Wood expansion worries

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Forum topic by Marlys posted 07-04-2012 04:12 AM 1299 views 0 times favorited 5 replies Add to Favorites Watch
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23 posts in 2156 days

07-04-2012 04:12 AM

Topic tags/keywords: expand contract

I am waiting on hardware for another project and thought I would finish up a project I started earlier. My back room has a fireplace and I wanted it to “fit” more of a bungalow style with built-in bookshelves on either side. The shelves are just about complete and I need to address the mantle. It will span the width of the room and will be ~15’. I would prefer to use solid wood, either in segment or spanning the entire 15’. I could use plywood and may have to if i can’t resolve the expansion/contraction issues. I know wood will expand and contract, especially with such a long run, I just don’t know how to combat it. I am using red oak and would appreciate any advice before I glue it all up and regret it later.

-- The difference between genius and stupidity is that genius has its limits. ~ Albert Einstein.

5 replies so far

View WDHLT15's profile


1741 posts in 2472 days

#1 posted 07-04-2012 11:11 AM

Wood expands and contracts across the width, but does not expand or contract along the length.

-- Danny Located in Perry, GA. Forester. Wood-Mizer LT40HD35 Sawmill. Nyle L53 Dehumidification Kiln.

View jmos's profile


827 posts in 2365 days

#2 posted 07-04-2012 12:08 PM

Yup. Unless we’re missing your concern, you don’t have any worries about width. It it’s anchored to the wall, let it expand outward and you’ll be fine.

-- John

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Bill Davis

226 posts in 3920 days

#3 posted 07-04-2012 02:06 PM

Red Oak, as all woods do, swell and shrink in three dimensions (radially, tangentially and longituinally) as the moisture content(MC) of the wood increases or decreases. The dimensions of the wood will remain stable if the MC of the wood remains constant. The MC of the wood will vary as the relative humidity (RH) in the surrounding environment changes. For a given RH the woods MC will reach what is called the Equilibrium MC (EMC) if given enough time. As long as the RH remains at the value as determined by the EMC no dimensional changes take place.

In a nutshell, the atmospheric humidity determines the MC of wood, and the MC, in turn determines the dimensions of the wood.

Having said that and presuming that the longitudinal (along the length) dimension is what you are concerned about as far as dimensional change, that is the one of the three directions that changes the least – typically only about 0.1%. For Red Oak Tangential shrinkage is approx 9% and radial is about 4%.

So what can you do?
1. Know and/or control the RH change in the environment. It will vary over the course of a year usually being highest in summer and lowest in winter.
2. Measure the woods MC and stabilize the piece of wood at the average yearly RH for its intended environment before you cut it to size.
3. Apply a finish to the wood that will slow the pieces absorption/desorption of moisture. You cannot stop that process, only slow it depending on what finish you use.
4. Just cut the piece a little short to allow for expansion.

An example of longitudinal shrinkage.
An 8 ft wall stud that is installed green (~30% MC or greater) and allowed to dry to an average 8% MC would shrink only about 1/16” along its length.

View Marlys's profile


23 posts in 2156 days

#4 posted 07-04-2012 05:30 PM

Wow! Thanks for the great responses. I was worried that glueing up several long pieces (for the top flat surface) and then putting a front face on it would cause buckling or splitting. Based on your suggestions, I should acclimate wood, seal and finish to prevent absorption, and be prepared for it to shrink 1/16 during winter.

I guess I’m still struggling with logistics of getting 15’ long boards I position and looking right. I hate the thought of plywood. Back spasms are pretty much keeping me from anything right now. Ugh.

-- The difference between genius and stupidity is that genius has its limits. ~ Albert Einstein.

View MonteCristo's profile


2099 posts in 2184 days

#5 posted 07-04-2012 10:27 PM

Given that your red oak will be fairly dry already, one assumes, along the grain dimensional change is a non-issue.

As an aside, it is worth adding to Bill Davis’ reply that the radial dimension change is always less than the tangential. In some cases the ratio is close to 3 but for most of the hardwoods used in furniture and the like it is about 2 or less. Even then, sometimes one uses so- called vertical grain boards to help minimize the amount of dimentional change, as, unless you own a museum, your RH will change over time.

-- Dwight - "Free legal advice available - contact Dewey, Cheetam & Howe""

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