How can I create a planked wood table top with a plywood core?

Question

I was looking at this table made by Harp Design company Melissa Table

The description describes the top's construction as:

Each top is crafted from reclaimed old growth pine which is planed down to approximately 5/8″ thickness. It is then joined together using ship lap joinery on top of a solid plywood core to make the finished top approximately 2″ thick.

I was wondering how I could make a table similar. For example, if I used 1x4 pine planks and joined them, how could I properly attach the planked top to a 3/4" plywood sheet? (I'm thinking about how to allow for wood movement).

And, how could I trim out the edge of the table. It looks like in this table they used veneer or really thin wood to cover the edges of the planks and plywood.

Answer 1

I was wondering how I could make a table similar.

You can't really, and shouldn't try. Making a table that looks similar is easy enough, but actually laminating solid wood to plywood is inherently a flawed methodology. Even when you do this with veneer it can cause bowing if it's not done correctly, and veneer is extremely thin, so it's easy to imagine the problems that might arise with a much thicker wood bonded to the plywood.

That is not the only issue with this design if we can see the details of the construction correctly from the pictures (the other major one shows up at the edges). As a result I very much doubt examples of the Melissa Table won't see structural problems down the line, it seems a certainty.

Off-topic but I also think the "reclaimed old-growth pine" thing either describes just some of the wood, perhaps individual tables in the series, but possibly none of them at all... unless their definition of 'old-growth' is wood from tract houses from the '60s or '70s! Look at the short lengths, the abundance of knots and most importantly, the wide growth rings in the example pics on the site. The lack of narrow growth rings alone is suspicious, but taken all together you can be certain someone is fibbing or at least stretching the truth :-/

It looks like in this table they used veneer or really thin wood to cover the edges of the planks and plywood.

That does look like how they did the edges but even at that it's not sound construction, the short sides are the particular concern.

So how to build a table that does look somewhat like this, but do it stably? The three best ways I can think of are:

• use solid wood of the full thickness (thicker wood is more costly but in pine this wouldn't be exorbitant);
• laminate thinner boards to the final thickness (there shouldn't be issues with movement as each layer will have approximately equal response to changes in moisture level);
• use plywood that you veneer with shop-made veneer (and importantly, counter-veneer also), then do the edges just as this table appears to have done it.

Answer 2

There are several issues to address in fabricating such a table top.

First, as you pointed out is wood movement. The plywood base will not have any appreciable movement in either direction while the pine planks will expand across their width. If the two are attached, the difference must be made up at each board, where it is very small. I suspect that is exactly what the table manufacturer has done since they specify a lap siding connection between the boards. Assuming the table is indoors, its much more problematic if you intend to use it outdoors because of the extreme exposure to moisture, a liberal calculation for the expected expansion of a 5' board is 5" x 5% (assumed percentage of moisture variance) x .0029 (liberal expansion factor for yellow pine) = 0.0725 inches or approximately 1/16". That means that the lap joints should be installed with 1/16" of slop in the joint to accommodate expansion of the pine without affecting adjacent boards.

As to how to attach the boards, there are several methods you can use. They can be nailed. Apply the nails under the lap so that the nails are covered by the next board. When you reach the final board you can nail it in from the side so that the top surface is nail free. The apron trim around the perimeter will hide these nails. When you nail the side trim you can fill any nail holes with wood filler before finishing.

Another alternative is to glue the boards to the plywood, but not to each other. Since the width of each board is narrow the expansion will be absorbed within each board. The problem with gluing is how to apply pressure evenly to each board along its length as it sets up. This will require a very flat surface to lay the plywood on and a lot of weight to hold each plank in place as it is glued. I would recommend proceeding one board at a time.

Of course the best solution is to both glue and nail. The nails will act as your clamp holding the boards firmly to the plywood until the glue sets (wait at least 24 hours to be sure the glue is fully set before moving the table top.

I would recommend using 1x2 pine boards to match the top surface cut to length and mitered at the corners glued and finish nailed to the top surface (nails alternating between the plywood and the top surface. IF the span of the table is greater than a couple feet I suggest you increase the depth of the trim to between 3 or 4"