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Wood movement in Butcher Block tops

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Forum topic by Mark Colan posted 408 days ago 1068 views 0 times favorited 14 replies Add to Favorites Watch
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Mark Colan

209 posts in 1479 days


408 days ago

I am trying to work out the best way to fasten a butcher block top to my metal bench support. Wood movement is new to me (I mostly use sheet goods) but I need to understand it, and I think I do.

I would welcome any comments from you more experienced folks.

I found this article about wood movement, and it seems pretty good. For the picture below:

.

They say: ”Wood is fairly stable along its length, moving only 0.01 percent as it loses its bound water. However, (on the average) it moves 8 percent tangentially and 4 percent radially.”

How does this apply to butcher block?
  • First, it says it should move very little lengthwise.
  • Second, it will move up to 4 or 8% in the width (but note that these numbers are for drying green wood; butcher block tops would be made with pre-dried wood, so the movement would be considerably less)
  • Whether it is 4% or 8% depends on whether the butcher block has the face grains or the straight grains glued together (where “face grains” here means the nice broad pattern you see on the face near where it says “longitudnal”, and “straight grain” here means the top face in the picture above). To minimize the impact of movement, it would be best to glue face grains together, and I think that is what they do.

By my reasoning, movement should be least along the length and greatest along the width, assuming the individual slats run lengthwise. There could also be considerable movement of the thickness, but that’s not a problem to me.

Do you agree with my analysis and conclusions? Please correct me if I am wrong.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA


14 replies so far

View Sergio's profile

Sergio

402 posts in 1326 days


#1 posted 408 days ago

The “movement” of wood is due to moisture, as the article you pointed out says.
If you seal the wood in a way that it does not absorb the moisture, it will become stable.
My chemical supplier said that for food related wood products, the best way to go is Tunge Oil. It goes deep into the wood and dries (or cure) after some days, becoming a hard resin. It would keep moisture out and make the structure stable. Tunge oil, after have cured, is food safe.

-- - Greetings from Brazil - --

View Charlie's profile

Charlie

1008 posts in 919 days


#2 posted 408 days ago

If I’m reading this right and you’re attaching a wood top to a metal base, slot the holes in the metal base in a direction perpendicular to the grain in the wood. Then put 2 fender washers (or large washers) on a screw and run it up into the butcher block, The slotted hole will allow for wood movement and the double washer makes it a little less likely to “stick”

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404 - Not Found

2544 posts in 1602 days


#3 posted 408 days ago

It will move with seasonal changes. Charlie’s method of attaching the top to the base allows the top to move without splitting or shearing the screws.
Different species expand/contract at different rates, see this link for movement rates in different timbers.

http://workshoppages.com/WS/Articles/Wood-Movement-Charts.pdf

Tells you how to work it out and all.

The good stuff is on page 2.

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Mark Colan

209 posts in 1479 days


#4 posted 408 days ago

Charlie, I’m really puzzled by what you said. It seems to me that the movement perpendicular to the grain would be maximum if the diagram I posted above is correct.

I had been thinking of using screws along the center line of the length of the bench top, which is parallel to the grain.

Can you explain please?

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

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PurpLev

8476 posts in 2282 days


#5 posted 408 days ago

this image shows you where the movement will be (width wise across the table top):

-- ㊍ When in doubt - There is no doubt - Go the safer route.

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PurpLev

8476 posts in 2282 days


#6 posted 408 days ago

here’s the suggested hole in the metal base – elongated in the direction of the wood movement to allow the wood to move in-out:

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View Charlie's profile

Charlie

1008 posts in 919 days


#7 posted 408 days ago

You can use screws down the center, parallel with the grain and you can actually screw those through holes… tight.
And then let the edges “float” with no fasteners if the butcher block stays down, that’s fine. If you put any screws out from the center, line then the slotted holes in the metal base should run perpendicular to the grain so that the whole thing can expand and contract.

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Mark Colan

209 posts in 1479 days


#8 posted 407 days ago

Many thanks to both PurpLev and Charlie.

Now I know what the “clips” are that I have heard of for holding down the top. And at the right of the diagram from PurpLev is confirmation that movement occurs perpendicular to the grain. PurpLev, nice to hear from you, been awhile!

Charlie, thanks for confirming my plan of tightening down screws along the length.

Question: if I use slotted holes in the metal base to let me hold down the sides, do I have to be careful not to tighten them down, lest it prevent the wood from moving? Is the two-washer method enough to allow movement even if it is somewhat tight? Ideally, I would like to tighten them at least somewhat to prevent cupping, or flatten a cupped bench top.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

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Mark Colan

209 posts in 1479 days


#9 posted 407 days ago

Also, where would I find metal cleats of the sort shown in PurpLev’s diagram? Do they have a more complete name?

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

View Douglas's profile

Douglas

304 posts in 1193 days


#10 posted 407 days ago

Those are “table top fasteners” and you can get them from Rockler and Woodcraft. Just search for them on their sites.

-- Douglas in Chicago - http://dcwwoodworks.com

View Charlie's profile

Charlie

1008 posts in 919 days


#11 posted 407 days ago

Table Top Fasteners

You can tighten one row of screws running in a line parallel to the grain. The center is suggested only because you’ll get relatively equal amounts of movement to each side away from center.

I have a walnut top on my island. 8 feet long and 42 inches wide. I drilled 5/8” holes at my fastening locations and ran a screw up into the countertop from below. I used the 2 washers and you can tighten them down pretty good. Think of it as just a little tighter than “good and snug”. I do NOT want to tighten enough to sink one of those big fender washers into the wood.

I just built it a year ago so I occasionally get under there and have a look, but I’ve now been through all of the seasons and no movement that occurred put any stress on the screws. So it appears I got it right :)

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bondogaposis

2491 posts in 985 days


#12 posted 407 days ago

You’re on the right track here and have been given some good advice. The 4% and 8% movement figures are way high for dried wood, 1 or 2% would be plenty for kiln dried. You can tighten the screws until they are just snug but not extreme.

-- Bondo Gaposis

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Mark Colan

209 posts in 1479 days


#13 posted 407 days ago

Hi Bondo, yes, I know that the 4% and 8% numbers apply to drying green wood. I did not have numbers for kiln dried, and I thank you for those.

If my objective is to tighten the screws to pull the edges down, make or keep a top flat, will “snug but not extreme” do that?

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

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Mark Colan

209 posts in 1479 days


#14 posted 407 days ago

From the Rockler link supplied by Charlie:

“These Table Top Fasteners provide a sturdy connection between the tabletop and apron, while still allowing the top to expand and contract with seasonal humidity changes.”

This provokes yet another question. I assume that I can tightly fasten an apron along the front and back of the bench top, because of the lack of movement along the length, and especially if the aprons are the same bench top material, which should move in the same way. I envision a but joint over the edge of the bench top.

Is that assumption correct?

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

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