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I see that some builders choose to not use a center strip re enforcement on the back. For example I’m pretty sure Larrivee doesn’t use one. What is the general consensus? I think I would feel uneasy about leaving the seam not backed up by a strip.

Do you feel lucky?

I always use one. It might hold things together with a less than perfect glue job. If a Factory that produces thousands of high quality guitars still does it, there might be a good reason for it.

I had some factory second backs from Martin. They had back strips glued on....they were laminated backs, no center seam....go figure?!?

I suspect it's more psychological than anything. If the seam fails despite the reinforcement, the wood is blamed. If it fails and there's no reinforcement, the builder is blamed But IME if a seam is going to fail, the reinforcement probably won't stop it. Although it does hold the back together so the failed seam doesn't actually cause any problems aside from being ugly.

I've repaired a couple of instruments where the back seam had opened up and there was no seam reinforcement. (Now there is!)

the reinforcement is important. I have seen to many none reinforced backs open. Martin doesn't use one on some models that have no back strip purfling. I still find it is cheap insurance. Use one

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Traidtion dictates transverse grain to the long grain of the back . In the past backs were glued with HHG, If they seperated, the center seam allegedly slowed down the process. . I prefer a long grain to long grain center seam, with the back plates glued with titebond and the center seam with fish glue or hide.

Wouldn't the long grain reinforcement strip just split along the grain line if the seam separated? The reason for the tranverse grain across the long grain center seam is for resistance to separation of the seam. What am I missing here?

On some of my early guitars I've had twice that I can remember back seams open up a bit. It's not something you ever want to see. I now use Oak and Teak epoxy to join backs AND a center seam usually with Titebond but sometimes HHG. Aint gonna get no back separations any more!

We use back center joint reinforcement on everything we build, but if the joint is a straight long-grain to long-grain joint, it should be as strong and just a bit stiffer than solid wood.

Where the graft is desirable from a structural point of view is where a piece of marquetry trim is used, which weakens the un-reinforced joint, reduces across-grain stiffness and strength, and for trim which is routed in to the back to half depth or so, creates a stress riser at the edge of the trim. The graft restores cross-grain stiffness and strength, and mitigates some of the stress concentration under load at the edge of the inlaid strip. A graft with grain aligned with the back's grain will also help with the issues mentioned, but will not be as efficient as the cross-grain reinforcement for restoring cross-grain stiffness and strength, but will add some stiffness along the grain direction.

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James , Never had a long grain to long grain seam open up. I like the fact that it bends easily in the vertical and transverse directions when it is glued in . As woodie mentioned above ,it all depends on the wood your using, the glue ? and your preferred method. I do use cross grained strips of spruce, but am not as fond of it . It seems ok on small instruments where there is not much vertical or transverse deflection. After trying many glues I prefer fish for the center seam. If i screw it up I can reheat and reclamp . It has great initial tack unlike titebond, and it has a 1/2 hr window before it hardens, So I can double check my alignment joints and see if there is solid contact on the reinforcement seam. For long grain reinforcement strips I like 1/4 sawn sycamore, it is pretty with ray flecks, can be shellacked, bends easily, readily available.

Grandma always said . "Its better to Have and NOT need , Than to need and NOT have " I use them , easy application for the potential of an issue .

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I usually get the cross grain strips from the offcuts of the soundboard. Most soundboard material seems to be an inch or two longer than the soundboard. I glue it with epoxy because it doesn't make the strips curl up like water based glues can.