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Please, I need help:
I glued on the fingerboard on my 1st build (flamenco guitar) and after removing clamps I noticed it's not flat
The part between 1st and 12th fret is flat and the part between 12th and 20th fret is flat, but the 1-12 part has a slight back angle (picture 1).
The gap between the ruler and the 20th fret is about 1mm.



I thought about flattening the fb - it's about 6mm thick now, the plan says it should be 4.5-6mm thick, I should be able to flatten the whole board
(removing most at 12th fret and less and less toward both ends).

But: The soundboard has a built-in curve in it. It's flat from heel block to sound hole and then it goes up toward the end block.
If I made a line parallel with the 1-12th fret part of the fretboard, at the place where the bridge should be,
the line would be only 3mm above soundboard(picture 2):



The strings should be about 6-7mm above soundboard at the bridge, according to my plan. If I want <3mm string clearance at 12th fret, now it would be about right, yes?
But if I'd flattened the whole fingerboard I'd effectively decrease the back angle thus decrease the clearance between strings and the soundboard.
As I cannot make the bridge less then 6mm high I'd have to increase the clearance at 12th fret, is it right?
So flattening the fb would in this case result in higher action at 1-12th part of the fretboard? Not flattening the fb would only result in higher action at 12-20 frets where the guitar would be seldom played.
Am I thinking right? Or should I include neck bow under string tension into this?
English is not my primary language so it's possible I mixed it up a little - hopefully the pictures are making it clearer.
Thanks for any help!

If you have 3mm at the bridge on a bare fretboard, you will have 4mm when the frets are installed. Add 2 times the average 12th fret action 2.75x2=5.5mm and you are up at 9.5mm
I aim for 8mm, which allows you to come down a bit if neck or top move.

I would probably take about 0.5mm off the 12th fret, leaving it full thickness at the nut.
A good way to check is to make up two shims
One at the nut 1.25mm thick representing frets plus 0.25mm clearance
one for the 12th fret 3.75mm thick (frets plus 2.75 action)
Then you can use your straightedge to directly measure string height at the bridge

theres a lot going on there, at least to my eye. ive never built a classical and only two steel strings, so take this (as hesh would say) for what you paid for it.

the soundboard has a strange shape to it, at least from the photos. it almost looks like it is concave. not sure how lightly it was built or if there are possibly any humidity issues to deal with. is my view of the shape of the soundboard correct?

the neck angle is quite flat. it seems 3mm above the soundboard is way too low. difficult to adjust the angle on a classical. you might have to plane/sand a taper into the fretboard to get the neck angle up to an acceptable level.

its hard for me to say how all of that will change the action at different frets when its all said and done, but youve got to angle that neck/fretboard up somehow.

hope that helps some.

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sweat the small stuff.

Quite a different beast from a steel string Heath.
The neck could actually do with being a little more forward. but it will be fine with a little work on the fretboard.
The soundboard often looks like that with a flat or even concave upper bout and a domed lower bout. That is quite nornal and intentional.
You are actually very close Kominac, just a little off at the 12th will make it perfect.

im perfectly happy to concede if im wrong, but id say hes a fair ways away from being close. hes 3mm above the soundboard, not the bridge.

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sweat the small stuff.

If his goal is 8 mm he's only off by 1.5 mm at the bridge with the present configuration. Tapering the fingerboard will give you the space you need, if you are comfortable doing that.

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Waddy

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Heath, the rules of thumb for a Steel string do not apply here
Desired 12th fret action is nearly twice that on a steel string
Desirable Bridge saddle height is about 4 mm lower on a Flamenco than on a Steel string.
Plus he has no frets installed.

yeah, i suppose the bridge is a hair lower on a nylon string.

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sweat the small stuff.

Thanks guys! Jeff, that's a great tip with shims - I'll definitely try that. I'm not very good at imagining things in 3D...
Heath, the concave curve of the soundboard is intentional - there's a lot of tapping on the soundboard when playing flamenco and this is supposed to make it easier (brings strings closer to the top near the bridge).

i realize i definitely showed my lack of knowledge on the subject, so i apologize for that.

it didnt seem like there was much response, so i thought id give it a crack.

i learned some stuff in the process, so thanks!

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sweat the small stuff.

Hey Heath,
I only built my first flamenco a year ago, and had to ask about neck geometry.
Flamenco is different from classical as well as from Steel String.
I found a lot of contradictory advice and different approaches along with realizing that the flamenco guys are very fussy about string height at the bridge to allow easy tapping on the plate.
It is one thing which is very easy to stuff up and hard to repair, so it is best to understand your goals and how the relationships between neck angle,action, fretboard thickness and bridge height work.

Big thanks to Jeff - it seems I managed to get it just right!
Of course it was also thanks to a happy accident that neck had some back angle in the first place
How fine do I need to sand the fb before fretting? I leveled it with 80 grit sandpaper.