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I have some red cedar tops,and I understand that they should have a finshed thickness greater than red spruce when building a steel string guitar.Is there a guideline to use on how much thicker?

As you will discover there are many factors involved in thicknessing a top. In general, my cedar tops are around 10% thicker than spruce tops.

This is dependent on the stiffness of the specific top your dealing with, the body shape and size of the guitar, and bracing specifics. That said a good stating thickness at brace glue up is .125"-.130

and it depends on how you brace it .... there are too many variables to just say it should be 120 or whatever ... Larrivee tops are around 108-110, and he uses a 8x16mm X brace at 90 degrees ... if the red is stiffer than average sitka, you could go less .. or brace smaller .. or possibly both .. depends on body size, string gauge ....

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Red Spruce, which you specify in the thread title, and Red Cedar, which you mention in the thread, are generally at opposite ends of the stiffness range...

Peace,
Sanaka

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...imagine there were no hypothetical situations...

That has me condused as well Sanaka. Isn't "red cedar" what come folks call eastern cedar?

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Formerly known as Adaboy.......

There are no rules of thumb worth adhering too. I have used (single thickness) Adi tops in the .060" range all the way to .100". It all depends on the long grain / cross grain stiffness, density, bracing system and more importantly the size of the box and the player's attack. Read up on deflection testing.

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tim...
http://www.mcknightguitars.com

I know it as western red cedar !!!

My bad on the topic vs what was in the post ... again though it still depends . i ahve cedar thats as stiff as sitka, some thats not ... and the other way round as well .. it all depends on the piece .. thickness accordingly !!!

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Generally 15 to 20% thicker than I would do red spruce, not only because it is never as stiff (IME…), but also because it sands much quicker than red spruce when doing the finish prep.
As an example my red spruce tops for a 000 are usually between .105" and .115, WRC would be at least .120" up to .135".
Of course it varies with builders, depending on the bracing, style etc.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

What Tony said.

It turns out that the Young's modulus (E) along the grain of all softwoods varies with density in the same way. That is, any two pieces of _any_ softwoods that have the same density will probably have the same stiffness in bending along the grain at a given thickness. If you know the density, you can assume the stiffness, and be within about 10% most of the time.

I have a WRC top that is an exact match in properties for a Red spruce top in my stash, except the WRC has lower damping. The density, and stiffness along and across the grain, are the same within the limits of my ability to measure them. That should make an interesting pair of guitars some time.

If you can't measure the density of the wood, I'd make a cedar top about 10% thicker than I'd make a spruce one. Remember that stiffness goes as the cube of thickness: that extra 10% thickness adds up to 30% more stiffness.

Cool! Measuring density is something even I can do! (I've not gotten around to building a deflection testing jig, yet.) Weigh the top, measure length, height and width, and do the math for g/cubic whatever.

So Alan - does this mean that one could map out a curve of optimum thickness for tops based on the density of the top, regardless of the species of top, and be, at most, 10% off the theoretical optimum?

Hmmm. But 10% off - so would that mean that I could conclude that the right thickness should be .100", but in fact the optimal thickness for that top is .110"? (all other factors being equal, of course)