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I am working on a OM that I have had the box closed up for a couple of months, sitka sprice top, 28 ft radius , the UTB is flat and not radiused, I used a simpson jig to cut the neck tenon and discovered the top is flat, when I take my 28ft template to it it is flat, the top was built in a radius dish, so when I set up the jig there was no angle offset for the neck so I routed it, when I lay a straight edge across the fretboard it comes out a little high over the bridge, about a 1/4 inch, the bridge I am using may be slightly thin at 1/4 inch thick, my question is that since the braces are radiused , humidity in north texas is not a problem, 61% today, should I reduce the neck angle so the straight edge is flush on top of the bridge , or should I ecpect the top to radius up when under string tension??? hope I am making sense...

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MK5acoustics.com

I'm not sure I want to jump into this one, but if I understand you correctly... The top you built with a 28' radius is now flat. If that's the case, I would want to know what happened to cause it to flatten out.
Many thousands of guitars were (and many still are) built with flat soundboards. If you figure the top is stable, adjust your heel angle for correct alignment to your bridge & carry on.
The only time I had a top flatten out (the back did the same thing) was when I built the box at very high humidity levels.
That guitar inspired me to build a small storage & assembly room in my shop & keep it at 42 - 48% RH at all times.
If you built under controlled humidity conditions, I don't know what could have caused the problem... Was your brace stock fully dried?

This sounds like a humidity control problem. Before you continue, you must understand humidity, how it affects wood, and how to control it in your shop.
You might need to take the top off and replace it with one built with proper humidity control.
Tell us about your humidity control in your shop.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

So is the entire top flat, both above and below the soundhole? Or is the straight UTB holding the section
above the soundhole flat- which is what you want, BTW. If the entire top is flat, you probably do have an
RH problem. Have you been having extremely low RH the last few weeks? In California, when that very cold
Arctic air was with us a couple of weeks ago, we had very low RH. In general, low RH will flatten out the
top, high RH will cause it to dome more.

If it's just the upper bout that's flat, I'd make my bridge 3/8" instead of 1/4", and sand the upper bout a bit
so a straightedge extending off thethe fretboard will end up kissing to 1/16" above the bridge.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

I build inside my house, so humidity is influenced by HVAC, the room adjacent to my garage where all my power tools are kept,, I checked the last two builds and they seem to be fine, here are some pictures...probably hard to see

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MK5acoustics.com

would carbon fiber laminated in the x braces keep this from happenig??? all the wood used in this build was stored in the same room I build in, for several months before buiding..

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MK5acoustics.com

I'm sure numerous things can cause a braced top to flatten out. I'm thinking that if the braces are not tall enough to have the necessary stiffness to not flex under the tension of the top itself and thereby negating the radius shaped on the bottom of the brace, then it could flatten out. When I glue the top - and the back for that matter - I hold the plate in a vacuum fixture that is dished at the proper radius and then glue the rim to the plate. That helps maintain the dome of the top and the back.

Good luck,
Chuck

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"To live a creative life, we must lose our fear of being wrong"
- Joseph Chilton Pearce