Roubo Split Top Gap Dimensions and Wood Movement

[Brent VanFossen]

I'm at the point in my split top Roubo bench build per the Benchcrafted plans that I need to cut and fit the short rails of the base. These are the cross pieces that will support the two halves of the top, which are mortised for the leg tenons. Since these substantial top pieces are fixed at the front and rear of the bench by the leg tenons, any wood movement will cause the gap in the middle of the top to change width.

My top is laminated from Douglas Fir, flat sawn boards, with rings oriented more or less vertically. Per the Benchcrafted plans, the gap should be 1 5/8, while the gap filler is 1 9/16. That only leaves 1/16th of an inch for wood movement.

Is this really sufficient? I live in Oregon where the summers are dry and the winters humid.

Brent

[Joe A Faulkner]

my guess is this will be too tight. But why not build it to this spec. Keep an eye on it over the winter months and if the filler get's too tight take it down a 1/32 or so as needed.

[Cliff Polubinsky]

You might want to consider widening the gap a bit. I believe the gap in the plans was designed for the original Bessey heads. The newer head's are a bit wider.

I made my gap 2" just to be safe.

Cliff

[Brent VanFossen]

You might want to consider widening the gap a bit. I believe the gap in the plans was designed for the original Bessey heads. The newer head's are a bit wider.

I made my gap 2" just to be safe.

Thanks for that, Cliff. I was shooting for 2", but on my first glue-up, I ended up with some bow in my filler, and I had to cut it apart and glue it again. I'm at 1 7/8 for the filler now, and the gap will be a bit above that. I just don't know how much, yet. Right now, the perfect gap would be 1 15/16.

And Joe, thanks, but because my Jet clamps are 1 13/16", I can't really afford to lose much more and still use my clamps through the gap. I was hoping to find someone with experience with how much the gap will change through the seasons.

[Kesh Ikuma]

Timely topic. I just finished my split-top Roubo at the end of the spring, was still dry in Louisiana humidity scale. I didn't follow any plan and made my gap 1-1/2" wide (a mistake, the filler's slit is too skinny for most hand tools). Anyway, I made the filler to fit fairly tight (well under 1/16") well, because I didn't think about the bench top wood movement that you described at the time of the built... Then, the humidity hit hard (along with the flood) in my area late summer. This past weekend, I had to hammer the filler from the bottom to raise it. lol

I'm with Joe on this. Go with the 1/16" as the plan says, and keep your eyes on it during the winter time and shave off more as needed.

If my bench (2' deep, SYP, short stretchers are pegged, and minimal initial gap) could withstand the outrageous humidity swing of Louisiana (I have no idea how much of a swing you experience in OR; hard to imagine it's worse than LA. Still a struggle for me after 9 years here), I suspect it will not break your bench during the first winter if it gets too tight.