Wet vs dry timber for outside use

Question

Take for example a pergola. Beams and rafters are outside, but under a roof. How would dry and wet timber behave in regard to twisting etc.?

I read that dry wood is more stable, but if built with dry timber, would it not become wet from the humidity outside and expand, thus cracking?

Than we have winter and summer, would not both types, dry and wet, become dry and wet in a corresponding season?

Am I missing something? This is about pressure treated wood. I feel like weather will turn timber into dry/wet in summer/winter regardless of what timber is used. I want to build a carpot and trying to understand what type of timber to get and why.

P.S. what I understood from responses is that dry and wet labels talk about moisture all the way to the center of the timber. And when Dry timber gets wet, it is still Dry timber because moisture hasn't penetrated all the way to the center.

Answer 1

I think you're being confused by the terms wet and dry. These terms are usually used to describe the condition of wood after it's cut from a tree or pressure treated, not the climate changes of the season. In your case the wood you'll be using will be pressure treated. The P.T. wood you get in home stores has been recently pressure treated and needs to dry out before it will be stable to use. Otherwise, it will warp, twist and split and ruin your project. As Harper indicated, it takes months to dry out. Once they do dry out, you can stain/ seal them and build your project and the climate changes will have little effect on them.

When I built my deck I had 25 - 5/4" x 6" x 14' planks that had just been pressure treated and were very wet. I laid them out in position on my deck joists for 6 months before screwing them down. I flipped them over often. By the time I was ready to screw them down, I needed three additional planks to make up for the shrinkage due to the drying.

One final note. You can buy wood that has been kiln dried so you don't have to dry it yourself. You just have to find a dealer in your area and be prepared for sticker shock because it's expensive.

Answer 2

TL;DR:

When you purchase regular (non-pressure treated) lumber, consider it dry enough to work with for any household construction project.

When you purchase pressure treated lumber, expect it to feel damp to the touch. You may want to allow it to re-dry for 3-4 months before using it to avoid the majority of the warping/twisting/shrinking effects of drying lumber. You can, if you desire, purchase a moisture content meter to ensure the wood has dried back to your desired moisture content, if you feel the exact moisture content is critical to your project.

When designing and building your project, plan for some amount of seasonal wood movement. Know that the vast majority of expansion will be in width (the 4" dimension of a 2x4), a bit of it will be in the thickness (the 2" dimension of that 2x4), but almost none in the length (not enough to ever really be concerned about).

The long story:

As I covered in the answer I linked to in my comment on your OP, wood is a natural product and will expand and contract seasonally. When you build with wood, whether for an outdoor project like a pergola (as you've asked) or a solid plank door (as was addressed in the linked question), or an indoor project like a solid wood dining room table, dresser, or decorative storage cabinet, you must allow for this expansion and contraction or your project will end up warping and splitting. (And it might do one or both, even if you allow for it in construction.)

"Wet" v "dry" wood has very little to do with droplets on the surface, it's all about the moisture content on a cellular level within the board itself.

When a tree is cut down, it is "green" or "wet" wood - each individual cell has a very high moisture content because it was (just moments ago) a living thing that needed that moisture to survive. Immediately after being harvested, the cells begin to lose their moisture, this is a process called drying. As others have discussed, it takes about 1 year per inch of thickness for boards to dry out to the point where they're "stable". While they're drying, the wood can warp, twist, split and crack, which is why you see so many pieces of wood at the local home center that look more like the keel of a boat than they do framing lumber.

All the wood you buy at your local home center will be dried lumber. The lumber mill will have stored it for roughly 12-18 months to allow it to dry. Before they ship it on, they will test for moisture content as Lee Sam noted in his answer and mark it as such. When you buy it, you can be confident that you're buying "dry" wood, though the moisture content may be higher or lower (by a couple of percent) than when it left the sawmill, depending on where the mill is, where you're buying it, the season it left the mill and the season you're buying it. It will be more "wet" in the summer, and it will be more "dry" in the winter, but again, only by a couple of percent, and this is well within the expectation for "dry" lumber.

If you were to go to a sawmill/lumber yard you could, if you desired, buy green lumber. You would only do so if you had a specific task in mind for working with green lumber (some people turn with green cuts, then let the warping become part of the look of bowl), or if you wanted to save some money by spending your own time & storage space to allow it to dry before using.

Answer 3

Our shop (not me) does historical restorations of wood things. Our rule is to allow wood to stabilize for moisture content, and our rule of thumb is one inch per year. Allowing for double-sided drying if the wood is stacked to allow airflow on both sides. So "two-by" stock, 1-1/2" thick, means 9 months if stacked for double-sided airflow. It also helps to weight it to deter twisting.

Where do you do this? In the same environment as the wood item will ultimately be serving. We are fortunate to have passively climate-controlled storage.

So buy the wood, stack it up, cover it from rain (I assume your structure will have a competent roof)... and wait.

Answer 4

Lumber is divided into two main categories: 1) framing lumber, and 2) finish carpentry. (There is also wood siding, paneling, etc., but is outside this discussion.)

Framing lumber nor finish lumber is not randomly allowed to dry before use...just as it’s not randomly graded. (You can’t just store it under a tarp and expect the material to be acceptable, as one answer suggests.)

The Code requires and Architects and specification writers specify exactly what is to be used, including moisture content for optimum performance. Likewise, the manner in which each piece of lumber is graded and reaches its moisture content is carefully controlled.

We use the West Coast Lumber Inspection Bureau (WCLIB). They have specific requirements for drying lumber, including controlling humidity, stacking with strips at certain intervals for air circulation, loading individual boards to hold them in place during the drying process, etc.

Then based on designated quantity of moisture the lumber is assigned a moisture content value and grade that the architect can use for the structural design and stability of a structure...such as No. 2 & Better with a moisture content of 18% or less for framing lumber (often called “Green Lumber”) AND such as Clear VG, “C” Select & Better with a moisture content of 15% or less for exterior fascia, exterior trim, etc. AND such as Clear VG, “C” Select & Better with a moisture content of 10% or less for interior trim, etc. (Each piece of lumber is stamped with a grade and moisture content or you can get a letter that certifies its grade and moisture content if you’re going to stain it with a light penetrating stain.)

Without these grading rules, the performance of lumber products would be random.

When lumber is in contact with the ground, then it is pressure treated which adds moisture to the piece of lumber and reduces its stability, strength, etc. All this is accounted for in the Building Code, Architect’s calculations and ultimate performance. (More moisture content then more shrinkage.)

More shrinkage is allowed in rough framing because appearance (shrinkage) is not as important as in trim AND exterior trim can have more shrinkage than interior trim.

Summary: I tell you all that to show there is exact methods of drying which lead to expected outcomes. That is to say, lumber dried before installation will be more stable than “Green Lumber”. Yes, all lumber will grow and shrink depending on the weather, exposure to rain, etc., but dry lumber will perform better because most of the “inner” moisture has been removed and rain will not penetrate to the inner core of beams, etc. and this less movement.

Moisture mainly enters lumber through “end grain” not “side grain”. To protect a piece of lumber, seal it’s end grain.