My husband and I built the Modern Farm Table using Douglas Fir. Instead of using six 2x6 boards we opted to use three 2x12. They were a tight fit in the top while building and looked wonderful on the table top. Now, less than a month after building the table we noticed that the board have shrunk and we now have large gaps between the boards. Any known reason for this?
Wood shrinks across its width as it looses moisture. Most home centers are selling wood that is too wet for anything besides framing. The usual solution is to let it sit in your shop for a while to let the moisture escape.
The simplest test is to feel the wood. While it's wet, it will feel slightly cooler than the surrounding environment. When it's dry, it will feel the same temperature. It's kind of a subtle thing, so a lot of people buy a moisture meter. I don't know if the big box stores have them, but Harbor Freight sells an inexpensive one that, while not super accurate, will probably be good enough to get the job done.
I wrote a short blog post on it for one of my projects http://claydowling.com/articles/11172011-0954/wood-acclimating which will at least let you see what one of these drying wood piles looks like.
While I'm no expert on the matter, I do know that the type and cut of the wood you use matters especially on a larger piece like a table top.
There are countless articles ranging from easily digestible to Bill Nye the Science Guy level regarding the differences between "flatsawn" and "quartersawn" lumber. In short, if you bought your wood from a big box store, chances are it was "flatsawn". On the end of the board, you can see the end grain which is essentially parallel to the face of the board. It's cheap and easy to mill lumber in this way, but it is all highly susceptible to shrinking and expanding across its width with changes in humidity levels.
There are workarounds prior to assembly, but unfortunately, I don't know if there is anyway you can easily remedy the issue at this point. Using a wider board may have exaggerated the issue as well.
Please let me know if you want more detailed information and I can try to point you to good resources.
Construction grade lumber is cut and processed wet and dried to a minimum standard. If the wood you purchased was stored outside, it would have acquiesced to about 10-15% moisture or more if rained upon. At this level of moisture, the wood is essentially not furniture quality. Many of the projects made by Ana disregard this aspect of furniture building. Wood shrinks radially the most and the interior of most homes will dry the wood to 7% or so. Basically when wood dries, it shrinks and oftentimes, it will split on the ends because it is drying faster on the ends and the rapid drying or rapid shrinkage is powerful enough force to pull apart the grains. If you wanted the fir to get to a furniture grade, you could have held it inside for several weeks or a couple months to get it to dry down. Modern furnaces dry the inside winter air even more than the older ones. Using a wider board did not exacerbate the issue.
If no glues were used, this table could essentially be rebuilt, but it would be rather arduous/difficult.
The table is rustic enough that it will look great as it is in my opinion. If the openings really bother you, another idea would be to get a paintable or clear (depends on the finish) caulk and you could caulk all those seams, but it would require excellent caulking skills and although I could do it, I would not.
There is nothing horrible about those openings; they can be finished with oil/paint and if a crumb falls though, c'est la via!
I agree with Dan. What we build is best described as rustic or farmhouse furniture. If you are building something that you are really concerned with wood shrinking, warping or cracking just let it set around in your garage for a couple of weeks . I actually think the misadventures we face are kind of nice. I have a kitchen table with 2x4s for the top and I just filled the gaps with a good wood putty and stained the top. If more crevices develop I will just add more putty and stain it again.
I also do not attach table tops to the frame with glue and screws...only screws. That way if the top warps or cracks and you want to fix it just unscrew it and make the ffixes or replacement.
Actually, Jake is incorrect to a degree. My cousin is a trim carpenter and he absolutely hates if the trimpak gets placed in the garage. The garage, unless heated, is essentially an outdoor environment. If it is humid weather; it will be such a problem, my cousin moves the trimpak and tells the gc he'll have to wait a few days or the work is not guaranteed against cracking. Sorry to be a nit Jake, but even my garage has a higher humidity than my house and it is heated. Opening the doors does that. And the best place to leave the trimpak or the furniture wood you intend to use is in the environment it is to be used in. So, if you want to build a dining table that doesn't move after build, take the pain of leaving the wood in the dining room or a more practical location in the home.
Now, a caveat... I keep KD furniture grade in my garage in neat piles. My garage is heated, and if I am making a tabletop or something, I may put the top in the house for a few days to see if anything cracks on me.
Wood movement is the constant; reducing it is the challenge. Most of the projects we see here are of the nature that a little movement isn't horrible.
I'm curious about your comment that "using a wider board did not exacerbate the issue". Not to nitpick (as that is clearly not the intent of helping the original poster), but just wanted to see if we were on the same page.
It has been my understanding that just like you said, wood moves radially (or across the grain) the most. And, since most if not all big box lumber is really not well suited for fine furniture making (to mean minimal clearances in joinery), most here will see relatively significant amounts of movement as the wood reaches EMC in their home environment. I know some people even go so far as to air dry their lumber after cutting to dimension since the freshly cut wood will absorb and release moisture at a new rate until it stabilizes again.
Anyway, with the nerdiness behind me, it is also my understanding that wood will move radially proportionally to its width (if we are talking flat sawn). I.E., a 4" board has fewer cells to expand and contract than its 8" brethren. Now, the difference may be nominal depending on the species and countless other factors, but I think the general rule of thumb still stands.
Using http://woodshopwidget.com/index.htm?ip=75&s=movement&f=1&em=1&ms=Imperia... may better illustrate what I'm trying to get at. Simply use fixed values for start and end moisture, but vary the board width.