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I am about to build a Jumbo with a true flat top, meaning no top radius, everything flat like nylon stringed guitars. Is there anything I need to watch for when doing this? Do braces have to be stronger when building the top completely flat?

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Nylon strung Guitars aren't flat or rather some of them are, some of them are not. Modern Classical Guitars are virtually all domed. Just saying.

If you do end up with a perfectly flat top, it won't stay that way long. Humidity changes and string tension will cause movement. On a top that is slightly domed, an increase in humidity will increase the dome. On a top that is flat, an increase in humidity can cause it to go either way, up or down. A decrease in humidity has a better chance of causing a crack since there is no dome to give way to the shrinkage.

I've never built a Jumbo before but for my small body design I started building true flat and I didn't change the bracing at all except that it's not arched. In fact I've been building even lighter. Don't worry your true flat top will be domed before too long. Others with experience may chime in however.

I built a jumbo with a flat top. However, I built a 1/8" ramp in the top, per suggestion from John Greven. So while flat, the top arcs from the waist to the neck block down 1/8". this creates an approx 3 degree neck angle.

Seems to be working well. the guitar us a year old, and my main playing guitar these days.

Glenn

I used to use a 25 foot dish on the top but found it caused a huge gap between the fingerboard and the soundboard. I thought by making it flat it would eliminate the problem. I can eliminate it by creating a neck angle but it would require a very tall saddle.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

That gap you're describing is called falloff, and it's caused by the mismatch between the neck angle and the top. There's various ways to eliminate it, but simply making the top flat won't.

You can sand a facet into the sides at the neck joint, and literally bend the top, creating the 1.5 degree neck angle with no gap, which would likely work well with a truly flat top which is what Glenn described, or you can match the dome geometry to the neck angle.

If the problem is the falloff gap, there's ways around it which don't involve eliminating the dome.

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No, that will actually make the problem worse. You need to work out the geometry of the way the fretboard joins with the top, just dropping a flat fretboad onto a domed top won't fit, something needs to be adjusted. If you make the top flat and drop a flat fretboard on that, the string height at the bridge will be the fretboard (0.25") plus the frets (0.05") plus the action (0.10"), or 0.40" without any top deflection, which will reduce that amount. Putting in enough neck angle to get the string height to 0.50" make an even bigger gap. You can put a wedge under the fretboard, or sand the angle into the top (if it's thick enough) and lower the fretboard.

Todd Stock has posted his method several times, in involes reducing the radius of the UTB to 60' and is pretty simple.

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If you already have the top glued on... Why not make an insert to fit under the fretboard to get your geometry right?

I did that on a small body guitar - and it came out pretty nice... The key to doing this is to make it large enough that it looks like it's meant to be there, rather than looking like you were trying to fix a mistake....

Thanks

Just be careful not to radius any of the braces

Honestly, there are hundreds of thousands of guitars built with the domed top with no gap, just learn how to build it correctly.

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I would not start off building a Jumbo with a true flat top.... The Flat flat top tends to produce a top that runs more flexible than a domed/curved top with similar dimensions... and Jumbos already tend to be woofy/bassy unless you are real careful....

I would start off your Flat flat top efforts with a smaller body guitar - so you can gage the differences on a model that tends to need a more flexible top producing a bit more bassy sound... Then - work your way up to larger and larger body sizes....

I personally found that I needed to be more attentive how you deal with bracing between the center of the X-brace and the headblock - as Flat tops really do want to cup/potato-chip more than a similarly constructed curved top guitar...

Neck geometry wise... Not really an issue... I have built a couple true Flat tops... never had an issue getting everything to line up for string height and bridge dimensions...

Thanks