# How flat is flat enough when laminating boards?

I'm making a workbench top using 2x4s of douglas fir laminated face to face and a hand plane.

I know that the quality of the glue-up is going to be a function of the clamp strength, the flatness of the glue face, and the glue coverage.

The question is, how flat do I need the glued faces to be? I'm measuring with the corner of my plane across the boards at various points along the length and looking at the light that makes it through. But I'm not sure what is a "flat enough".

Any tips or rules of thumb?

• Just to check, you're asking about the glue faces? If so anywhere from easily closes with clamp pressure to dead flat will be fine. Get dead flat if you can, but don't obsess about it too much. Jul 23, 2019 at 18:50
• The other thing to consider is if you're able to do the glue-up in one shot or not. If you're gluing up sub-assemblies then they will need to be jointed "dead on" flat, since you won't get very much bending at all when you clamp them up. Jul 23, 2019 at 20:42
• Oh darn forgot to check, so P.S. are you joining them face to face or edge to edge? Makes a big difference to your query so we need clarification on this point. Jul 24, 2019 at 6:40
• @Graphus: I edited the post, I'm doing face to face joining. Jul 24, 2019 at 15:42

The question is, how flat do I need the glued faces to be?

Since you're joining face to face pretty flat is good enough.

If you were joining edge to edge it would be more critical, because boards are stiffer in that direction and it's harder to deflect them when clamping to account for any unevenness. But boards readily bend in the opposite axis, so flattish will usually be fine because only light clamp pressure is needed to close up any minor gaps1.

Something to bear in mind is that many people doing these sorts of laminations with 2x material don't plane the surface of the boards they've bought, they don't even sand them. It's quite common for boards to be glued together after being cut to approximate length and no other prep work. This isn't the way to ensure a good result, but lots of benches have been made this way and it's a testament to how good glues are that many of them have remained solid despite the shortcomings of the construction method.

So essentially what you're doing with your hand plane is cleaning off the surface of the boards (a good thing) and barring any major twists or bows2 you could be good to go after a light skim with the plane.

I'm measuring with the corner of my plane across the boards at various points along the length and looking at the light that makes it through.

You hopefully won't need them here but for future reference you could do with having a long straightedge and a pair of winding sticks for checking for flatness and wind (twist) in boards that you're planing. Even more so for glued-up panels.

You can buy commercials versions of both but they are easily made by hand in the workshop, in fact they have often been given in woodworking guides as good early projects for the learner woodworker. You can make them from plywood for dimensional stability and resistance to warping (use good ply) but both can still be made from solid wood as they used to be. Select good straight-grained stock, with no knots and ideally with rift-sawn or quarter-sawn grain3.

Once you select the wood for the qualities mentioned the species becomes less important, even decent pine could be used. But they'll last longer and remain in better condition if made from a tougher softwood or from hardwood. After completion application of a good finish is advisable to help them remain stable.

Or if you don't care about them being made from wood you could just use lengths of aluminium extrusion. Aluminium is easily cut with any modern saw.

1 But use higher clamp pressure when you're doing the glue-up! You must use high clamp pressure to ensure tight, strong glue joints if using PVA glue of any sort (white or yellow).

2 Which ideally you shouldn't. It best practise to select boards carefully at time of purchase to avoid any with these defects (and others, including too many knots), even if this means having to buy from more than one location or building up stock from repeat visits over a period of time.

3 The wood along one edge of a wider plain-sawn board will often be found to have suitable grain that's close to vertical, saving having to buy rift- or quarter-sawn wood and pay a premium for it.

My tip will have to do with the boards.

I did the same thing about 10 years ago and congratulated myself for making a really thick and sturdy bench top 3.5" thick, by laminating 2x4's from the big box store. It was cheap wood. I could tell it was not very dry, but hey. How far could it go? I glued and screwed the bejeezus out of it.

Within a year it was filled with splits, gaps, and wasn't flat anymore. I don't use it for assembly so this is pretty much OK with me. I'd say, you can make it perfectly flat now, and have the prettiest glue joints ever, but if the stock wants to move, it will.

If this possibility doesn't bother you, use the cheap department store lumber. Otherwise, spend the extra for decent kiln dried material.

• I would suggest purchasing wider boards such as 2x10 or 2x12 and cutting them to the required depth. The larger boards are usually taken from the center of a tree and will often have straighter grain and less knots.
– Ashlar
Jul 23, 2019 at 23:07
• You can always stack the cheap wet 2x4s and allow them to dry further until they reach equilibrium. Put some weight on the top ideally and get them off any concrete surface. Cover them if it’s outdoors. Even 2x material labeled as kiln dried can be plenty wet enough to twist a lot so I do this with all 2x material that I need to be flat and straight. Just need to plan ahead because you can’t use it right away. Jul 24, 2019 at 23:45