Cabinets, Furniture, Woodworks/bowing table top
We recently had a large table built. The builder used walnut which I provided it was very dry about 10 years.The planks run across the top and consist of two halves and three leaves. The halves are 40 inches wide by 42 inches long making the table 40 X 84 without the leaves. we had the table about three weeks when the two halves started bowing upward. I noticed that in the construction he had used biscuits and then glued and nailed a 1/4 inch plywood cover over the entire underside. Would this construction of plywood on the underside cause the tops to bow up or would they bow down? How could this be fixed as we have a large investment in this table thanks Jerry
WOW WOW WOW - I almost fell off my chair when I read about your table problem. Your table top has serious issues. Depending on how big your large investment is, and how much energy you have, you need to pursue this with the person who made it.
That table top construction violates three major rules of woodworking, and that top really doesn't stand a chance. It will never be right, even if the maker tries the one thing I am going to suggest at the end of this answer.
First, you ALWAYS (did you get that? ALWAYS) treat both sides of wood the same. That means if you veneer one side, you have to veneer the other side. If you put plastic laminate down on one side, you have to put laminate down on the other side. The reason for this is simple - wood absorbs the humidity in the air. So if you treat one of the sides differently than the other, the two sides will absorb different amounts of ambient moisture. This is a very common cause of bowing, like you are experiencing. That plywood skin on the bottom is one of the main reasons that top is bowing.
Second, you never use nails when building fine furniture. Never. They pull out over time, and are worthless. They are also a sign of a poorly educated woodworker, as almost any pro knows this fact. This may sound like it's not a big deal - trust me, it is.
Third (and the worst) - and this sort of goes along with the first major blunder, but gluing plywood to solid wood is a recipe for disaster. Just reading this makes me want to call that maker and ask what the hell he was thinking. See, when wood acclimates itself to the seasons, by absorbing the humidity in the air (as I explained above), it moves. It shrinks and swells with the seasons. Ever have a drawer that sticks in one season, and is fine later that year? Ever have a door that sticks during certain seasons? This is why.
Wood can shrink or expand as much as a quarter-inch over a twelve inch span. You didn't mention which way the grain runs, but I'm guessing it's the 42" direction. Still - if the width is 40 inches, that means the top can potentially move an inch or more from a dry season to humid season. Gluing that plywood means the wood isn't free to move without being bound, and it is simply a recipe for disaster. It cannot be fixed, unless that plywood is removed.
OK, in the beginning of this answer, I mentioned a possible solution. This is just a short term solution, because trust me, this table top has issues. But it's the holidays and you might be entertaining, so you obviously don't want to be serving dinner on a cupped table top. This is only a short term solution, but it might work.
If your furniture maker attaches some cleats to the underside of this table with clamps, and pulls the table top flat, he can possibly put a few screws into the plywood/table top and pull it flat temporarily. Now - I don't mean to insult the guy, but he has to apply the cleats properly, or they won't work. Let me explain.
Say you put a couple of bricks about 8' apart, and then lay a 2x4 on top of them. If the 2x4 is laying flat, and you step on the middle of that span, it will bow. There is no strength on a board laying flat like that. But if you turn it up on end, so you're standing on the skinny edge, that board will not flex.
If your guy applies cleats to the underside of the table, he will have to do them vertically, so that the sheer mass of that wood will (hopefully) pull that cup flat. He will need clamps, and for god's sake, make sure he doesn't use screws that are too long and could possibly pop through the top. Well, would it matter at this point? You need a new top.
OK, that's all I have for you. Reading about your problem makes my head hurt. Good luck, and if there's anything I can do, let me know. Sorry if I came on sort of strong in this answer, but damn.. this is Furniture Making 101.
Keep in touch,
Jamie in Vegas
Wood It Is! Custom Cabinetry
Las Vegas, NV
My woodworking blog: www.wooditis.blogspot.com