Work surface top - advice on glueing to plywood please

[AJB Temple]

I have been asked to make a work surface Island top 3m by 1.2m that is to a specific architectural specification. Which is as follows.

Top in 1" rock maple. Very high quality hand selected (by me) boards, various widths but mostly about 200mm wide or a bit more. all at about 8%. I bought them sawn and they have already been thicknessed and edged by me. Dead flat and currently acclimatising in the room where they will end up.
Edge jointed with dominos.
Entire work surface glued to 25mm marine ply.
The whole lot edged with dominoes and glued Wenge (again - top notch stuff, selected by me. I have some very wide boards. Eye wateringly expensive even with trade bulk discount).
Epoxy finish.

I have no issues with making the work surface (which might get reduced to 2.5 metres long - there are in fact two of them) but I am a tad worried about glueing to the plywood. The architect (who is a well respected chap) says this is his preferred method, as in his past experience it has achieved great stability. I am a bit worried about the maple moving about and cracks appearing.

There is no guarantee exposure or anything like that - I just want to make sure we are not making a bad decision, and do the best job. He is not a dogmatic man, and will listen to reason. These are expensive worktops needless to say (will end up costing more that granite or quartz).
Any thoughts would be appreciated. I am not a professional joiner making a living from this. The architect has been a good friend for 30 years and he is going to work with me as he wants to have a hand in his own build. We are helping each other on our respective house refurbishments.

Thanks, AJ

 

[profchris]

I'd say your worry is humidity changes. I ran the calculations here for 1.2m of big leaf maple and got a width difference if the air humidity swings from 40% to 70% of between 9 and 17mm, depending on the cut.

http://www.woodbin.com/calcs/shrinkulator/

You'd certainly want to glue to the plywood in low humidity, otherwise there is a good chance of the maple cracking as it shrinks, though with thick boards that might take a few years.

What I don't know is (a) how fast the maple would react to humidity changes (epoxy will slow this but not completely unless it's really thick), and (b) if the plywood would resist cupping enough.

A solid wood substrate would move with the top, and you could pick a species with similar shrinkage/expansion rates.

With luck someone will know whether a 25 mm board can overpower 25 mm ply, but I'd guess it might to some extent.

 

[MikeG.]

Blimey. I'm not sure that trying to subdue the natural movements of timber like that has ever worked well. That's absolutely not an approach I would be taking, either as an architect or a woodworker. Could you suggest, say, sliding dovetail battens, instead? Obviously one edge can be screwed into place through the carcass of the units below, and the other edges could be retained by buttons in the usual way, but if the architect is so worried about cupping or twisting, then maybe the sliding dovetail suggestion would put his mind to rest. You could also suggest relief cuts to the underside (stopped, of course, to not show).

 

[Steve_Scott]

No experience here other than fitting solid (40mm finger jointed) wood worktops but My gut feel is either put a thick(ish) veneer onto plywood for stability or make a solid wood top that you allow to move.

My fear would be a 1” top would want to move on the ply substrate and induce a stress. Might be unfounded?

There’s nothing wrong with doing what you’re instructed to do and why it goes wrong blame the plan

 

[MikeG.]

......There’s nothing wrong with doing what you’re instructed to do and why it goes wrong blame the plan

There's more merit, though, in helping the architect (or whoever) understand the issues and known alternative approaches which might have a higher chance of success.

 

[Jacob]

......There’s nothing wrong with doing what you’re instructed to do and why it goes wrong blame the plan

There's more merit, though, in helping the architect (or whoever) understand the issues and known alternative approaches which might have a higher chance of success.

Well yes. If it goes wrong and it's his fault you still get the blame!
I did similar with 18mm ply doubled up to 36mm and formica on top. But the lower 18mm is only around the edges and across the width where needed, so most of it is 18mm but you couldn't tell from looking at it. Saves timber and allows a bit of flexibility/movement.
PS why glue it to the ply it's not going to go anywhere is it. Attach the edge strips to the Maple but with a clearance gap - ply base smaller by 5mm all round? Or just glue it down the middle, along the grain not across.

 

[Jonathan S]

My 2 cents worth....if its glued to the plywood, at some point in its life the maple will split......if the wenge edging is going across the grain of the maple and is glued, somewhere something has to give......years ago I did something similar with 60mm wide Canadian maple, the maple didn't split but the joints opened up!

Solid timber Islands are a nightmare because of there width......I have two in the workshop that where pull out and replaced with man made stone.

Good luck......remember the client is always right!






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[Sgian Dubh]

Top in 1" rock maple. Very high quality hand selected (by me) boards, various widths but mostly about 200mm wide or a bit more. all at about 8%. I bought them sawn and they have already been thicknessed and edged by me. Dead flat and currently acclimatising in the room where they will end up.
Edge jointed with dominos.
Entire work surface glued to 25mm marine ply.

Hm, asking for trouble I suspect. Hard maple (same as rock maple) has shrinkage factors of, tangential: 9.9%, and radial: 4.8% from FSP (fibre saturation point [30% MC]) and oven dry (0% MC). Assuming seasonal RH varies between about 40% (winter) and perhaps 55% (summer) resulting in an MC range in the wood between about 7.5% MC and 10.5% MC, the maple is likely to want to vary in width by about 1% tangentially, and about 0.5% radially. So, take a median point of 0.75% movement because your boards are likely to contain a mixture of radially and tangentially sawn wood, and being imported from and sawn in the USA all the boards will have been sawn tangentially, where there will be parts of each board approaching rift sawn, or quarter sawn, as well as tangentially sawn.

Some sums indicate a newly made 1200 mm wide panel starting at, say 8% MC and moving towards about 10.5% MC is likely to want to grow in width by up to 9 mm, and shrink back towards 1200 mm during winter when RH is generally lower at about 40% MC.

I don't think gluing your maple panel to a piece of plywood is really a practical technique. I'd devise a panel that's able to float independently of whatever is underneath, as in typical table tops fixed to a framework of rails and legs, and other similar panels that are fixed down flat, but still allowed to expand and contract.

To be more succinct than in the windy response above, I would gently decline to make it the way the architect wants, unless your friend/architect is willing to sign a waiver absolving you of all responsibility for possible failure in the future. On the other hand, it doesn't sound like your architect friend is unreasonable, and he may be willing to talk about alternative solutions given the problems likely to be encountered with the current construction proposal. Slainte.

 

[Inspector]

There is a reason for not glueing veneers over 1/8" thick and that is because wood as thick as you are doing will crack or cause the plywood to curl like a potato chip/crisp from moisture induced wood movement, especially if you don't do the same to both sides to balance the forces out. No amount of glue and/or fasteners will hold it to the base below and stay flat. Even coating all sides, both seen and unseen, with thick epoxy is no guarantee it won't move. If I understand the edging it will not hold either because the piece glued to the ends will break off because it won't shrink or grow while the maple does with moisture changes. If they are mitred corners they are going to open up when the maple swells when it shrinks and the maple crack at some of the glue joints or break free of the edging as it shrinks. Using Dominoes to align for gluing is fine but it won't add any strength to the joints except to hold edging to the ends, but those joints will still with movement. Mike's suggestion of dovetailed battens running across the slab with proper breadboard ends is probably the best way to approach it with solid wood and veneer if the plywood is insisted upon. Not both.

Sorry to be negative but those are the realities.

You could prove it to yourself and glue 100mm long samples of your 200mm wide pieces to a strip of the plywood simulating a cross section of the counter. Set it maple side down on the lawn for a few hours and see if it is still flat.

Pete

 

[MikeG.]

.......Solid timber Islands are a nightmare because of there width.......

There's no good reason that should be so. Solid wood table tops 4 feet wide or wider aren't a risk because they are built to allow for the movement. Applying the same principles to a worktop on an island unit isn't complicated.

 

[Jacob]

Or: frame it up underneath and around the edge. The trick is to glue/screw members along the grain where no movement is expected, but cross grain members no glue, slotted screws, loose half housings where they cross the longitudinal ones.
i.e. Roughly how round tables were made with 1" boards 4ft diameter. I know this from restoring a very old mahogany one but gluing the cross grain members too, and the top split first time in its 100+ years of life! Top was two 2ft boards. Frame pieces and edge apron were about 1" by 2" vertically

 

[AJB Temple]

Thanks all. Any further ideas welcome.

The reason I asked this question, is that I would not contemplate making a top this way normally and I wanted some impartial, experienced input.

My preference was to use 40mm thick maple and just make a conventional top. We couldn't get Maple he liked in enough quantity at that thickness. The job is a bit more complicated than I said in my simplified post. I need to do cut outs for tepinyake plate, and an induction hob, and a big sink. The thick ply is essential apparently (and might get doubled up) as he is installing massive Gaggenau rotisserie below it.

He is a lovely man and will not blame me if it goes pear shaped. But I do not want to do something that will go wrong. I've worked with maple a lot, making musical instruments, but never used it for a very large table. Hence worrying a bit. I would far rather make the Maple top float.

Also, although the wood was sold as kilned to 8/9, my meter says it is nearer 12.

 

[Jacob]

Passing thoughts:
Why use marine ply, it doesn't have any particular useful attributes for this project, or is it cheaper than birch through ply?
Why not veneer directly on to the ply? match grain with top layer of ply and eliminate shrinkage probs entirely. Visually no different.
Or if solid wood just forget the ply altogether. Frame up underneath as aforementioned. Think "Georgian round table" or "guitar soundboard". 40mm very OTT, 25mm would do.
In other words; either solid wood, or ply, but not both. Both is the problem.

 

[AJB Temple]

Thanks Jacob,

He's already bought the wood and it's on site, He wants solid maple not veneer.
I think the marine ply is because a sink is being let in and he is worried about water ingress. I will seal the edges anyway.
I agree with the framing approach, and will revisit this.

To be quite frank I don't really understand the ply aspect and will have to ask him again. It is something to do with fitting this Gaggenau rotisserie oven which is about a meter wide and will get up to commercial temperatures of circa 300 C apparently. I would have thought the weight would be taken on the bottom, but there is also some kind of cradle that mounts from the top.