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I'm in the middle of building my first acoustic and I've got a question regarding the fingerboard extension over the soundboard.
I'm using a mortise and tenon joint (via the lmi jig) and so far so good.

However since my neck is angling upwards to achieve the desired height at the bridge, I'm getting a small gap underneath the fingerboard extension where it goes over the soundboard (See pictures attached).

Is this gap expected? What are the best steps forward? Glue the extension to the soundboard or shim it?

It should be noted that I radiused the sides and braces at the top to 50', however above the soundhole I left it flat, including the braces...I thought I came across a few videos/articles that recommended this but it's possible I misinterpreted it. Would it have been better to leave the entire top domed at 50'?

I know this guitar isn’t going to be perfect; it’s only my first and this more of an educational process for me.

You can make a wedge to go under the FB extension. If you use the same material as the fretboard people may not even notice. It's also not at all uncommon to pull the end of the FB down, giving some 'fall away' to it that helps avoid high fret buzzes later on. I don't think I'd pull it down that far, though.

That area needs to be planned out as part of a system. There are many different ways to achieve the right geometry.

Mine always have a bit of gap (maybe not that much, what is the measurement?) which I bend down to create fallaway...

Thanks! Its 1.5mm right above the soundhole.

You could make a shim, but that gap is small enough that it's probably ok to just clamp it down and level it out while sanding the radius.

FYI, your fret slots look too shallow (assuming you are going to sand a radius)

EDIT: On second thought, 1.5mm is probably enough to warrant a shim, since leveling it out will lower the neck projection a bit, so then it needs to be angled a bit higher, which makes the gap even bigger.

It looks to me as if you could clamp and glue that down to the soundboard without difficulty, and it would be an appropriate amount of drop-off. That fingerboard still looks thicker than you want it to be - is that your intended final thickness, or still over height. About 6mm is typical for a steel string. And I agree that the fret slots might need to be a bit deeper if you are planning to radius the board.

Thanks! I will deepen the fret slots as well!

Thanks! That's not my final thickness yet, I'm still going to radius it and bring it down some more.

this should have been taken into account before the body was put together.
I sand an angle into the rim to account for this
Assuming you are at the correct bridge height the only thing you can do us put the wedge under the board.
You can push the board down, but I feel it's a too much drop off.

There are several things you could do - sand the top under the fingerboard extension area (depending on the thickness of the top), taper the fret board thickness slightly from end to end, add a small amount of fall away, or lower the finished saddle height (as long as it is within reason for the type of guitar you are building). A little bit of each might avoid the shim, but the shim is easier and not really a big deal.

Could you lay a straight edge along your neck and body. It looks to me that there is some space at the neck joint also. Your fingerboard could be slightly warped showing you an incorrect line. I would work this out with a straight edge first. Then apply the fingerboard.

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

That looks fairly minor. With the fretboard off and measuring with a bare neck, what is the clearance using a straightedge to project the neck plane over the saddle location? If it is in the 0.085"-0.100" range, I'd say you'll be in good shape once the fretboard is glued down.

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I left the fingerboard extension unglued on my first guitar with the same situation as yours. Since I was going to keep it for myself, within about 2 years it curled downward on its own and touched the top. It's 6 years old now and has remained the same. It may be that guitars with cracks in their tops adjacent to and parallel to the fretboard extension may have resulted from some pressure put on the top from this happening while most say that it's caused from string tension pulling back on the neck and was originally counteracted by Martin with the addition of the popcycle brace.

Also to add to what others have said, assuming conventional x bracing and heel block, some of this gap will be negated by string tension once strung up. Some builders choose to build in a little fall off as well. The slight faux pas you have there would be more difficult to contend with if it were near the heel. Think you should be ok.

To me, this is one of the most complicated part of guitar building. Before you glue it up, make sure to ask a LOT of questions. Hopefully your transverse bar was not shaped. Just flat. Its not uncommon to have to pull the extension down for "fall off". U def do not want to have to deal with this after glued. Some folks prefer the fall off. Like I said, this is complicated. Final setup is dependent on this. I've learned the hard way. I strive to have no top curvature in the upper bout. If you are using a bolt on neck, do not glue the tenon. Just the finger board extension.

But wouldn't an arched upper transverse brace have prevented this? Bump the soundboard up to reach the fingerboard.

That's not a bad gap considering it's your first build. You should see the gap on my first guitar. HA!

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-Marcus
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Dennis, this is an oft discussed subject. I'm sure someone else will jump in. I do know that in the final setup, I have WAY less issues with a flat upper bout. I've seen Martin videos here (and I think Medlin is a fan) where they flatten the rim and kerfed lining. I hear what you are saying. I have three plansets for Small Jumbo, Medium Jumbo, and OM. All specify a flat transverse. I missed that in the past. Had endless troubles.

Dennis, I think intuitively I know what the issue is. We all have become accustomed to thinking of the top as the surface a sphere of some large radius. If the top were mostly flat, then a shaped transverse would help. Problem is, the top is rounded. The high point (I think) is somewhere near south of the sound hole, meaning the bridge is lower down on the curvature. That puts a hill between extension and bridge. In the end, I end up with strings too close to frets on extension. Pain in the a$$. This thread does seem a good place to discuss this, but if OP prefers, we can move to a new thread. I REALLY would like to discuss it. In my opinion, the OP does not have a problem. Pull it down and glue. Finger board looks thicker than 0.25" IMHO