How flat is S2S?

Is there a standard definition of what it means to S2S?
Standard technique to get 2 faces surfaced is to joint an edge, then joint a face, then plane the opposite face. Thus to S2S such that a board is flat, you've really S3S (or S2S1E as I've seen used).
Seems to me that if you just S2S you've got 2 faces parallel but the board isn't necessarily flat - it could have a bow or twist that isn't removed by planing the 2 faces.
So what I really want to know is ... is S2S lumber flat enough for precision work without further flattening, such as to make a picture frame or cabinet door frame? If not, is S4S any flatter?
Thanks, Michael
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Surfaced 2 Sides. Usually means planed both sides.

If you want your lumber jointed, you must specify it that way, and likely pay a good bit extra. S2S does *not* mean jointed on one face and planed on the other.

That is correct. For that reason, I buy my lumber rough whenever I can, and joint and plane it myself.

No.
Why would it be? That just means the edges are approximately straight, and approximately square to the faces.
-- Doug Miller (alphageek at milmac dot com)
How come we choose from just two people to run for president and 50 for Miss America?
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Yes, maybe, sort of. Coming off the planer at the mill the wood may be perfect. After sitting for an hour or a month it may move.
Probably everyone here can tell you a horror story of how wood looked perfect, but went to hell overnight. I bought some a few months back from a very reputable source. They planed it to thickness for me and it was perfectly flat. Got it home and ripped the edge to get the width I wanted. Nice flat wood, ready to use. Had my lunch, went back to cut it to the lengths I wanted. Nice big split right up the center of the board.
Bought some S4S to make a bookcase. Cut the length I needed and finished for the day. Next morning you could have used the boards to make pipe as it was curved that much. It was stored in an outdoor shed where I bought it. I moved it inside. Ed
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...The wood is roughsawn and banded, or dimensioned and then banded. The wood can warp, twist, cup, etc. but if it has been surfaced to S2S the surfaces, no matter how distorted the board is, are parallel....

...Yes it is, and no it's not. A little quick lesson here. Wood has a specific cut: "To grade", and "Through & Through". The wood is then hand graded for grain, figure, color, and defect. At this point the wood is usually still flat. The wood is then surfaced to grade: "Hit & Miss, S1S, S2S, S3S, S4S, SS-PTS, finish (the later two are plywood grades). Not surfaced is called rough sawn. The grading can also take place after the wood is surfaced, and just before it is packaged for delivery. Depends on the mill, how they process, age of the mill, size of the operation.
The wood can then be air and or then kiln dried. The wood is dried by weight to a specific moisture content, when drying, it is stacked, stickered, spaced, and banded. The period of initial drying subjects the wood to much tension. The banding is to keep the wood from distorting. As the wood dries, it shrinks and the banding is thus loosened. The movement at this point is reduced, but the wood still prone to distortion. A surfaced board can distort but still be parallel.
My preference is to select my trees, assist the sawyer and cut for grade. Air dry to about 25% (I live in the desert). The wood can then be further dried to 12% in a solar kiln. A solor kiln is not used comercially because they are too slow, however, they heat and then cool. This daily cycle help relieve the internal stress in the wood. This makes it possible to keep degrade as low as 3% (yes, really!). By air drying the wood myself, I can set the thickness of the spacers and stickers to determine how fast the wood is to dry. I can also keep checking the wood for weight and shrinkage and keep tensioning the chain binders. This helps reduce degrade. The advantages of processing my own wood are that I know exactly what wood I have (ever bought Honduras Mahogany? You may have bought H.M, but were actually sold African Mahogany). I can also select the cut, and dimension. This greatly reduces waste as I get the wood sized to the projects. Strict control over the drying gets me a better wood. Usually no case hardening (though I have had some problem with Black Walnut) or internal stresses. Also, the cost is much lower. The down side is that I need a way to fell, transport, cut, dry, and store the material. This all takes time, knowledge, tools, and space. I also need the knowledge of how to grade if i intend to sell any of my private stash.
To finish answering your question, no, S4S is not any flatter. That is a matter of wood movement, not surfacing. "S4S means Surfaced 4 Sides". If the surface is dimensioned, it is dimensioned. If it distorts, it distorts.
To me it is always better to select and process my own wood, but then I have the background (the lumber industry, followed by the construction, and then millwork industry) and the resources. Most of my fellow woodworkers are stuck with lumber yards or worse yet the big box stores with high- priced wood that is processed to look saleable, and generate the highest dollar.
Just my 2 cents... -Rick Buchanan

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be parallel, and jointing the other face won't help.
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True, but the second face is then planed. The downside in the board will be thinner. (obviously). -Rick
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Michael Press wrote:

You have lots of answers already. I'll give you the simple answer I don't see yet.
"It depends."
I don't have a planer or a jointer, and my skill at surfacing with hand planes is such that I really try to avoid having to flatten the entire face of a long, wide board by hand. I can get it pretty close to flat, and get the opposite faces pretty close to parallel, but I'm much better off to leave well enough alone and use it straight from the mill as much as possible.
The wood I get is S2S, but the faces are only barely planed. To get a smooth board, I have to shave a little off. I prefer to do this *after* I have cut what needs cutting and have done almost everything else, and I only plane what needs planing. (For example, I leave the insides and bottoms of boxes semi-rough, since I'm covering them with velvet anyway.)
I've found that the key to this game is *careful* lumber selection. Every board has a slight (or pronounced) warp or a twist or a cup in it somewhere, but if I'm very, very picky and spend a good 45 minutes going through the entire pile, sighting down every board from both ends, laying them flat to see if they rock, I can almost always find what I need.
So far, it's worked out fine. I adopted this practice, however, after making the mistake of buying a really pretty board that was twisted a little to the left at one end. Danger Will Robinson, danger! I used this board to make some large poster frames. The frame pieces were all about 1" wide and up to 40" long. A great many of them started curling toward the kerf on the far side of the blade, and I was very grateful I had my splitter installed that day.
I salvaged the frames, but it was an ugly business. A couple of the posters are nailed to the wall to hold the frames flat. Ugly. Shameful. Lesson learned.
If it ain't straight when you buy it, it ain't gonna get any better when you start cutting it up. Get it as straight and flat as you possibly can, and let it sit in your shop for a bit before you work it up. That way you'll get some idea what's going to happen before you commit to using it. Even then, it might start moving on you as soon as you cut it.
Hope this blind leading the blind advice helps. I've only been using real lumber for somewhere around six months or so. Before this, I always bought S4S at the BORGs. I never had any of these sorts of problems to deal with back then, but then again my choices of wood were limited to red oak or poplar, and all the large boards were actually glue-ups made from 1x2s or similar. This new stuff is cheaper, better, and of course I'm using walnut for everything, and hopefully will never have to use a piece of red oak or poplar for anything again. :)
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Michael McIntyre ---- Silvan < snipped-for-privacy@users.sourceforge.net>
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