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Tabletop (made from floorboards) cupped

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  1. #1
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    Default Tabletop (made from floorboards) cupped

    Got some leftover blackbutt floorboards 130x14mm and i glued them up, hoping to make a table top over my tv cabinet 2500mm long.

    Unfortunately they cupped and dont sit flat.

    Soaking water on the concave area see significant results they become straight but returned back when left to air.

    I tried to seal up the boards immediately using danish oil when are flat but still cup later.

    I know thin wide boards tend to warp. How can i fix the problem

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    Generally, you can’t. Timber will do what it wants to do and it will succeed in doing it sooner or later.

    You will see a lot of solutions on the net which involve rehydrating the concave side and then sealing the moisture in. Don’t expect long term results.

    Other suggestions are to fix the boards down to some type of framework or battens underneath. The problem here is that the forces involved are immense and you still will end up with ugly twists unless you use very thick cross braces. Also, if fixing to members lying perpendicular you will have a problem with expansion/contraction which will prevent you fixing the boards down rigidly anyway.

    So, if it were mine what I would do is :

    Rip the boards apart on a table saw, along the glue lines.
    Leave to acclimatise in your home environment for a couple of months.
    Reject any that are really bad.
    Rip or plane along the edges again making sure that the edges are exactly 90 degrees to the faces (they may be distorted after acclimatising).
    Lay out the boards to plan for the glue up. Make sure to alternate the orientation of any crown cut boards - what this means is that if any boards haVe a U shaped grain (when looked at end on) then alternate them pointing the apex of the U up and then down.
    Glue them up with sash clamps. Use lots of pressure to drive the air out of the joints and the glue in. Don’t use biscuits or dowels or anything else - they are not required because the gluing surfaces are large enough to give a perfect bond and they only complicate the glue up. Use Pva woodworking glue - long open time, good joint strength and a tiny bit of creep where needed.
    Prep and finish. It won’t make much difference what finish product you use just make sure you do top and bottom the same.
    Danish oil does not seal moisture in, you can only really do that with a full envelope coating in someth8ng like epoxy, as in boat building, and even there it is more myth then reality.

    It is difficult to do a wide glueup with 14mm and make it flat, but it is possible.

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    Lay them on a substrate, I have used yellow tongue in the past.

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    Unfortunately i need them as a table top to lay over something. Cant lay over substrate nor anchor it down.
    I thought using floorboards are better as the tongue and groover provide more surface area to glue, rather than gluing square edge boards.
    I did clamp well using pipe clamps top and bottom and clamp flat too but still happen

    i tried to put a damp cloth over the concave side and shortly it uncupped, but i dont know how to keep it that ways. Cant go against forces of nature




    Attached Thumbnails Attached Thumbnails ac7888e9-4801-477a-828e-3b21d5bf542b.jpg   f11c295d-46f0-49c3-8ecd-a33bb0779551.jpg  

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    I wonder if polyurethane or any other seal like lacquer or varnish will seal the wood in from external forces?

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    Looking at your photo, the one showing the boards end-on, I can see that you have 5 or so crown cut boards, all aligned the same way. With boards of this type and dimension, that’s pretty much bound to fail.

    In the attached image you can see the good and bad way to glue up.

    What I’m trying to show with the semicircles is how the grain lies.

    Imagine each board cups by 2mm. In the ‘bad’ way illustrated the total cupping will be between 20 and 40 mm (sorry, I forget how to calculate it). In the good way, the total cupping will be 0 mm because they cancel each other out.

    And no, polyurethane, lacquer or ‘varnish’ will not seal out elemental effects. If they did, there would be no need to make allowance for expansion/contraction and everything like cabinetmaking and laying floors would be so much easier.

    If they were mine, I would rip them apart and do again. The possible advantage of tongue and groove will be lost but that is irrelevant as gluing up properly with square edge boards will give you all the strength you need - in fact way more then you need.

    Don’t forget - always acclimatise timber. Preferably after ripping to approximate dimensions. Just learning to do that will eliminate 90% of this type of heartache. Here you would have seen that the boards were starting to cup, and realised that you weren’t going to get good results if you went ahead.

    I actually think only one board is cupping - board number 4 from left. Rip it up the middle and use as two narrow strips, one either side of a wide centre board for best appearances.

    7df50bd2-a9b3-4e7e-b5dc-5dcb63067629.jpg

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    Thanks. Totally understand why the crowns should be opposite. But for my floorboards to make the crown opposite, i need to flip 180deg. For floorboards, the bottom part are grooved and cannot be used.

    These were leftover floorboards I thought i could make something out of it.

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    You could also put anti-warp grooves to the underside of the boards.

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    Find a carpentry shop that has a drum sander thicknessing machine and have them flatten one side or both. Then mount on a table frame with solid legs and strong apron to counteract the warping.
    You will never achieve a flat stable surface out of 14mm floorboards without support. I thought floorboards are minimum 19mm
    Alternatively find 25 or 32 thick material properly aged. For 14mm thick, I am surprised at how good it is now.
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    There are thinner floorboards actually, called overlay. I meant to lay over my white tv cabinet as an overlay without securing. As you can see, there is still some cup but unnoticeable from the overview.

    From a woodworking point of view, is it harder to glue up square boards that are thin? I imagine more difficult to glue clamp thin boards without the tongue and groove.

    Maybe it is just my floorboards issue. I wonder if it is possible to find floorboards of opposite crowns so even if they cup it balances out.

    Left side cup on the first floor board, and right side cup on the back-most floor board. Or are longer boards more of an issue? I wonder if I use shorter pieces I can spread out the issues to be less visible.

    781e2f43-b517-4b17-9804-6898d8721b1c.jpg

    4a01e451-ba21-42d2-a8cc-7a47510a3903.jpg
    Right side

    76b11899-86a4-4489-9b07-20f68beb1691.jpg
    Left side

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    Shorter won’t make much difference, it’s the thin timber section, only way to keep it flat is to stick it down to a stable substrate.
    Alternating the grain is the usual way of counteracting the cupping. Most modern boards are uneven tongue design so you can’t just flip them but you could re-machine the tongue and groove, or biscuit join, or just butt join.

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    When doing a glue up, tongue and groove, biscuits or dowels are best thought of as an alignment solution, not as a bonding solution. In other words, they make it easy to align the glueup but they are not regarded as necessary in terms of adding extra strength. With modern glues, the bond achieved by butt join is perfectly sufficient.

    Tip: have a close look at your current glueup with a straightedge in your hand. Are all the boards cupped or is it just one or two. From the picture it’s hard to tell but it looks like there is really just one real culprit. If so, rip it down the middle, square up the edges, flip one side, and reglue. It won’t be perfect, but pretty good for 14mm boards.

    To answer your question about alignment of thin boards; of course it is easier with the tongue and groove in place but if you don’t have it then it’s not a big loss. With a long open glue like pva, you should be able to get all the boards aligned with or without the tongue and groove. Good flat glueups of 14mm boards are possible but you need to be sure the boards are seasoned, straight and square and you are not embedding stresses.

    Going for shorter boards will not help. Going for narrower boards helps a great deal.

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    With gluing tongue and groove I have found that on the groove side the lower protusion is often set back a fraction. This almost guarantees cupping unless stuck to the substrate, the answer is usually to run the boards across a jointer to remove the grooves and a thicknesser to remove the tongue (or vice versa) No need for dowells or anything else. Your problem does look more like actual layup of the boards, however the onle fix will be to run a saw down the joint lines, square up the cuts and reglue. Even then the underneath grooves will not help much either, quite the opposite.

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    Quote Originally Posted by Jspan View Post
    There are thinner floorboards actually, called overlay. I meant to lay over my white tv cabinet as an overlay without securing. As you can see, there is still some cup but unnoticeable from the overview.

    From a woodworking point of view, is it harder to glue up square boards that are thin? I imagine more difficult to glue clamp thin boards without the tongue and groove.

    Maybe it is just my floorboards issue. I wonder if it is possible to find floorboards of opposite crowns so even if they cup it balances out.

    Left side cup on the first floor board, and right side cup on the back-most floor board. Or are longer boards more of an issue? I wonder if I use shorter pieces I can spread out the issues to be less visible.




    Right side


    Left side
    The boards are to thin to be left unsecured, they will cup every time.
    I would be securing them to a substrate be it a good quality ply (not the rubbish you buy from Bunnings), or even MDF if you are really trying to spend nothing.

    You would be best to glue them to the substrate and maybe a few screws to secure it while the glue dries
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    Can you screw it down to that cabinet? Or add some solid 2"x3" timber inside the cabinet secured to the sides to then screw down to? It dors not look too bad. Just a bit of warping. Shouldn't be too hard to bring it down

    As for t&g gluing, for another time, if you want a flat tabletop from t&g, you don't glue the edges, you glue it down to another surface or a frame and leave the edges free ... or ... you put the boards through a thicknesser on the edge using a jig, and square them all.
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    Thanks for all the suggestions.

    Well I cant glue to another substrate as that will be seen from the side. Dont want to screw from underneath either I’m not sure maybe the forces of nature will overcome the screw strength. Further more thats an Ikea cabinet. Cheap stuff

    What I can foresee if I do it again in the future, I should
    1. Use narrower floorboards
    2. Get rid of the tongue and groove, square it up then glue (Not sure how I can properly align)
    3. Use 19mm thick boards

    With gluing tongue and groove I have found that on the groove side the lower protusion is often set back a fraction. This almost guarantees cupping unless stuck to the substrate,
    Dont quite understand this. Do you mean the T&G glue up is causing the cup? I can see that while they fit, it is not 100% and there are still small gaps

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    What Johnc is saying is that some t&g boards groove, is not machined equal. If you put a square on the edge you can see that the top side of the groove is longer than the underside. This is to maximise the chance of achieving little or no gaps when installing the floor. No good for gluing whilst exercising lateral pressure. Yet not all boards are like that. It is more an old fashion profile.

    As for mounting your top on plywood, that is easy done. Cut the plywood 20 or 30 mm short all around and glue a strip of flooring on the edge. Job done.
    And you Ikea cabinet can be reinforced with strips of pine along the inside, if you want to screw the top flat.
    The way I would do it is clamping the top to the cabinet with long clamps and 2 90x45 top and bottom at both ends.
    When it is down flat, then you screw and leave it clamped for a day or so.
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  18. #18
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    What Marc says about mounting the boards on plywood should fix the issue.

    If you want to use these boards, go for a good quality plywood that's flat, 12mm would be the minimum thickness as long as you also screw it to the ikea thing to keep it all in place.

    Either PVA spread / rolled over both surfaces, or direct stick flooring glue is the cheapest way to do it and you just apply it to the plywood, something like the below glue is the cheapest way to do it.
    Then screw from behind the plywood into the flooring to secure it hard onto the board, make sure to put enough screws around the perimeter and in the centre to make it pull tight onto the plywood.

    To cover the edges you can do what Marc said by placing another thin piece of timber around the plywood edges, or keep the plywood flush and cut a thin piece of the same timber and glue it around the perimeter like a veneer.
    Marc's version is easier if you have limited tools.

    Once the glue has dried, you can sand the edges and it will look like a thicker piece of timber.
    Then fix it onto the entertainment unit with screws from the inside, if the top is hollow which most ikea stuff is full of air, use Marc suggestion of a few pine battens painted white to stop the screws from pulling through the top.

    If you don't want to fix it to the ikea with screws, I would opt for a 17 or 18mm ply, and undercoat the back of the plywood to ensure it doesn't warp, it shouldn't but I have seen it happen as the glue can exert a bit of pulling pressure on one side o f the ply, enough to warp the ply.

    You can do the fix with the existing boards, you will just need to do a bit of sanding and recoating.

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    Quote Originally Posted by Marc View Post
    What Johnc is saying is that some t&g boards groove, is not machined equal. If you put a square on the edge you can see that the top side of the groove is longer than the underside. This is to maximise the chance of achieving little or no gaps when installing the floor. No good for gluing whilst exercising lateral pressure. Yet not all boards are like that. It is more an old fashion profile.
    Most modern tongue and groove are machined with uneven depth for the purpose of secret nailing, it leaves material under the tongue so that the hidden fasteners do not simply split the tongue off.
    Older tongue floor was machined even because it was always top fixed.

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    i'd be screwing from the top (or underneath) into the cheap ikea TV unit

    if by some miracle it warps or damages it.... go back to ikea and get another one, but i'm pretty sure you'll be ok.
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    For the last month a bloke who is a good carpenter was making a couple of tables at our mens shed from silky oak timber tongue and groove that came from a church in Mt ISa that was built in the 1920's so it was well seasoned. He assembles it and the top of the tongue he cut out with a router and made silky oak strips to fit and glued them in place.
    He finished them last week and we admired them and they looked perfect. Came in yesterday and two places there were two seams opened up probably about half a mm.
    It seems no matter how much effort you put into it that could still move probably dependent on the environment.

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    Quote Originally Posted by Bros View Post
    For the last month a bloke who is a good carpenter was making a couple of tables at our mens shed from silky oak timber tongue and groove that came from a church in Mt ISa that was built in the 1920's so it was well seasoned. He assembles it and the top of the tongue he cut out with a router and made silky oak strips to fit and glued them in place.
    He finished them last week and we admired them and they looked perfect. Came in yesterday and two places there were two seams opened up probably about half a mm.
    It seems no matter how much effort you put into it that could still move probably dependent on the environment.
    If you try to build a large surface out of strips of timber glued at the edge, in order to keep the top stable, you need either a larger gluing edge (thicker) or keep the edges free from each others forces and glue to a substrate or fix, screw to a frame.
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    Quote Originally Posted by Jspan View Post
    Well I cant glue to another substrate as that will be seen from the side.
    You could cut a strip of the hardwood boards and glue to the sides/front.
    If you have the top "overhang" by the thickness of the side strip, this will hide the substrate and give the illusion of a thick top.

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    Quote Originally Posted by Bros View Post
    He finished them last week and we admired them and they looked perfect. Came in yesterday and two places there were two seams opened up probably about half a mm.
    It seems no matter how much effort you put into it that could still move probably dependent on the environment.
    Yep, have a hardwood top on my sit/standing desk - 25mm thick and 45mm wide strips glued together and screwed to a metal frame.
    Has held up well for the last 18 months with the exception of where my laptop is placed. 2 Boards have opened up 2mm the width of the laptop where the fans extract out the heat.

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