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Thread: Glued up Bartop Slab Attachment... Help?

  1. #1
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    Glued up Bartop Slab Attachment... Help?

    Hello,

    I'm an amateur woodworker and small business owner (not related). I own a bar/coffee house and we are moving into a new bigger space.
    In the new location, I've hired an experienced trim carpenter to build the bartop. It has a fairly deep overhang customer side (15"). Resultingly, for substrate, he has used a 1/4" steel plate topped with two layers of 3/4in plywood (seams overlapped). Plywood is screwed to the steel plate from below, and the two sheets of ply are glued and screwed together.

    I've specified an edge glued up red oak slab countertop (He will be biscuting/dominoing them as well). He has built several bars and indicated he has always used troweled on glue to glue down the slab to the plywood and has never had a problem with separation or seasonal wood movement. However he normally builds it more like a floor and doens't edge glue he indicated.

    Now, I'm an amateur, but I've read dozens of books and Fine Woodworking/etc. for the last decade of my woodworking hobby. Everything I think I know is that a glued up slab should NOT be screwed or glued to plywood. That with a table for example, we use some sort of attachment to the aprons that let the seasonal wood movement happen.

    The big question: How should a glued up slab such as this (32" wide all said and done) be anchored to the plywood substrate? I'll take some pics when I'm at the jobsite later today. Being a $15,000 bar project, I want it to last for at least 10 years and not be pulling open at the seams or failing otherwise.

    Any advice?

  2. #2
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    That is not the right way to support a slab, it will have issues.
    Bumbling forward into the unknown.

  3. #3
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    You mention he builds it like a floor and doesnít edge glue. What does that mean.
    I have my ideas that could be the reason your carpenter isnít worried about cupping or cracking.
    Its always good practice to account for wood movement.
    Good Luck
    Aj

  4. #4
    Here is a link to the Shrinkulator. It will show you how much the slab could move over the plywood that won't. I don't think much of his proposed method, I'm an amateur too though. I would consult some other shops to see how they would propose doing it and what they would charge. Lowest bidder isn't always the best.

  5. #5
    Your choice of wood could be better. Red Oak is open pored and you will have to work really hard t get it completely sealed so it won't suck in spills. White Oak, Maple, Walnut or Cherry would be better. You can actually blow bubbles in a glass of water through a short piece of red oak.

    I wouldn't want a floor type construction on a table or bar. The wood will expand and contract seasonally and there will be gaps just like there is on a floor. Dirt and crud will get in them. I would insist on and 32" edge glued top and then allow for seasonal movement by anchoring the edge behind the bar so it can't move and then the screws up through the substrate should be in slotted holes so the wood can move. the fact that the over hang will move a little doesn't matter. Your customers will never notice it.
    Last edited by Lee Schierer; 04-29-2019 at 5:46 PM.
    Lee Schierer
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  6. #6
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    I think I would bolt the slab directly to the steel plate from below with elongated holes across the grain to support seasonal wood movement. While I would certainly use Dominos/Dowels/splines/T&G during the slab assembly, I'd also edge glue it so it becomes a single surface, especially for this application. I agree about not using red oak...white oak is a better choice for moisture resistance if you want the look of oak.
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  7. #7
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    Slotted / elongated holes from underneath is the proper way to attach something like this to allow for seasonal wood movement. You can go with one fixed point of non slotted screws, but all other areas should be slotted to allow the wood to move. It will move, no doubt. Maybe your trim carpenter has somehow never been called back about a glued down counter / bar top moving seasonally, buckling, or gapping but that's definitely not the right way to attach a wide, solid wood slab like this.

    When you say floor construction, do you mean tongue and groove along the edges? As pointed out above, the wood will still move seasonally and gaps will open up at the edge seams in the drier months creating a nasty place for crumbs and detritus to gather in, which is a bit of a food safety hazard at a restaurant. It should be edge glued 100%, then the grain sealed, then some type of protective, film forming finish (waterlox, spar varnish, poly, epoxy, etc)

    I also agree that there are many more appropriate choices than Red Oak. If you're spending $15k for the bar, you might as well make it something that's well suited for the application. White oak would be a better choice and most would say that it would be more aesthetically pleasing as well.

  8. #8
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    Thanks All,

    Question on the slot: If a screw is tight with the steel plate at the bottom will it really be able to move in an elongated slot to allow for movement?

    In answer to above, when I say built like a floor, I mean he was describing a sort of rebate/overlap with each board glued and nailed to the substrate.

    As for finding another contractor, probably not enough time. My GC trusts this gentleman's work. I think I'm just going to insist on a method of attachment that will let me sleep at night RE: wood movement.

    PS. I'll see if we can switch to white oak. I was wanting red because originally I wasn't planning on staining, and the slight pink of the red oak just looks better to my eye. Now I've got a bunch of stains to sample on scrap this week. I may well be going that route, so less concerned about white.

  9. #9
    I build a lot of butcherblock and slab type bars. We never use a substrate. Your 15" overhang should easily be self supporting. To be honest, your guy sounds like a hack. Flooring nailed to a substrate seems like DIY junk to me. For that type of money you should be getting a solid wood bar that should easily outlive your grandchildren.

  10. #10
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    Quote Originally Posted by johnny means View Post
    I build a lot of butcherblock and slab type bars. We never use a substrate. Your 15" overhang should easily be self supporting. To be honest, your guy sounds like a hack. Flooring nailed to a substrate seems like DIY junk to me. For that type of money you should be getting a solid wood bar that should easily outlive your grandchildren.
    The plan is to edge glue up a slab probably also with biscuits/dominos. I insisted on edge gluing. We have a 1/4" steel plate supporting most of the 15" overhang, and two 3/4" plywood glued together on top of that (seams overlapping). I thought it was overkill, but I've never built a bar and this is a pretty long run (~28' of the 15" overhang). At any rate it is already there. So forward I march.

    The slab is to be built in place on top of the substrate. At this point I just want to focus on the attachment so we don't have problems in the future and staining/sealing/grain filling (I have another thread going in the finishing section on that).

  11. #11
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    Quote Originally Posted by Erich Weidner View Post
    Question on the slot: If a screw is tight with the steel plate at the bottom will it really be able to move in an elongated slot to allow for movement?
    The fasteners are tightened enough to hold things flat and a washer is used, too. Believe me, the lag bolt will move in the slot. A piece of oak like that can move an impressive amount seasonally. Doing it like a floor like you describe is asking for trouble, IMHO.
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  12. #12
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    Using the Shrinkulator, given a 32" wide slab. I'm only seeing about 5/16" of tangential movement across the slab. Shouldn't need to do too much slotting then, if I'm reading that right.

  13. #13
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    As long as the slots are longer than the maximum extend of movement and are placed such that they take into account a edge that might be constrained...such as by a wall or post, you'll be fine. Too long is better than too short...
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  14. #14
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    Quote Originally Posted by Jim Becker View Post
    As long as the slots are longer than the maximum extend of movement and are placed such that they take into account a edge that might be constrained...such as by a wall or post, you'll be fine. Too long is better than too short...
    The screw holes in the steel plate were already drilled and countersunk. I may look to see if we can use an undersized flathead screw with a washer so as to allow for movement. But again, if it is screwed up tight will the wood movement really be able to drag the screw in the slot as it moves? Or will the screw be too friction tight to allow for movement?

  15. #15
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    The screw doesn't need to be and shouldn't be "super tight" for this application. Snug so it holds things down, but not so tight that it totally constrains movement. You don't want the top to crack from that! 5/16" might seam like a small amount, but consider just how much stress/pressure that can create. Slots for movement are better since for that kind of weight AND application (a bar) you want to use fasteners that are strong. Downsizing to small screws might not be the best idea.
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