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I have never made anything out of wood before. This is all new to me.

I want to build a computer desk table top. My goal is to just make the table top right now and then purchase some electric legs that move up and down. I have some wood that I was given from a building that was torn down. I believe it is oak. There are several holes in it from where it was nailed down before, so I want to cut it up into several pieces, effectively removing the holes, and then glue them all back together in a nice pattern.

I am concerned about the load capacity. I want to make sure it is strong enough to hold 300 lbs. I have easily 150 lbs of computer that I want to set on it, and I expect I will lean on it plenty, so it needs to be pretty strong. However, since the pieces will all be glued together, I'm afraid of the joints coming apart.

How can I calculate the maximum load capacity?

Bellow I have include a simple diagram of my design. It will have several 9.5" squares at 45deg surrounded by a 2" border. The result will be 71.175 x 30.87 x 1 inches. For added strength, I want to rabbet the joints between all of the squares, but probably not the 2" border. I think a centered 1/4" thick 1/4" long rabbet on every joining edge should work. I'm not certain I'm using "rabbet" correctly, so I will describe it in another way. Basically, I want to router off the top and bottom of one edge so that there is a piece along the whole side that is 1/4" thick and sticks out a 1/4", and then on the board it will connect to I will cut a 1/4" groove to match so they connect together.

Basic draft of table top design

  • There are approximately 3845.56 previous discussions about tables and load-bearing capacity. did you check the previous Q&A? Also, static load is the least of your worries when making a table. Have yo considered dynamic loads and racking forces? A usable table is more than the top surface -- just see how many different ways a table will attach legs, have aprons, etc. – jdv Jun 19 at 15:48
  • If I understand correctly you want to glue together a group of 9.5" x 9.5" wood squares. How thick is the wood? – Ashlar Jun 19 at 16:55
  • @jdv I looked at a few discussions, but I had a hard time following and remained uncertain if my specific design would work. Thus, I wanted to ask about my exact situation. I don't know what dynamic loads and racking forces are. I will look them up this evening and see if I can apply them. I don't want any aprons because of how I plan to use the desk. Essentially, I want it to rest directly on my lap when sitting. – Trevin Avery Jun 19 at 17:36
  • @Ashlar Yes, you are correct. The wood is 1 inch thick. – Trevin Avery Jun 19 at 17:37
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    Maybe you should ask "how can I make a 70 inch long desk out of 56 in long boards?" instead! This is a bit of an XY problem in that respect. – FreeMan Jun 22 at 18:20
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You haven't shown the leg arrangements, so a more immediate problem might be racking forces. That is, depending on how the legs are attached and arranged, a ~300lb. weight on the top may cause the entire table to swing like a pendulum and maybe even fold up where the legs meet the table.

However, it is my guess that, even at a nominal 1-inch thick, this table will eventually sag. Even if it doesn't, it might fall apart because of seasonal movement or flexing anyway.

Those joints are not going to be as strong as a nicely glued plank edge, even if you really sweat the details when gluing up and clamping. Expecting tongue and groove to support a span is unfair, as this is not what that joint is good at. Much of the gluing surface is missing, and in most cases there are going to be lots of voids where the tongue and groove doesn't quite mate for PVA glue to work its magic. And half those joints are going to be and grain on at least one of the pieces, which makes for a weaker joint.

So, it probably won't fall apart immediately, but I bet it'll keep flexing until some of the joints start to open up.

Note that, traditionally, tongue and groove surfaces were never intended to be glued anyway. They are a way to mate surfaces so they stay put, but still allow for seasonal movement. At most, you might blind-nail the edges together in a few places just to keep the joint tight to the material below it. If you glue these joints, the top might just pull itself apart by next year (depending on where you live.)

My advice would be to design a sturdy table using quality plywood (or solid wood, though plywood is hard to beat for its mass-to-strength ratio) then covering that with your solid wood "marquetry". Your edging will then hide that construction detail along with the raw edges of plywood.

Care would have to be take to make sure it is stiff enough for the static loads, and that the leg arrangement resists swaying when presented with racking forces. Sometimes you can combine leg attachments so that they offer both extra table stiffness along with racking strength. This is why I suggest you visit the previous Q&A on table design to get some ideas.

If you want to forego the plywood idea, I advise you experiment. Make a smaller version of your joinery top and see how it flexes under weight. Work tables useful for piling computers and equipment on top need to be able to hold up a medium sized person without cracking or flexing; you must be able to stand on it and not feel like it is going to fold up, or without hearing any ominous cracking sounds.

More specifically in your case, look at your design. Imagine your 300lb. static load right in the middle, at the joint where four pieces meet. These four joints are going to involve end-grain, and some of those joints the wood is going to want to move perpendicular to the grain (which often amounts to the same thing). My estimation of the failure mode is a crack starting at that corner, and then zig-zagging through any other of the joints to another point, where the stress pattern repeats.

When you lay flooring or bricks or wood siding, one of the important rules is to not have matching joints. That is, if you rotate your table top 45 degrees you'll see you have (essentially) 1x3 rectangles that your are combining to make a slab, but you are joining those 1x3 sections such that the joints are all along the same line. You are not using the strong glue joints to support the weaker wood joints. Thinking about the construction as a series of 1x3 sections you join together staggering the joints would go a long way toward getting an ideal slab strength.

We haven't even talked about material yet! You don't mention what sort of wood you want to use, or how well it is seasoned. But 10-inch squares are going to move, and they will move perpendicular to the grain. So, even if you carefully prep the mating surfaces and clamp ideally, after some time you might find that the top starts to buckle or crack.

If you used very stable and seasoned wood, and you carefully matched the grain so you minimized additive movement, and you glued up the top in sections ideally, staggering the joints, it might still move with seasonal changes.

In short, don't just focus on the top of a table build. Success relies on you getting a full idea of how static forces are handled through the top and transferred to the floor, but also how everyday use introduces dynamic forces that operate on completely different axes. The whole table needs to be designed to work together to give you the adequate strength in all these directions you need.

In conclusion, I don't want to suggest you give up the whole thing as a bad idea. I will point out that this is a very challenging project for the novice woodworker because you really have to know your material and your techniques.

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  • This is pretty close to becoming the cannonical "Will my table be strong enough" answer. It's got specifics on marquetry, but will apply to them all and can either have that removed or have sections added to cover other construction techniques. Most importantly, it nicely covers legs and racking which all newbies seem to forget about. We should either copy/pasta or just point to this guy for all future questions of this sort. +100! – FreeMan Jun 20 at 13:38
  • Would flat edges be stronger than tongue and groove edges when glued? I didn't mention leg mounting because I was planning on using prebuilt electric lifting legs like these. I don't think there is anything I could do if they can't handle racking forces. – Trevin Avery Jun 20 at 16:16
  • Flat edges, if glued up appropriately, would be very strong. The only caveat being that many of the joints would involve end-grain. Take a look at Ikea slabs made from softwood: where they have end-grain joints, they have sandwiched that joint between two other pieces. That is, they never have non-staggered joints. In your case, all you have is non-staggered joints, some of them end-grain. My gut tells me you will not get the strength you need. – jdv Jun 20 at 16:23
  • Commercial trestle style legs will be fine, assuming you get them positive connection to the slab top you need. It all depends on how stable from deflection and stiff the top is. – jdv Jun 20 at 16:25
  • What exactly is end-grain? Is it any edge that is not parallel with the grain? Is deflection the same thing as racking forces? – Trevin Avery Jun 20 at 16:30
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I think what you're proposing will turn into a very attractive tabletop.

The joint you're describing is a tongue-and-groove joint (don't worry about mis-describing it as a rabbet joint; I'm very sure that a couple of years ago I made similar mistakes).

Truthfully, I don't know how to calculate the load bearing capacity of those joints. What you might try doing with some scrap material is create a "test coupon" that has the same joint with the same material and apply weight to it until it cracks or tears. Alternatively, build the table's underframe with beams that the tabletop will rest on (kind of like how a floor is built) that will bear the weight instead of relying on only the strength of the joint.

I'm sure you've already done this, but in the interest of safety I'll say it anyway: make double-sure that you've removed all nails before cutting into the wood. Visually inspect it and also use something that will respond to a hidden nail, such as a strong magnet or metal detector.

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