Official Luthiers Forum!

My first dumb question today: The conventional wisdom is to reinforce the plate join seam on guitar backs with a strip of crossgrain wood. We don't do that for soundboards.

Why not?

Thanks!

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Peter Havriluk

I think the strip on the back comes more into play if there’s a decorative back strip either inlaid into the center seam, or inserted between the two back halves.

If there’s no center strip, the back reinforcement simply isn’t necessary IMO. I’ve done hundreds of guitars without one without issue over decades of work without issue, and the old shop I used to be at did tens and tens of thousands. You rarely see a glue joint open up…

Wisdom ^

I’ll add that some folks do put small cleats on the top between the sound hole and the tone bars, between the tone bars, and below the tone bars (or some combination) on an X braced top. I don’t do that but I’m a free spirit, and lazy.


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Brad's far from lazy, but he's right that it is a common thing to put small crossgrain reinforcements on the top center seam. The Cumpiano/Natelson book certainly calls for them, and Robbie O'Brien's building course includes them.

In the 1920's and before, Martin would put a diamond cleat on the seam just below the bridge. That is the most stressed part of the joint.
Failure of the seam joint is often a result of overheating the guitar.
I have always felt that softwoods like spruce glue better than dense, oily woods like rosewood. The worst case is when both parts are a dense wood. That can be addressed by using an intermediate strip of a softer wood like mahogany, or by using a marquetry strip between the halves.

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John

Not everyone uses a BJR (back joint reinforcement) with Larravee being one example. We've serviced hundreds of Larravees and never for the back joint coming open.

I'll add as well that as John says Martin would use diamonds below the bridge but there were two things then that are different from now. First the tops back then tended to be thinner and some of the late 1800's Martins could have tops as thin as Somogyi is known to sometimes go. So less surface area to glue. Next back then we did not have some of the decent humidification systems for the guitar, case and our homes and facilities that we do now. We also did not have inexpensive and widely available hygrometers (even though the digital ones suck...) back then either. And there was no Internet for us to learn that buying the acoustic guitar also should be along with an RH solution.

I don't see any harm in it but I'm very much in the minimalistic camp when it comes to getting a stick of wood to vibrate.

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Ann Arbor Guitars

I expect it's probably not necessary but having come up with Martins around everywhere a guitar with no back reinforcement would just look weird to me so I'll keep using them

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Steve Smith
"Music is what feelings sound like"

That is like buying insurance, so you hopefully never need it, but helps when it is needed.