After 25 years of building exterior shutters, my question is, is a 3/8" x 2 1/4" shutter louver any less likely to bow than one that is 5/16" thick? Lengths range from 12" to 30" on average. I am talking about exterior fixed louver shutters, and the wood type may be cypress, red grandis or cedar. I am also asking in regards to a louver that has already passed inspection during the cutting and assembly process. These are all woods we typically use, or will be using. Stress relief and uneven moisture has already been dealt with, as much as possible. For years my biased thinking has been that the thicker wood would somehow be more stable, but many of the things I read here on Woodweb lead me to believe that 5/16" is just as good as the 3/8". Yes a small difference in thickness, but a huge difference (to me) in the cost of making the thicker louver. It also seems like the length is not a huge factor either.

My opinion is that the thicker stock looks better in the bigger shutters. Not going to add any stability, just appearance and proportion. The cut of the wood will make the difference in bowing. Quarter sawn would my cut of choice for real stability.

I've only made one house set of shutters in my career.

Who decided that 3/8" was superior? If it has been the "standard" for the last 100 years, it still doesn't really mean anything.

Often the dimensions of such things was determined by ease of fabrication, not the physical properties of the material.

Its kind of like making a structural beam larger than it needs to be out of convenience. The math says it could be smaller, but you would have to cut it down.

Based upon my experience, I would not expect those woods to have stability problems that would be helped by keeping the material a wee bit thicker. If you are having a problem the thickness is not going to be the reason.

In short lengths ,no. Longer I'd say yes as it's easier to flex, therefore it takes less force to bend it.

Tim has made a point that the thicker material has more resistance to force, and I think that is probably the main reason why I always believed the thicker material was better. While a few of my customers believe that closing their shutters can add a level of storm protection, we certainly do not sell them with this in mind. Sure they can deflect a few small limbs, but nothing serious. So with that in mind, for the most part, these louvers basically are made just to sit there year after year, with no force being introduced. So for the most part, I think this is a minimal factor in my deciding which is better, with the exception of orders where a thicker louver is specified.
I agree that quarter sawn lumber would be more stable, but seldom do my suppliers have it available in the wood types I use. Thicker does look better at times, in proportion to the type and size of the panel. I can resaw 4/4 lumber to get 2 blanks for making 5/16" louvers, but not 2 of the 3/8" blanks. Around here, 5/4 lumber is getting harder to find, which I use to resaw, to get the thicker louvers, even up to 1/2". But when I have to plane down a 4/4 board to get a 3/8" louver, that is a lot of waste. Too much labor, too much time and too much waste and all for too little gain. Other factors like moisture or defects will cause problems and bowing in even 1/2" louvers, so that is now not a strong factor in deciding.
Thanks for your thoughts and input, 5/16" it is.

I would say that thickness doesn't matter. Now, our esteemed Presidential candidates will have no trouble debating the merits of thickness, since "we all know what that means...." But we know better.

If the wood is going to move in response to changes in the environment/MC, then it will do it if it is 1/4" or 2" thick.

Another cause is internal stress caused by the tree's growth or by improper drying. In this case, if the piece if left in the size/shape it was when dried, it may stay that way. But once it is cut, the stress will come into play and the wood will move at the saw. Jointing flat will work until it is planed, when its internal stress overrides that flat imposed on it, and it moves.

Generally, thicker means better quality, and in woodwork, it can mean stronger shadow lines and better mass. We all have seen products where the accountants have thinned the stock and flattened and shrunk moldings to where the product looks nothing like what it should, but it does hit a certain price point.

I don't know that I could detect a 1/16" in slats, so I have no real answer. I do think wood quality and proper drying are the key factors that will affect the flatness of your slat stock.

So you have no real answer but pipe up only to throw in politics.

Nice.

So isn't the one liner making a comment against David piping in?
Anyway, thank you David for your informative response, which contained good information which confirms much of what I was thinking. Rich, Adam and Tim I thank you as well. I find this forum to be very helpful and I for one really appreciate that. Even when we have to separate and step thru the petty crap. Some people.

Mitch , besides woodworking we have also

been in the window coverings business

for about 25 years . We don't fabricate our own shutters but we do sell them .

Thinking about say a 2 or 2 1/2" blind slats

They are rather thin even maybe about an 1/8" give or take some are thinner.

The most stable wood slats are made of Basswood . So the thickness may not be as important as the properties and stability of whatever material used or the slats would be thicker if stability came with thickness.

D, interior window treatments played a big part in my thinking prior to my posting the question about the thickness of louver slats. While I also build interior shutters, I have been carrying a few lines of interior blinds for quite a few years. I have always been amazed how stable those thin blind slats are. Ours too are mostly Basswood. Just as interesting to me, is the fact that I have observed about the same % of defective parts in exterior and interior products. Totally different environments, or so it seems. The side of a closed blind in an uninsulated window gets exposure to some very high temperatures during the day, plus I see some higher than expected humidity in some poorly sealed windows. But overall, I would bet maybe 1 or 2 slats per 1000 end up bowing to a degree that is not acceptable, about the same with exterior shutters. I often laugh when customers start telling me how stable plastic faux wood slats are. They may not bow, but I find they sure do expand and droop, even worse than real wood.

From the structural point of view you don't have any need of extra thickness, but if you just want to add to its thickness for change in the look or some other design need, then that is your choice.

Wood warps because of grain angle. Wood warps because of uneven MC, face to face. Wood warps because of juvenile wood, tension wood, and compression wood.

So, straight grain (like we use in high value pool cues) with a finish to slow moisture changes, and with avoidance of special wood will behave excellently. From a resource point of view, basswood meets all these criteria, so is the preferred species.

Note that thickness is not an issue, but if things are not perfect, thin pieces will warp more easily than thick. The reason is that a small MC change or other issue requires more force to warp than in a thin piece.

Have you considered using a thin kerf multi blade frame saw and 10/4 material?

That way you would essentially have quartersawn material and the least waste possible. Yes, I know 10/4 is more than 4/4 per BF, but I bet the reduction in waste makes up for it, and you get the additional stability of quartersawn for "free".

It would also give you the ability to change thickness as required and still use the same material.