Drying rough-sawn timbers

      A large outdoor project raises questions about drying large, rough-sawn timbers for long-term use. June 20, 2000

Q.
I have about 2000 board feet (BF) of rough-sawn ponderosa pine timbers that are going to be used for an outdoor pergola. They were cut by a log home supplier, and I'm not sure what percent moisture they have, but they are not very dry. They are mainly 6 x 6 x 16 feet, 3 x 8 x 12 feet, and 3 x 12 x 12 feet.

I want to apply a good penetrating preservative/sealer prior to building the structure, and have selected Menco 200. The company suggests applying it only to wood that has been dried to 18 percent moisture content (MC) or less.

My dilemma is that this is a one-time project, so I can't really build a kiln for it. What are my options? How long would it take to air dry, sitting in my garage? I am in Salt Lake City (SLC), Utah, where it is already pretty dry. (Humidity is pretty low almost always.)

Would it be viable to build a temporary kiln out of visqueen plastic and use fans/heaters to dry it? Could it be done with just fans?

Maybe it is overkill to even try and dry it before it is built. But it is a very elaborate structure, complete with stainless steel fasteners, etc., so I want to do the job right.



You would probably be better off drying the timbers out in the open rather than under your garage, unless it is wide open on all sides. Stack the material on at least 1-inch stickers, with space between the pieces as well.

Orient the stacks to make the best use of the prevailing winds (face the broad side of the stack to the wind so that the air flows well through the stack). You might want to make some rooftop covers with the visqueen (to keep rainwater off), and let them hang off each side about a foot, but not in a way that would impede air flow through the stack.

If you want to keep the lumber bright, you'll need to apply an anti-stain chemical, but if you want the weathered look, just leave it as is. In your dry climate, you should be good to go in four to six months. The 6-x material will take a little longer than the 3-x.



You might want to look into having the timbers custom dried.

We have been using a radio frequency (RF) vacuum kiln for the past year to dry our large timbers, and the results are fantastic. Drying time is reduced to three or four days, (for 8 x 8 douglas fir) and there is little to no defect or stain.



That sounds interesting. Is that a service that is usually available most places? What kind of company would have the equipment to do custom drying in Utah?

Any idea as to a ballpark price to have wood custom dried? I haven't a clue.

I do need to find some other alternative. The timeframe of the project won't allow four to six months of air-drying time.

I still wonder about a low-tech drying method. What if I stack it right with stickers, and put it in an enclosed room, or tent it off indoors with visqueen and add a couple of recirculating fans and a heater? With good circulation and a temp of 90 to 100 degrees, how long would that take?



Be careful. Timbers may seem to be big and tough, but drying them incorrectly can cause as much damage to them as if they were dimensional lumber. I know this from experience.

I put green posts and beams in my front room just before winter. We had wood heat very near to them. Those timbers have to be replaced. I now wish I had left them outside, in open air, under cover (no sun at all), for at least six months. That is definitely what I am doing when I replace them.

Vacuum and RF kilns by design are supposed to be good for drying large stuff. Listen to Gene's advice here and you will save your timber.



I have experimented with drying timbers quite a lot and have one trick to pass on: You will get more checking in the timbers if the heart is still in them. I found that wrapping them in plastic allows the timber to dry more evenly, therefore you don't get as much checking. You still have to dry pile/sticker and cover with a roof.


In SLC you can easily and quickly dry the timbers to a low MC suitable for outdoor use. Just stack them in a shed outside, protected from rainfall and direct sun, but open to light breezes. You would not need to treat them with preservative unless they will be subject to getting wet, because dry wood does not rot.

Further, I have yet to see a preservative treatment that you can safely apply, without pressure-treating, that provides any substantial protection. You must get the preservative deep into the wood, but a brush or spray on the outside will not penetrate. Some penetration is achieved by soaking, but then you need to use hot and cold temperatures -- heat to expand the air in the wood and then the cold to contract it, causing the wood to "suck in" some of the preservative. However, even this treatment is rather shallow, except for the ends.

The most effective procedure you can use is to keep the wood dry by architectural design and the use of water repellents.

You might consult Chapter 14 of the "Wood Handbook," US Department of Agriculture Handbook No. 72, for more info. Here are a few quotes: "Dip applications provide very limited protection..." "...they do have value for exterior woodwork and millwork that is painted..."

The text discusses the benefit of cold soaking using pentachlorophenol, but that is not widely available due to the high toxicity and long life of the material if it ends up in the soil. You would not want to use it where people might contact it.
Gene Wengert, forum moderator



The structure is an open-air pergola. Because it has no roof, all wood is exposed to the elements. So trying to keep it dry is not possible.

I understand that a surface treatment cannot achieve the same results as pressure-treated wood, but it isn't possible to build it with pressure-treated lumber.

It was mentioned that it is possible to air dry pretty quickly in SLC, to a fairly low moisture content. How long is quickly?

What about my idea of a temporary, low-temperature kiln? I have not studied any of the kiln info regarding temperatures and humidity, but I could easily make a structure around my stack, using insulated foam boards or visqueen. I would install circulating fans with baffles to get full air flow through the stack. It would be pretty easy to get the temp to 100 degrees. Is this a viable method to dry? Any ideas how long this would take?

One last idea: If a surface treatment isn't going to do much, is it even going to help much to have the wood dry before I apply the surface treatment? Maybe I should just build it with green lumber, and then try and apply something at the end of the summer?



I built two, 18-foot signs for the town where I live back in 1992. They have 8 x 8 frames around them, similar to a raised-panel door, with the exception that I used moulding around the inside edge and let the field float.

The first sign is framed out of built-up, southern yellow pine. It is not holding up as well as the second sign, framed with 8 x 8 western red cedar.

I ordered and finally got cedar without the pith, "the heart" as some folks call it. I put the frame together with an open-pinned mortise-and-tenon joint. Very challenging, mostly hand work, and used a shoulder plane for final fitting of the tenon.

Since I ordered and got no pith, I had flat- and edge-sawn surfaces. I put the frame together to get all radial and tangential surfaces on the same side, if you will. My thinking of course is that I wanted the same amount of wood movement, in the same direction.

Then, under the directions of the U.S. Forest Services' Forest Products Lab (FPL), I brushed on a coat or two of 'Wood Life' water repellent. Wood Life is similar to Thompson's Water Seal, but has less wax. After the Wood Life dried, I applied an oil-based primer, letting it dry of course, and then a good quality, latex-base paint.

The cedar sign looks good from the road, some 50 feet, today.

If I had it to do over, I would disregard the Wood Life, as I did this on my recently built shop, and the paint has not adhered very well. Now the FPL suggests rubbing the wood down with steel wool after the Wood Life dries, but this is not practical on large and rough-sawn surfaces like Western red cedar.



What you say about using free-of-heart timbers is a very good idea to prevent the inevitable checking that happens to air-dried, boxed-heart timbers. The only other aspect that you might want to consider is having the timbers cut with four faces showing vertical grain.

We use this configuration on all of our structural timbers, and have had very few problems associated with differential shrinkage.



If the issue is only how long to air dry until the surface is dry enough to get the preservative into the wood slightly (longer air drying will not help get more penetration, as the amount of penetration is nil), then the answer is to air dry for three to four weeks.

If the issue is how long to air dry so that you will not get cracking later on, opening up new, untreated wood inside the cracks and thereby get decay within a few months if there is any rain, then the answer is more like six months.

It seems a shame that you will be spending time and money on something that will not last very long, plus it seems a shame to waste our natural resource.

The joints will begin to decay very quickly IF THERE IS RAIN. Treated wood or naturally decay-resistant wood is essential if the item is to last.

Certainly, building with green wood and treating with a water repellent after it dries and after the wood has had a week of dry weather to open all the cracks is a good idea. Forget the decay-resistant chemicals, especially if people will be touching the wood.

However, green wood will shrink, so you will have to retighten all the fasteners.

Cracks and joints may open up as the wood shrinks. Nails will have only about half the strength if driven into green wood which then dries, compared to nailing dry wood.

Have you also considered that non-kiln-dried (under 160 degrees F) pine wood will ooze sap?
Gene



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