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]I started my current guitar in February but have not had enough spare time to complete it in the time I expected. I now have all of the peices ready for glueing but I noticed a few weeks ago that the spruce soundboard has a curve in it that wasn't there a month or two ago (all of the braces and struts are in place). I'm not sure if this happened due to having less humitity in the air now that it's summer or from the wood and braces simply having settled. My shop is in the basement in a cool, closed environment.

The curve isn't bad, in fact it looks like a typical curve found on back pieces and it's even from top to bottom (so not a warping/uneven curve). I wasn't counting on having a curve on the front (even though some guitars have them) however and I'm not sure how to attach the neck/finger board on a curve so I'd rather have the soundboard flat.

Has anyone come across this before? Should I try and rehydrate the wood in case it dried out, I was thinking maybe running a hot shower and letting the top sit on the bathroom counter for a few hours in the hope it would straighten long enough to glue. Or should I leave it and take this as a lesson learned in dragging my heels in guitar construction?

If the face of the soundboard is "convex" then the humidity is a bit higher than when you braced it. Indoor R.H. is usually a little higher in the summer, so this what one would expect. Returning it to a lower humidity (dry box?) so it will "flatten out" might be a good idea before gluing it to the sides.

First off, it is almost certainly humidity related.
Second, I would think that your humidity would be HIGHER now that it is summer. That's the way it works most places (at least places that experience winter).

I definitely wouldn't suggest upping the humidity. In general you want to brace and close the box when the shop has had a steady RH at or slightly below the average RH of where ever the instrument is going to live. For most folks that means keeping the shop at around 40-45%. Having your shop too dry, though not necessarily ideal, is ok but too humid will definitely cause you problems.

If you brace or close the box when it's too dry then the wood will expand across the grain when subjected to normal humidity, causing the top to rise a bit. This may cause some issues with your setup, etc. but not really a major structural concern.
If you brace/close the box while it's too humid then, when the instrument is subjected to very low humidity (usually in the winter) it will shrink across the grain. In shrinking, though, it has nowhere to go so it cracks.

If i were you I'd find a hygrometer and figure out the humidity in your shop. Again, it's probably high (basements are notorious for this). If it is high then I'd figure out some way to lower it (dehumidifier, etc) or relocate your setup to a place with lower RH.
Let your top acclimate to the new humidity before trying to close the box-it'll probably go back to it's original shape (more or less) so long as it hasn't been cupped too long.

Yes it's convex. So it definitely seems like a jump in humidity then and not a lack thereof.

Ok I'll try bringing it upstairs to see if that makes any difference over the next couple of weeks. Hopefully it works out. If not then I'll have to come up with something else.

Thanks for the advice/info!

If you don't have one, you MUST have a dehumidifier in your basement shop and ideally keep it running full time at about 40-45% humidity. If you arent able to run it full time you should for sure at least bring the humidity down to 40-45% when gluing up. If you glue up in a humid environment your glue joints will fail.

I keep a large dehumidifier running all the time in my basement shop. It does a great job at keeping the humidity down and it gets very humid in TN in the summers. Sometimes I turn the dehumidifier off if I want some quite time for a few hours while carving braces or something. Sometimes I forget to turn it back on when I leave the shop and the next day the humidity in my basement can get up 75% or higher! Extended periods of high humidity will cause your wood to swell, and most certainly cause bad glue joints.

Chris have had the same problem here in KC.My soloution was to remove the braces resand and then stack the top with a weighted box in a drier environment 40-45% for 2 months ,an wait for a dryer day to reglue the braces, the lower ones into a 25 ft arch.Since I am from MTL, my guess would be to wait til the cooler drier month of sept , if you lack a dehumidifier.June july aug humidity in mtl/ottawa runs abt 70-80% so your taking a big risk if you want to close your box ,or whatever? I spoke to another luthier who recommended a 5 ft arch on his backs to counter the effects of high humidity in his location, sw mo.He says he has never had a sunken back since swithching over,I haven/t tried it ,but kevin ryan I believe ? is an exponent of this method of countering high humidity locations.Especially putting in the braces immediately after thicknessing the back, so that it does not have a chance to move.

When a top goes concave on me it is always because the humidity was higher when I glued the braces on than it was when it went concave. The differnces in humidity are usually at least 15% Once the humidity has returned to the value of when you glued them on the top should return to normal.

If the humidity it takes to bring the top back into shape is above the 'normal' the guitar will live in, then you may have a problem. If that is the case, removing and regluing the braces during lower humidity may be the answer.

Once a finish is on the guitar it will react slower to humidity changes. If you seal the brace side of the top it will react even less. Some builders will have a violent reaction to this suggestion but I have been doing it for years with no ill effects that my ear can distinguish.

_________________
Joe Beaver
Maker of Sawdust

It's convex NOT concave indicating that current humidity is HIGHER that it was when you braced.
You need to dry it before gluing to the rims or you risk cracking when it returns to dry conditions.

Putting the top (and the sides) in a drying box for a few hours prior to glue up should return the top to a flat condition. An enclosed space heated to 10 to 15 degrees above room temperature will probably accomplish this. you might even get away with gluing things up as they are, but why risk it? A drying box can be made from a light bulb placed in a cardboard box, if proper clearances are allowed to avoid combustion (I would not leave such a device unattended!!). A better, more permanent one can be made from an old cabinet. There is a discussion on the MIMF forum (tools section) going on right now .

Before I was hip to humidity, and glue ups and all,
I had tops go concave on me.
Now what I do is, glue the braces at about 40%.
Then I don't glue it to the rim until the humidity is 40%.
I don't think one needs a humidifier or a dehumidifier unless you want to build full time.
Just wait until the humidity is the same for the important glue ups.

When you look at the cost of the wood for a guitar . A de humidifier is a cheap tool.Humidity can ruin a guitar. You want to glue braces on the dry side. As for gluing the top and back on , the humidity isn't as much an issue . What your areas RH is should be your guide. The more the humidity is stable the better , but here in PA the swings can be dramatic.
I glue braces at under 40% . Early in my building I found that when I didn't control the humidity my guitars would have fits come winter. Now that I do control the humidity I have a much more stable product. Not only do you have tops that move , you may end up with cracked tops and backs.
A small de humidifier can be had for around $150. Cheap insurance when you consider the time and cost of a guitar. If you build one for a customer and it splits a top , it isn't a good thing.

_________________
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 had luck with braced plates staying put without gluing down to the sides. Every time I try to get away with it, I get burned and my entire shop is climate controlled year round. I don't brace until I'm sure I can close the box in the next several days.

Another good thing about full time dehumidification in a basement shop is it prevents the musty smell. Also, there is no way a basement would ever get to 40% RH in the warm months without one (in the South at least).

AC is all I need in the summer to keep the RH around 40 to 45% in the basement shop. My basement is pretty dry, however. That is probably due to the attempts at a moisture barrier when the house was built. One end of the basement floor is at ground level.
I have been lucky to have a relatively stable working environment. I started monitoring RH about 22 years ago, and my main concern has been low humidity in the winter. It's good for assembling guitars, but not so good for vintage guitar storage. I generally use a humidifier from December to March.
I am in East TN, at an elevation of 1200 feet.

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John