Official Luthiers Forum!

I am building a Martin 00 in maple with a redwood top. I have never used redwood before so I have some questions.

What thickness range am I looking for - thicker or thinner than spruce? than cedar?

Is bracing for redwood generally wider at the base? Narrower? Taller? Shorter?

What is a good material for redwood bracing? Spruce?

I got a 65 year old (at least) piece of the 1954 living room wall of a friend and resawed it this afternoon. The grain is absolutely vertical all the way across this 8-1/2 piece, and is 35-45 grain lines per inch

Redwood has the most variation from piece to piece out of all the woods I’ve ever worked with. I’ve sent far more back than I’ve ever accepted. I generally don’t bother with it anymore.

That being said, due to its variance it is imo especially important to be able to use data to ascertain the useability of any particular piece. Unlike spruce where you could safely say .110 is probably a safe target, redwood is so all over the map that I wouldn’t even tender a suggestion for thickness. I’ve had pieces come in at .150 to meet my initial target deflection, which to me is insane.

This is where deflection testing and general math based techniques really come in handy.

I hope the stuff you have works out for you, and sorry I can’t be directly helpful...

Redwood is an interesting material. As Ed Bond mentioned there is a lot of variation from board to board. According to the wood database "old growth" redwood can be as heavy, as hard, and as strong as red spruce. The biggest problem I noticed with redwood is it's tendency to split along the grain. A relatively tough wood like Sitka might be a good choice for bracing.
Checking it's stiffness across the grain by flexing it often leads to disaster, so resist the urge.

Thickness - as meddlingfool said, very variable.
I measure stiffness, as in young's modulus along the grain by deflection testing.
Failing stiffness measurement, density seems to be a good indicator for all top materials (+/~10%) as Alan Carruth has often mentioned, so you would use the same thickness as for a top of the same density spruce.
I prefer spruce for redwood top bracing, and brace much on the same basis as for spruce,
There are tables out there on the net, based on Alan's observations, of specific gravity vs stiffness vs top thickness vs measured stiffness.

Here's a quote from Alan with some specifics

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I have used redwood in several guitars, and treat it like spruce. That is to say I get it near .125 and start flexing it long grain. I pass it through the sander until it feels “right”. I guess that is the caveman’s version of deflection testing

Bracing I have always used spruce, using the same pattern as always. I shave down the braces by flexing and tapping, just like I do a spruce or cedar top.

Do I end up at different thicknesses and bracing than spruce? I guess, but no two spruce tops are the same either.


Sent from my iPhone using Tapatalk Pro

P.M. sent.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Redwood is great. Some of the finest sounding guitars I have ever built or played are redwood.

Brace with spruce. Hard to answer generalized questions about top thickness since bracing is a factor, build style, your level of experience voicing are all practical considerations. I have found redwood to be reasonably consistent in vertical grain. If unsure I'd suggest bracing with good spruce stock, just as you would with a spruce top guitar. Run top at spruce thickness - it's a 00 and, secondly, most people tend to overbuild tops.

Redwood can be brittle. Take the usual precautions you might take with any top. Make sure your tools are sharp and don't even think about any approaching a climb cut .

Seal all short grain with dewaxed shellac. Do not install with a rout down bridge patch - you will be better off taping the patch and not exposing any end grain for the bridge glue up.

You'll enjoy the finished guitar. The wood looks nice!

I love stuff like that. My brother in law who does fine carpentry and restoration work was doing a job that required a pallet of redwood about 15 or so years ago and picked up one piece that he thought I'd be interested in. I built 3 guitars from it. I didn't treat redwood any differently then spruce, braced it the same with spruce and so on.

agree that you need to do some testing. I start at about .135 in go from there. This is a cracky wood so be careful. I use a slightly bigger maple plater, .110 thick but about 3/16 wider

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I've never seen an issue with the plate, John, but I'd say if a builder's bridge plate is on the narrow side, this is definitely wise advise. I would also recommend chamfering the edge of the bridge plate after glue-up (so as to get a good clamp up at the edge) - and chamfer the edge to about zero. No sense in creating a stress riser.

Good advice so far.

As has been said, redwood seems to be more variable than most other softwoods, so measurement is called for rather than going by feel or by species. Density is generally a useful indicator of long grain stiffness, but I'm not as sure about redwood as I'd like to be. Over the past few years I've been 'salvaged' redwood, either from stumps or structures (particularly railroad stuff) that has had much lower long grain stiffness than the density would suggest. In one case the wood has also had very high damping: the tap tone was like cardboard. Since I particularly like redwood for it's low damping I sent that back. I'll note the famed 'LS' redwood was salvaged from a stump by Craig Carter, and has wonderful properties: the density and stiffness are like good Red or Sitka spruce, and the damping is as low as Brazilian rosewood, so not all 'stump' wood is bad. I strongly suspect that the bad stuff suffers from hidden structural issues caused by the stress of the weight of the tree, but it's hard to say since you can't know what the tree was like or where in the stump the wood came from. Again, it only reinforces the utility of doing some form of testing that gives you numbers to work with.

Redwood, like Western Red Cedar and Douglas fir, tends to be less tough than spruce; they are prone to splitting along the grain. The low peel strength poses a problem in glue lines, particularly between the bridge and the top. Make absolutely sure that there's no run out in the top in the spot where you glue the bridge. The bridge will generally start to peel on the side where the grain runs 'up' toward the front of the bridge, and when the opening reaches the other side the wood will split down into the top. A bridge with a larger footprint, particularly one that's wider from front to back, will help a lot in keeping it from peeling up, as the maximum stress at the back edge will be lower. I often use walnut for bridge on redwood and WRC tops, which allows me to make them bigger without getting too heavy. If, as many of us do, you finish the top and then scribe around the bridge to mark where the finish has to be removed, by very careful to only scribe the finish: don't cut into the wood, particularly along the back edge. We had a Lowden with a WRC top in the shop that lost it's bridge for that reason: the glue line was fine, with wood over the whole bridge surface, but the wood peeled up starting at the back edge.

Similarly, I would not try to make the bracing too narrow to give it some extra glue surface. If you're worried about weight, use some lower density spruce for the bracing, and leave it slightly taller.

That looks exactly like some planks that I got about 18 years ago. They were gym seats that were over 20 years old when they were replaced with new bleachers. The boards are 9" wide and a full 1" thick, with super close grain lines. I have had some re-sawn for tops and I have used some for bracing. I have always assumed it was Douglas Fir, but now I wonder if it could be redwood. I will post pictures next week when I get home. What I do know is that it makes a wonderful sounding guitar top.

Regarding bridge lifting, another useful trick is to make the bridge plate extend past the back edge of the bridge. Greatly reduces stress concentration on the glue line. Obviously a real soundboard won't bend so sharply, but hopefully this illustrates the idea.

Extending in front is not really necessary, except on curly redwood where a sharp bend at the front edge of the bridge can cause cross grain splits (actually long grain splits in what is essentially endgrain, due to the high runout of curl). But I usually do it anyway because it's only another gram or two of wood, and provides a nice platform for gluing pickups.

Denis - thanks for the drawing. It helps to visualize what is going on in there.

Everyone- thanks much for time. I have some nice SItka bracewood I got from a friend who built a cello - 5/16 X 6 X 60". Plenty for several instruments.

Ed

Even better is to extend the plate fore and aft of the bridge, and then taper out the front and back edges as thin as you can make them. You don't really need the full thickness except where the ball ends of the strings are, and thinning the edges out takes the stress riser down to almost nothing.