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By convention we install a crossgrain strip of wood across the plate join line of a back.

So...we don't do that for the soundboard. Why not?

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Peter Havriluk

Well, I only use one if there's a backstrip. I never inlay my top seams, so...

Because there is more "support" of the joint line with all the top braces vs the back. That's my reasoning.

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The top already has a lot of crossgrain braces that are glued across the center seam; certainly more than what the back gets. Include the bridge and the bridge plate in that accounting. Some people add small patches of crossgrain support below the bridge and between the lower face braces to deal with the largest stretches of the center seam that are otherwise unsupported. I think the idea of not using a continuous strip (as with the back) is because there is inherently a lot of crossgrain support already, and adding more could inhibit the sound.

There are builders who routinely build without a center reinforcement strip on the back plates joint so it could be argued the strip isn't really necessary on the back (except in some cases if there's a backstrip). Also, tops are generally significantly thicker (in the range of something like 25-40% thicker) than backs so the top joint has a larger gluing surface area than the back joint.

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Some do use a small “diamond” or 2 across the seam in the lower bout


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Thanks, folks. Topic clarified!

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Peter Havriluk

That’s one of the best questions I’ve heard I a long time. Thanks for asking it!

My guess is that in practice, unreinforced back seams fail more often than soundboard seams, and the design reflects that fact.

For one thing, soundboards are typically perfectly straight quartersawn softwoods with relatively low humidity expansion rate. Backs are much more varied, so you can have curved grain (changing runout on the joint edge- very difficult to plane), flatsawn, high humidity expansion, and so on. High-curvature growth rings from near the center of the tree are particularly troublesome, since they curl the plate edge when the humidity changes, putting peeling stress on the seam. But IME the reinforcement doesn't prevent the peeling. It only holds the back together despite the gap. So you really shouldn't join that way in the first place.

Straight grained quartersawn back woods most likely don't need reinforcement.

Another factor is that the softer the wood, the larger imperfections you can have in the joint and they'll squeeze out under clamping pressure. And on top of that, it's easier to cut a good joint in the first place.

Hard stuff like rosewood is tough to plane (especially abrasive ones like African blackwood), and the surface won't compress much at all under pressure. If you're using hide glue, then it has to be darn near perfect. So that probably helped establish the tradition of reinforcing the seam, although it's less of a problem with modern glues.

About cross-grain back reinforcements: Just do it. I fail to see what is gained by leaving them out. I made a batch of 3 guitars with 4-piece Brazilian backs. Perfectly vertical straight grain, very old and seasoned. Braced at 40-45% RH. No cross grain reinforcements. Pieces glued together with high gram hot hide glue. Every single seam opened up after a year or so in the field. By not adding the reinforcements during construction I made lots of extra, unpaid work for myself to repair the splits. Part of the repair was adding cross-grain strips. DOH!
So, theory is one thing, actual results are another.

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J. Romanillos use a vertical grain strip to reinforce the back. Before reading his book on CL gtr const. I have also used a spruce, fir or wrc back strip that was vertical grained,it easily conforms to the shape of the transverse and lengthwise arch of the back more readily than cross grain strip IMHO. See what works best for you by testing,

Crossing the grain as is done with plywood is stronger and less likely to split.

My tops (.110") are only 10% thicker than my backs (.100").

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One big variable in all of this is the relative stability of the wood used. Woods vary in the amount that they shrink across the grain with changes in humidity, for one thing. In my experience BRW i s one of the worst, paeticularly when flat cut, and flat cut woods in general will see more shrinkage quartered ones. All woods also undergo 'shrinkage hysteresis'; becoming a little bit narrower every time they go through a moisture cycle. I believe this is related to the progressive breakdown of hemicellulose, which is the part of the wood that takes up moisture from the air. Older wood takes up less moisture because there's less hemicellulose in it. Dense hardwoods seem to take longer to 'season' in this way, so backs will tend to shrink more over time. That shrinkage puts a tension load on the glue line, and you want to avoid that. The cross grain strip makes that into a shear load.

A violin making friend of mine pointed out once that you often see old fiddles with repaired cracks that have been cleated. Sometimes there will be a new crack along the end of the line of cleats: they're too stiff and can't move with the wood they're glued to, causing a stress riser. He's also seen plenty of cleats that have come unglued on one end because they were too stiff to move with the wood they were glued to, and peel up. He said that he has never seen a broken cleat. No matter how thin they are, they're plenty strong enough. Cross grain reinforcements are more a matter of getting a decent glue area than adding mechanical strength, and that also goes for the patch over the open side of the lap joint in the X brace.

A couple of years ago a student of mine restored an English 'guittar' from about 1780. The cross grain strip on the back joint was pretty haphazard. It was also only about 6-8mm wide, iirc, and not much more than 1/2 mm thick. It held, though, and because it was tapered out at the edges there was only a slight crack along one end of the cleat in one spot, which may have been a weak point in the back wood anyway.