Flooring Joint Profiles
From Carl Hagstrom, Systems Administrator at WOODWEB
You can use the WOODWEB wood shrinkage calculator to accurately predict the movement. Keep in mind that wood movement is controlled by a number of factors: change in humidity, species, and grain orientation. Typical assumptions for "in place" moisture content (MC) changes is 5% (from the drier heated winter environment to the more humid summer environment). Assuming that you'll have some flat sawn material, four inch wide alder boards will change 1/16th of an inch when the MC changes 5% (from 6% to 11%).
However, if you measure the MC of the wood when you install it (and you should definitely have a moisture meter and be using it), and the MC is 7.5%, then the boards will shrink 1/32 of an inch when they acclimate at 6%, or expand 1/32 of an inch when they acclimate to the higher MC of 11% in the summer.
Of course, if there is climate control year round that maintains a constant humidity, then the boards won't move at all. As appealing as that sounds, though, it's important to keep in mind that all it would take is a week or two of a malfunctioning humidifier to create movement.
As Contributor J mentions, the most important aspect is the MC of the material at installation (and maybe the client's perception of what is visually acceptable regarding movement). A link to the calculator is below - hope this helps.
From contributor S:
You didn't actually say you were using this for flooring, so I am going to assume you are. If you do a square reveal you are making a flat bottom channel, which dirt will pack into. Even sweeping will not always get all the dirt out. The "V" groove doesn't allow for the dust, etc. to get trapped, making it a little easier to clean.
From Gene Wengert, forum technical advisor:
Carl has indeed given you the correct info. Consider that the floor (if it is a floor) is about 20 feet wide and you have flatsawn alder. Further, if we assume that in the wintertime the EMC conditions about be 6.0% EMC (30% RH) and in the summertime about 9% EMC (50% RH), then the alder will change 3% MC seasonally. This is about 0.7% movement or for a 2" wide piece, 0.014" movement.
So, in the wintertime, you will have a crack between each piece of 0.014. Actually, what will happen is that three pieces or more will stick together, so the crack will be every third piece and will be 0.042" which is over 1/32". Now, if the floor has a stain to it, when the crack develops, you might see a white, unfinished area. Then as mentioned, if you get some dirt in it, then in the summertime it will not close and the next winter it might even be a bit wider. (Note that I assumed 30% Rh to 50% RH. It could be drier at times and wetter at times, which would just make things worse.)
I would be remiss if I did not mention that alder is not a good flooring material as it is quite soft. So maybe this is for a wall. If so, things may be even worse, as the cracks will be right at eye level. Is there any chance that you could edge glue the pieces together and then let the entire section float? The change in size could be covered with appropriate molding.
Note that wood can never be sealed well enough to prevent the moisture changes seasonally. An excellent vapor sealer (rare) will act as a buffer for extreme, short term conditions.
It is also important to understand that it will take perhaps four weeks for properly KD alder to adjust itself to the house's RH, once the HVAC is turned on and all the dry wall and paint is dry and concrete dry. For some reason, some folks think a few days will do the trick. I wish that wood dried that fast. We would not need lumber dry kilns.
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