Air Drying – Make stack covers for better timber drying

By Irwin Post

Covering drying stacks of stickered lumber with a temporary “roof” improves the quality of the lumber.

Air drying can be an effective way to dry lumber. It requires very low capital costs and is especially useful for construction lumber, where moisture contents below 19% (the usual target for kiln-dried construction lumber) are easily reached in almost all climates. Air drying is also useful for pre-drying lumber that will be kiln dried to a low moisture content, such as the 6% to 8% usually recommended for flooring, furniture, and interior woodwork. Pre-drying greatly reduces the time in the kiln and the amount of energy needed to reach the target moisture content.

The downside of air drying is that there is little control over the drying conditions – temperature, humidity and air flow. Drying too slowly can lead to problems with staining and even mold and decay. Drying too rapidly can lead to surface checking and other problems that may not be apparent until the wood is cut up or finished.

Ideally, air drying should occur under a roof, preferably in a shed with fans to control the air flow. The roof keeps rain, snow, and ice off the lumber, increasing drying tie as well as checking and the likelihood of staining and mold or decay. The roof also keeps the sun from shining on the top courses of boards in the stacks. If exposed to the sun, the top side of the boards gets heated and dried very rapidly, while the underside and core dry more slowly. This results in checking and lots of cupping in the top boards, decreasing their value and sometimes even rendering them useless.

Where a drying shed can’t be justified, next best is to have watertight covers over your stacks of stickered lumber. In effect, this gives each stickered stack a roof of its own. The covers keep rain, snow, and ice from wetting the lumber from above, as well as keeping the sun from shining on the top course of boards.

Tarps Make Poor Covers

I learned the hard way that tarps or sheets of plastic make poor covers for air drying. Unless you have a sacrificial layer of boards or plywood, the top layer will have drying problems: Moisture will be able to escape only from the underside, with the top surface effectively sealed. The high moisture content of this top surface, coupled with the high temperatures when the sun shines, almost guarantees the growth of microorganisms and staining problems. Next there is the problem with tarps or plastic sheets hanging over the top of the stack: If they drape over the edge even a little bit, air movement will be restricted for some courses of lumber. Also, there is the problem of keeping the tarp or plastic from blowing away. The only effective method I found was to use nails through strips of wood, leaving nail holes in the top course of lumber.

Even if one uses a sacrificial layer of boards or plywood for the top of the stack, there is one problem with using tarps or plastic sheets that can’t be overcome: holes. It is just too easy to get holes over the months that you are trying to protect the wood. Even a small hole can let a significant amount of water into the stack, especially if it is at a low spot where water pools.

Sheets of Metal Roofing Work Better

I therefore took to using sheets of metal roofing for covering my stickered stacks for air drying. Metal roofing has significant advantages: It is tough and not likely to get accidentally perforated; it can overhang the ends and sides of the stack (giving them some protection from the sun and weather); it is reusable; and it is faster to put on than tarps or plastic. I learned to put a lot of weight on top of the roofing to keep it from blowing away – with the large surface area and edges for gusts to “work,” a strong wind can blow off a sheet with quite a bit of weight on it.

Early on, I went the cheap route. Rather than buying new metal roofing, I scrounged used sheets and patched the nail holes. One time I missed a nail hole (or my patch fell off), and much of the sugar maple drying in that stack got rewetted many times. These boards ended out very stained, reducing them from furniture quality to “paint grade,” costing me about US$1,000 in retail value. I realized I could buy a lot of roofing for the cost of that one nail hole, and thenceforth bought new roofing.

I am not completely satisfied with the metal roofing, however. There is always the potential for water leakage between sheets. I sticker my lumber in piles 40 inches wide, and make the cover about 48 inches wide, which requires two sheets of roofing (I use the nominal 36-inch-wide roofing commonly available in these parts) with some overlap. I like to use roofing two feet longer than the nominal length of the lumber, so there is some overlap on the ends of the stack. When I don’t have sheets of roofing long enough, the lap along the length of the sheets is another potential place for leaks.

A different problem occurs when the metal roofing is much longer than the length of the lumber it is covering. The wind gets lots of purchase on the long overhangs, making it easier to blow off the metal roofing. And deep snow in winter would sometimes bend the overhanging ends.

A further problem with using sheets of metal roofing is the difficulty of placing them, and lots of weight, on top of a stack higher than about nose high. Using a ladder increases the danger and time it takes to do a good job of covering the stack.

An Even Better Way

Looking for a better way, a year ago I started to make pile covers that I place on top of my stacks of stickered lumber with the forks on my tractor’s front-end loader. These covers amount to an extra-long pallet with the sheet metal roofing screwed to the top. While they take some time to make (it takes me about an hour for a 10-footer once all the materials are assembled), this time is eventually more than paid back in the time I save over using loose sheets of roofing and weights.

I make my pile covers from low-quality wood – lots of wane, big knots, holes, etc., are no problem – using 4×4’s, 4 feet long every 2 feet for stringers and solid decks, top and bottom, of 1-inch boards attached to the stringers with nails. In an extremely windy location, it may be desirable to use thicker deck boards for more weight. I use screws with rubber washers to attach the metal roofing to the deck, and a bread of caulk or mastic (available in rolls from metal roofing manufacturers) wherever two sheets of metal overlap.

I wish I had thought of making these covers many years ago.

• They don’t leak. Period.
• They are fast and easy to handle, even on tall stacks.
• I’ve yet to have one blow off.
• The overhang helps protect the ends and sides of the stickered piles.
• They are so strong that an extra few feet of overhang is no problem.
• They put unsaleable lumber to good use.
• I can store unused covers on top of other stacks or in a separate pile.
• No sacrificial layer of lumber is needed: By placing stickers on top of the top course of lumber, the top course dried just as well as lower courses and the weight of the cover helps restrain warping.

I’m very satisfied with how well these covers work. My safety and productivity are both markedly improved, and I no longer have unpleasant surprises when I take apart a stack of lumber that I’ve air dried.

Irwin Post is a forest engineer, based in Chester, Vermont, USA.

Copyright 2006 by Sawmill Publishing LLC
Reprinted with permission.