tape to cover screws and cut edges, not for waterproofing, but to help cover the sharp edges that comes with cutting OSB. After I was finished with the construction, I moved it behind my barn.

Soaking

I loaded the tub with all eleven of my 4×4 posts. I had two rows of four and the top row had three. Remember, the posts take up quite a bit of space, so it’s best to put them in before you fill it with water.

The other thing you should consider is that the wood is very buoyant. Before I finished filling the tub, the top row of wood was completely out of the water. I tried to remedy this by adding large rocks on top of the pile to hold them down under the water. I only managed to keep the bottom two rows completely submerged.

After 6 days, I planned on pulling the posts out and rotating them so that the ones on top could then be at the bottom, but the eight at the bottom were already straight, so I pulled them out. I put the three that were on top back in the tub, along with a fourth post that I was on the fence about, for a second round.

Drying

This is probably the most important step, and I have to admit that I messed this up with my first batch, which I’ll explain later. The importance of the drying setup is to not only get the wood to a moisture content appropriate for its intended use, but to do it while preventing warping. Using a moisture meter helps to keep track of the drying process, but is not a guarantee that the wood will remain straight.

If you can keep the wood straight until it is dry, it has a better chance of staying straight. Obviously, if you expose it to moisture again and then let it dry on its own, you may end up in the same position. These posts are going to be surrounded by a greenhouse, and will be braced in several directions.

Improvised Method

My method for making sure the wood stayed straight was to use stickers between the posts, both horizontally and vertically. Then I would use four ratcheting straps, with 2x4s and strut channel to clamp the posts against the stickers. The strut channel is a 10-foot-long galvanized steel beam that can help prevent the wood from warping.

I left the posts outside for several days because the humidity was very low (between 30%-50%), and temperatures were in the upper 80s and low 90s. However, after a few days, I had to move the stack inside because we had several days of rain forecast.

Critical Errors

I made two critical errors in this attempt to dry the wood. First, I failed to put strut channel on top of (or underneath) the stack. I have them on the side, to prevent warping laterally, but I didn’t have any on the top to prevent warping up and down. This failure was compounded by the second mistake I made.

The second mistake was failing to support the stack along its length. Since I was unsure of how long it would take to dry the wood, and I have limited workspace, I decided to put the stack on furniture dollies so I could move them easily if needed. I only had two dollies, one on each end of the stack, which left the center unsupported. As you can imagine, the whole stack sagged a little in the middle. As a result, all of the posts have a slight bend. They’re still straighter than they were before, but not as straight as they could have been.

Another Method

To avoid my mistakes you could lay the wood on a flat surface, with stickers underneath, and pile heavy objects on top to hold them down. This will alleviate the need for strut channel or other braces to keep the wood straight. Or, if you have just a few pieces to dry, you could build a jig to hold them in place. Just keep in mind that a 4×4 post can warp in several directions.

Results

Does It Work?

In this case, yes. Using water and a controlled drying process worked to straighten these warped and twisted 4x4s. Despite the mistakes I made not supporting the material in the middle, the posts were fairly straight. By comparison, the six posts stacked on the right in the photo above weren’t soaked because they were the ‘straightest’ of the bunch. Now they look like the less desirable posts in my opinion (I know it’s hard to tell from the picture).

This method may not work for all woods, and won’t cure all types of deformation in wood. It is the ubiquitous “Results may vary” claim. However, I was pleasantly surprised by how straight the posts were right out of the water. They were straighter than when I bought them. However, the results depend largely on how well the wood is supported and how carefully the drying process is managed.

Is It Worth It?

Material Costs

I ended up doing this process on 12 posts. Each post cost approximately $21, so replacing all of them would cost $252. Of the materials, the only thing I needed to buy was the plastic. While I could have purchased a smaller amount of plastic, I did get a big roll that cost $80, because I will use the extra plastic elsewhere. The rest of the materials, I had on hand.

If you don’t have the materials lying around to build the tub (and don’t have a suitable tank), it isn’t worth it for a one-time project. However, if you are someone who collects or salvages wood on a regular basis, this could be a way of making some wood usable again.

Time and Productivity Costs

Another important factor is time. If you have to get a project done immediately, this is not the method to use. This method takes several weeks at a minimum. And to dry the wood fast, you’ll need a dry place, fans, and dehumidifiers. It is not a fast process, but if you have the time, you can salvage a lot of wood.

Along with time, you must also consider space required. If you have a large shop with floor space you are not using, then you can disregard this section. However, if you are like most of us, you have to consider the lost productivity of taking up floor space to dry wood.

Thanks

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