Neck block support and truss rod access

[Richard Warr]

My builds so far have been a single neck block with truss rod access
through the soundhole. But I've seen quite a few makers add
reinforcing blocks on top of the neck block that sit directly under
the fretboard extension. I see the benefit of this - a more rigid
neck joint and less need for a future neck reset.
What are your thoughts on this approach? And - how does it affect
truss rod access? I've heard that it then makes sense to access the
truss rod via the headstock.

[JohnParchem]

I do what you are describing, I treat the extension as part of the neck and route a slot. That way the truss rod wrench is able to reach the truss rod. I have never had a headstock adjustment. 

[johnc.marple]

I think I know what you mean, wouldn't you need an extra long Truss Rod ?

[Charles Tauber]

I use an inverted "L" shaped neck block with the "leg" of the "L" against the underside of the guitar top. The "leg" of the block runs from the inside of the side to the transverse brace. The end of the block - the extended "leg" of the block -  is used as a placement "stop" for positioning the top: the top is positioned by abutting the transverse brace to the end of the block. I use a 3/4" tall transverse brace and make the thickness of the "leg" of the block also 3/4". Making the block the same height as the brace eliminates the need for clamping cauls when gluing the fingerboard to the top: the clamp doesn't need to have clearance over a taller brace to contact a thinner block.  

I continue the truss rod slot in the neck through the top and block and install a truss rod that is adjustable through a hole in the transverse brace. The truss rod runs from about the first fret to the transverse brace. It does require a longer truss rod than is typically used for adjustment at the head. I used to make my own single-acting, double-rod truss rods and could make whatever length I wanted. Since switching to double-acting rods, I no longer make my own but have found that they can be ordered in suitable sizes, either custom or standard lengths.

Charles

[Charles Tauber]

I should have mentioned that I do NOT find using that arrangement makes any identifiable  difference on the need for neck resets. Making the block a little wider than the fingerboard probably does eliminate cracks in the top along the side of the fingerboard often a result of humidity issues. 

Having that support under the fingerboard can also make hammering frets easier in the area of the fingerboard cantilevered over the guitar top. I also find, subjectively, that having the added support under all of the higher frets tends to improve the sound quality of those notes, a little fuller, a little more even and a little more sustain, particularly with nylon string guitars. 

The arrangement I've described in quite different than the "popsicle" brace used in the upper bout by some manufacturers.