Is Kiln Drying Your Lumber Worth It?

You had a tree milled, or you bought a stack of rough lumber, and now someone has told you it needs to be kiln dried. Here is what kiln drying does, what it costs at our shop, and how to decide if your wood is worth it.

Why wood has to be dried

A freshly cut board is full of water. In many species the water weighs as much as the wood itself. Build with it that way and the piece will shrink, crack and twist as it dries out in your house.

Air drying is the first step. Stacked outside on stickers under cover, lumber here in South Carolina will slowly come down to around 12 to 15% moisture. It will not go much lower, because that is what our outdoor air holds it at.

The inside of a heated and air-conditioned home is much drier. Wood that lives indoors settles at about 6 to 8%. That last drop is the job of the kiln. Skip it, and the wood finishes drying after you have built with it. That is when table tops split, doors stick and glue joints open up.

Kiln drying also does something air drying never will. The heat kills any bugs, eggs and larvae living in the wood. Powderpost beetles can chew through air-dried lumber for years. A kiln run ends that.

We made a video on exactly this:

How wood shrinks and moves as it dries

Wood does not shrink evenly, and that is where the trouble comes from.

A board barely changes in length. Across its width and thickness it shrinks a lot, and it shrinks about twice as much along the growth rings as it does across them. For most hardwoods, a board 10 inches wide loses around half an inch of width going from freshly cut to indoor-dry.

Most of that happens while the wood is air drying. The part that matters to a furniture builder is what is left. A 40-inch table top made from air-dried red oak can still shrink about three quarters of an inch once it moves indoors. Kiln-dried wood has already done that shrinking before you cut the first joint.

Because the shrinking is uneven, boards change shape as they dry:

• Cupping. The face of a flat-sawn board curls away from the center of the tree.
• Bow and twist. Grain that is not straight pulls the board along its length.
• End checks. The ends dry faster than the middle and crack.
• Cracks around the center. Any board or slab that includes the heart of the tree will almost always split there.

Thick wood adds one more problem. The outside dries faster than the inside, so the outside shrinks around a core that is still wet and swollen. Dry it too fast and it cracks on the surface or tears apart inside. This is why thick slabs and dense woods like white oak have to air dry for months before they go in a kiln.

Some movement is normal in every load, no matter who dries it or how. It comes from the tree and the way the log was cut, not from the kiln or the person running it. We dry your lumber as carefully as we can, but we cannot be responsible for bow, cup, warp or cracks that show up as the wood dries. Plan to lose a little, and expect to flatten your boards afterward.

The kiln we use

We dry all lumber at M2 Lumber in an electric vacuum kiln. We used to run a solar kiln as well, but it was too slow, so we got rid of it.

A vacuum kiln works on a simple piece of physics. Water boils at 212°F in open air, but it boils at a much lower temperature when you pull the air pressure down. The kiln seals the lumber in a chamber, pulls a vacuum and adds heat. The water inside the wood turns to vapor and moves out without the wood having to get very hot.

That gives three advantages over a conventional kiln:

• It is fast. Most air-dried lumber is finished in one to two weeks.
• It dries from the inside out. The vacuum pulls moisture from the center of the board, so the core and the surface stay closer together. That means less stress in the wood and fewer cracks, which matters most on thick stock.
• It is gentle on color. Lower temperatures keep the wood closer to its natural color.

The chamber holds a stack up to 48 inches wide, 48 inches tall and 12 feet long. A full load is roughly 1,300 board feet of 1-inch lumber or 1,600 board feet of 2-inch lumber once it is stacked on stickers.

What it costs

You pay for the space your lumber takes up in the kiln and how many weeks it has to stay there. We do not charge by the board foot.

• Kiln space: a full kiln is $1,500 a week. Fill a quarter of it and you pay a quarter of that.
• Minimum: $150 a week, no matter how small the load.
• Stacking and stickering: $100 flat if we do it. It is free if you bring the lumber already stickered.
• Time: 1-inch lumber at 20% moisture or less takes one week. 2-inch lumber takes one to two weeks. Mantles take two.
• Air-dry storage: if your wood is still too wet, we can air dry it here first. Storage is $75 a month for a stack up to 4 feet wide, 8 feet long and 4 feet tall, and more for larger loads.

Here is what that works out to on five real loads. Each one includes the $100 stickering fee.

The pattern is easy to see. A decent-sized stack comes out between $1 and $2 a board foot. A handful of boards hits the minimum, and the cost per board foot jumps.

Is it worth it?

Sometimes yes, sometimes no. The honest test is to compare what your wood will cost you by the time it is dry against what the same wood costs already dried.

Add up what you have in it: what you paid to have the log milled, the drying, and a little extra for the boards that will crack or warp. Then look at the price of that species on the rack. At the time of writing, kiln-dried 1-inch lumber at our shop runs about $3 a board foot for poplar, around $5 for cherry, red oak and hickory, and $10 to $11 for white oak and walnut.

It usually makes sense when:

• The wood is valuable. Paying $1 to $2 a board foot to dry walnut or white oak is an easy call.
• You have enough of it. A few hundred board feet or more spreads the cost out.
• The pieces are special. Wide slabs, long boards and matched sets from one log are hard to buy at any price.

It usually does not make sense when:

• The wood is cheap to begin with. If you can buy poplar dried and ready for about $3, spending $1.50 to dry your own leaves very little room once you count the milling.
• The load is small. Five boards at the $150 minimum cost more to dry than to replace.
• It is pine. Kiln-dried pine is some of the cheapest lumber you can buy, and we do not dry pine at all because the sap and resin damage the kiln.

Real value and sentimental value

There is one case where the math does not matter. If the tree came from your grandparents' yard or the family farm, you cannot buy that wood anywhere. Drying it right is what turns it into a table or a mantle that lasts.

That is a perfectly good reason to kiln dry a small load of ordinary wood. Just go in knowing the number, and decide what the wood is worth to you.

Ways to bring the cost down

• Bring it all at once. One larger load beats several small ones that each hit the minimum.
• Sticker it yourself and save the $100. Keep each stack within 48 inches wide, 48 inches tall and 12 feet long.
• Air dry it properly first. Drier wood spends less time in the kiln.
• Cut boards to 8, 10 or 12 feet. Length is billed rounded up to those sizes.

Before you bring it in

Air dry it first. Freshly cut lumber cannot go straight into the kiln. Anything over 24% moisture has to air dry before we can run it. How long depends on the species and the thickness:

Thicknesses in between fall in between. Mantles over 4 inches thick need at least 3 months.

We check the moisture ourselves. We take our own reading when you drop the lumber off, even if you bring one with you. On thick boards the moisture at the center is what matters, and most low-cost meters cannot reach it. No price is final until we have checked.

Not dry enough yet? You can leave it with us to air dry. Storage starts at $75 a month for one stack and goes up with the amount of wood. It goes in the kiln when it is ready.

Old, dry lumber with bugs. If your wood was cut years ago and only needs the heat to kill insects, ask for a sterilization run. It takes one week at any thickness and is priced by kiln space like any other load.

Wood we do not dry:

• Pine. The sap and resin come out in the kiln and damage it and the other lumber in the load.
• Reclaimed wood, such as barn wood, old beams and used flooring. Years of oils, finishes and treatments cook out in the kiln and do the same.

Rather watch than read?

The video on our home page, You want to mill your own wood. But don't from Lincoln St. Woodworks, talks through milling and kiln drying your own lumber. It makes a good summary, or an extra reference if you want to dig deeper.

Still deciding whether to have a log milled in the first place? Read What It Costs to Mill Your Own Logs.

Get an estimate

Our kiln drying calculator gives you a price in a couple of minutes. Tell it the species, the thickness and the size of your stack, then book a time to drop the lumber off.

Not sure whether your wood is worth drying? Call us at 864-468-9663 and we will give you a straight answer.