Message Thread:
Thick Veneer Bar Top
3/30/16
Last year I built a very large bar top out of solid 8/4 Zebra Wood - against my better judgement but trying to please a very good client. It looked absolutely awesome the day it was installed. You know what's coming... 6 months later it's coming apart in slow motion. I don't need to troubleshoot what went wrong- it's probably a poster child for how to design and build a bar top with the absolute maximum potential for failure. What I do need to do is replace it, so I'm hoping for a reality check on a replacement that will let me sleep at night.
Here are my basic thoughts and questions (see photos and drawings):
- A base layer of 3/4 plywood or MDF (or particle board?). It sits on a very narrow center wall, so stiffness/flatness of this layer is critical.
- A second layer of 3/4 plywood or MDF (or particle board?) that gets a veneer top. I'm thinking it should be screwed but not glued to the base layer to accommodate any movement between the two layers?
- A veneer of 1/16 thick Zebrawood planks. This is to get the look of the glued up boards of the original, something I don't think is possible with paper-backed veneer. My thought is that this is thin enough to be stable and thick enough to be durable. I don't know if Titebond III is a good choice, or something more rigid like Unibond. I also don't know what substrate would be most stable to glue it on - plywood, MDF or particle board.
- Built in 8 or 10-foot sections in my shop, sanded and finished all 6 sides and assembled on site with splines and countertop bolts.
- I have heard many times that these assemblies should be balanced with veneer on the bottom as well, but I'm thinking that wouldn't apply to this double-layer substrate?
Any suggestions or cautions you may have are welcome. After watching the original $16,000 bar top self destruct, I really need the replacement to be bulletproof. Thanks in advance.
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3/30/16 #4: Thick Veneer Bar Top ...
need more info. no reason it would fail, except maybe- bad attachment, bad glue, dramatic change of moisture between shop and or on jobsite, lumber defects etc. I've made a lot of tops like that 8/4 and thicker. the edges always look different when veneered, even thick veneer. against all forum advice i have fabricated many tops with ply core, 1/4 plus thick veneers both sides pressed with moisture cured urethane flooring cement. 1/8 veneers I've used titebond II or even contact cement , for decades and have not had to replace a top.
3/30/16 #5: Thick Veneer Bar Top ...
Very sad to hear about that top coming apart.
Your plan is sound. Here is what we would do:
A solid core of MDF at whatever thickness you need it to be, minus 1/8". Then apply the waterfall edge band with the 1/16" veneers, then the top and bottom with the 1/16", preferably at the same time.
You want to have a balanced panel, with equal finish on both sides to keep everything nice and flat. Edge band after the faces may catch clothing and start to pull off, so it is better to have the face over the edge band.
The faces should be edge glued and taped into a single piece when laid. The edge glue will help keep joints from opening while being laid and pressed. We use a vacuum bag. Glue? low/no water. I'd lean towards polyurethane or Unbind 800. Not epoxy - unless the wood demands it. I have no track record with Zebrawood.
The method we use allows nice win miters and other details that can't - or shouldn't - be done in solids. But you can now teach that class, eh? Best of luck.
3/30/16 #6: Thick Veneer Bar Top ...
What is the size of the top in plan?
3/30/16 #7: Thick Veneer Bar Top ...
Thanks for the comments so far. The main reason for the failure is wood shrinkage. I warned the client that I really needed a few months for the lumber to reach a good MC before starting to machine it and glue it up, but that just wasn't an option. Also the boards are glued up perpendicular to the length of the bar, maximizing the number of joints and cross-grain shrinkage. The whole point of doing it that way was to get the dramatic end grain showing. I expected those problems, but I hadn't figured on the S-shape of the bar adding extra resistance to shrinkage and exacerbating cracking, checking and joint failure. The radius of the two curves tightened measurably due to shrinkage. Kind of fascinating to watch in a sort of train wreck way.
The bar top is 23" plus 4" drink rail, a full 2" thick at the front edge. I can't imagine trying to handle 2" thick MDF, which is why I thought a build up was more practical. Plus it allows for the integrated drink rail. If I need to balance the assembly with a veneer on the underside, does it really need to be the 1/16 Zebra planks as well? That's double the material expense and labor for something that won't be seen. Would HPL on the underside suffice, or must it behave identically to the Zebra wood?
I take your point about having the face overlap the edge band. Zebra wood tends to get brittle when thin, so when I trim the end grain of the face flush to the front edge, there may some minor splintering and tear out. Applying the edge band last would hide these defects.
3/30/16 #8: Thick Veneer Bar Top ...
You can use end grain slices for the edge band, but you will not be able to match them to the face grain. I don't know that most people would read that as what is going on. I think they see wood, they feel it, and they enjoy it. Only us anal retentive types worry about some of these details.
I know the safe thing to do is to balance with the same species/cut/source material, but the veneer shops will back with a lesser species that has the 'same characteristics'. Good luck there. The expense may be cheap insurance unless you can match the characteristics.
I understand you want this one bulletproof. I have been there.
You could do the build up, but still use the balance backer thick veneer, but I would not trust 3/4" or maybe even 1" MDF to support itself over time. 2" could warrant a torsion box, but then only if weight was critical, and it won't be once it is in place. A torsion box would certainly complicate everything, for a little weight savings. Plywood can save some weight.
The drink rail could be separate and set into a machined rabbet on the underside of the top. And the edge band on first, then the faces will be better for resisting catching.
You could go to 1/8" both sides and edges. That would give you a good solid corner to ease hard to prevent any snags. Those hard square corners get hit and then they open a bit.....
As for that slo mo train wreck - any photos? The best thing to do with your pain may be to share it.
3/30/16 #9: Thick Veneer Bar Top ...
I've done a fair amount of work with zebrawood veneer. Make-up your core thickness by gluing each layer together, there won't be any movement between layers. I apply veneers in a vacuum bag. Show veneer on edges and top face, and veneer the bottom with something less expensive like maple. This will keep the piece from warping. The other thing I'd like to point out is Zebrawood is open grained. So, in a potentially wet situation from a bar you may want to fill the pores and then topcoat. For this application I would use a UF glue, though you could use Titebond I since this is essentially a flat glu-up.
3/30/16 #10: Thick Veneer Bar Top ...
I'm with David on this one, edge band first. Substrate marine ply. Grain fill the Zebrawood to fill pours. For sure a balance veneer, look for something that has about the same movement. That has to be the widest glued up board I've ever seen.
3/30/16 #12: Thick Veneer Bar Top ...
You asked for train wreck photos, so here are some. The shrinkage has manifested in only a handful of places, as long sections have shrunk but stayed together as complete units, only failing where the adjoining section is prevented from being dragged along (if that makes sense). It seems that the transitions from straight section to curves are the worst. Here is the worst of the lot, pictures taken on 10/27/15, 12/2/15, and 1/13/16.
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3/30/16 #13: Thick Veneer Bar Top ...
Here's the bar soon after installation on 9/3/15, before the grief began.
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3/31/16 #14: Thick Veneer Bar Top ...
Is that the only place it cracked?
3/31/16 #15: Thick Veneer Bar Top ...
That's by far the worst split, but there are about 6 other noticeable ones. Plus several sections of 3-4 boards that have crowned a bit so that the original long flat expanse is somewhat hilly now.
At first I thought I could wait out the shrinkage until it was all done, then clean up the splits, draw them back together, sand things flush again and re-finish. But the scattered sections that are crowned, cupped, warped, etc. are just too much to hope to ever get the whole thing flat and straight again. And who knows when the summer humidity comes if it might begin to swell and start growing again.
3/31/16 #16: Thick Veneer Bar Top ...
Those are painful to look at. Good work, gone bad. Do you know the initial moisture content when you worked the wood? How about the MC today? A quick look at the Shrinkulator shows a 20' long (wide) top will move over 1.5" with a 2% change in MC. That demands a floating mounting method. The mention of cupping across width tells you that the initial MC was too high - too late.
I would be sure the veneer is of the proper MC as well as all the materials, acclimated together in the same room for a couple of weeks.
For the record, miters would most likely never succeed long term in solids like that. However, the curves and straights can work if they were mounted in such a way as to let the top move - expand and contract with the inevitable changes in RH in the room.
4/1/16 #17: Thick Veneer Bar Top ...
Website: http://www.factoryfurniture.co.uk/
You want to have a balanced panel, with equal finish on both sides to keep everything nice and flat. Edge band after the faces may catch clothing and start to pull off, so it is better to have the face over the edge band.
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