MIMF

What are the pros and cons of stacked vs one piece block?
I will probably be using Spanish Cedar and found some neck blanks, but not sure there will be enough material to make a stacked heel block or where to buy a single one piece block. Both guitars I have use a one piece block.

Thanks

"Spanish cedar . . . " So I assume you are making a classical guitar. Stacked heel is the traditional method here but one piece is good if you have the material. The reason stacked heels are traditional is Spain (where guitars developed) has very little in the way of "tonewoods", and consequently uses material very sparingly. There was none to waste. You can make a neck from a board of a table by stacking the heel and scarf joining the headstock.

Stacked heels have glue joints that can fail. It can happen. In the ideal world no instrument is subjected to extremes of heat or humidity but most instruments live in the real world and are occasionally subjected to unfavorable conditions. One piece necks don't have this issue so much.

I like to use a single block added for the heel - in between the stacked heel approach and the one piece neck approach. Fewer glue joints but still saves material. I also use a separate glued on peghead.

Of course, on the other side of the ledger, there are reasons why a one piece heel can fail as well. One advantage of stacked laminates is that you flip alternate layers so that the stress is not concentrated, and flaws don't line up. If you give alternate layers a little twist, so that the grain lines cross at, say, about 5 degrees, that's more insurance against splitting, and doesn't introduce enough stress to be a problem.

Glue lines mostly fail either because they're not made right ('cold' or staved joints with HHG, and so on) or because of moisture exposure. These days I'm using Titebond 3, which is waterproof, for this.

In any event, the main reason to not use a stacked heel is that buyers might reject it. Classical players are used to stacked heels, because of the Spanish influence, but steel string players are decidedly against it. They might find themselves having to make a choice at some point, as stocks of mahogany dry up, and thick pieces that can be sawed into one-piece necks become too expensive.

Don't forget the option to use two pieces of 1" thick mahogany laminated together vertically to eliminate the stacked or attached heel. Advantages are 1) looks better, 2) can add decorative center laminates, 3) less potential for separated joints.

Other arguments to add to Barry's:

His suggestion allows using flatsawn wood which, now rotated 90 degrees has become quartersawn relative to the loads and stresses the neck bears.
If slip matched from one board, if you then rotate one of the boards 180 degrees along it's long axis before glue up, twist and movement in the two sides of the subsequent neck will tend to cancel each other out.

I don't know anything about the structural or tonal differences but from an aesthetic point of view, I have seen some stacked heel blocks done in a different wood from the neck and the effect is really nice. Again, I can't speak to the structure of this choice, but the guitars with this done sure were nice to look at!

I've done it with stacked lams and I've done it Barry Daniels' way. These days, I do it Barry's way. It's not that I feel it's a superior joint. Not that at all. I just like the look.