Planning for Expansion due to Humidity Change

[MarkDarby]

Hi all. I'm a mid-level woodworker who is building a project that is larger than any I've ever done. I'm also in the middle of over-thinking it. I could use your advice.

I'm building a fireplace mantle (basically a box) out of black walnut (I know this will be expensive - but my wife loves black walnut). The fireplace is gas with a stone facade. We use it frequently. The mantle will be 12" deep, 6" thick, and 9' long.

The current plan is for the bottom to be a solid piece -- 3/4" thick. I plan to cut a 1/4" dado in the front, end and side pieces and a rabbet on the bottom -- like a panel. I plan to glue the bottom to the front along the full length. Since the bottom will be cross-grain with the side, I plan to glue only the front 2". I will not glue the bottom to the back.

My question: How much should I allow for expansion? My logic and assumptions:
-- a maximum humidity variation of 10% between summer and winter. I live near Raleigh, NC, and have natural gas, force-air gas heat.
-- the bottom will be flat-sawn.

I calculate the variation of the bottom's width to be (using this article: https://www.popularwoodworking.com/wp-content/uploads/2010/10/WoodMovement.pdf):

12" (board width)
x 10 (a 10% variation in ambient humidity)
x 0.00274 (the dimensional change coefficient for flat-sawn black walnut)
___________
0.329" (this is more than I expected -- but am not sure that I know what I'm doing)

Alternatively, I'm considering making the bottom as a piece of veneered MDF. This would be cheaper and eliminate the width variation issue.

Thoughts?

Thanks in advance...

Mark

 

[mdbuntyn]

You made a small mistake when choosing your numbers: You were supposed to use the change in the wood's moisture content, not the room's humidity level. With a change in MC between 2-5% (assuming a low of 7% MC and a high of 9-12% MC), you could expect between 0.066-0.1644" of movement.

 

[MarkDarby]

You made a small mistake when choosing your numbers: You were supposed to use the change in the wood's moisture content, not the room's humidity level. With a change in MC between 2-5% (assuming a low of 7% MC and a high of 9-12% MC), you could expect between 0.066-0.1644" of movement.

Thanks, Matt. I'm glad I posted. The more that I look at it, the more I'm convinced to go with MDF for the bottom.

 

[blackhawk]

If you seal that bottom piece on all sides with a finish, the moisture absorption and thus the movement will be less also.

 

[Rwe2156]

You're not going to get enough movement to be an issue.

You could veneer the whole thing.

 

[MarkDarby]

You're not going to get enough movement to be an issue.

You could veneer the whole thing.

Looking at what Matt posted above... you don't think I'd have an issue with a possible 0.1644" of movement? What's the threshold that I'd need to worry?

Veneering won't allow me to do the round-overs and dovetails that I want.

Thanks,
Mark

 

[Rwe2156]

Looking at what Matt posted above... you don't think I'd have an issue with a possible 0.1644" of movement? What's the threshold that I'd need to worry?

Veneering won't allow me to do the round-overs and dovetails that I want.

Thanks,
Mark

Movement is relative to the climate. Data is more relative to the species. Some wood moves a lot more than others.

Dont overthink it. In the end, you have to build it using methods that take movement into account. How much you‘re going to get is at best a guess. Walnut is a very stable wood I can’t see any problems here.

 

[HITCH-]

Is the mantle above a fireplace that gets used?

 

[MarkDarby]

Is the mantle above a fireplace that gets used?

Yes. Frequently. It’s gas.

 

[spitzerone]

The rate of movement will be different for the MDF than for the black walnut. So there's that. But my way of thinking is if you're going to use the black walnut, I know that I would be more likely to make it all black walnut if you are using this in your home and not as concerned about the cost of the wood.

 

[teesquare]

Seems it would be prudent to account for a drying effect beyond the normal humidity range - if this piece can be affected by any of the test from the fireplace. Even a "warming" from a fire is very dry....

 

[iclark]

If you want it to look more like you made the mantle out of a single slab, then laminate the ends using end-grain slices. If you can get the grain angles right, it can be hard to tell from a distance.

That should completely avoid the differential expansion and contraction due to moisture if you seal all the exposed surfaces (inside and out).