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I have a stairway that has the treads warping on the bottom flight which is between 3 concrete walls as shown in the pictures below , the walls underneath the stringers are unheated and uninsulated , I feel that there is a problem with moisture developing in the colder months as they stayed stable for the first few months, the top set stayed perfectly flat ,
It doesn't look like it ,but I'll ask-
And next- did you finish and seal the treads & risers all the way around?
I did not seal the bottom
You aren't going to be able to correct any cupping, it will happen every season. Unless you wait till they flatten out again, and then add the same amount of finish on the bottom surface as you have on the top. The surfaces are absorbing the moisture at different rates. Even if you circulate air under the stairs, the bottom surface will still change at a different rate than the top. Quarter sawn treads would have been a better choice.
I don't think any amount of finish on the other side is going to completely stop the warping. It might control it a little better but it won't stop it. In the winter months it is probably just above freezing on the underside and 70 degrees on the top . That alone is going to cause some moisture problems.
We are assuming the treads are cupping and not actually warping. And the hollow side is up, the belly side is down, with nosing lifting and the centers staying down. This would be in line with what has already been stated - the treads are absorbing more moisture - and giving it off - more on one side than the other.
Finish will slow this down to where it might make it less noticeable enough to pass, but mortising quartered treads into stringers would assure success. Rabbeting the lower edge of the risers and a tongue on the top of the riser to fit into the tread is also part of good practice. Modern work in most of the US is rarely done this way, but there is a reason it was done for a couple of centuries.
Depending upon who did what here, it will also be worthwhile to seal up that concrete and get some room air into that space. This may extend beyond your responsibility, to the GC, but is worth mentioning since if the humidity differential was not there, there would be no problem.
While temperature can play a factor, it is the Relative Humidity that needs to be better controlled.
Vapor barrier and some insulation sure wouldn't hurt. Who builds anything today that has freezing temps inside a structure?
We know 100% that this is a moisture content change issue.
It is hard to believe that the humidity, and therefore MC, on one side is greatly different than the other...different enough to cause this much warp.
So, what you are seeing is the natural higher shrinkage of the bark side of lumber when the lumber dries out. Experience shows that when the humidity increases in the warmer time of the year, the pieces will seldom become flat again.
The reverse will happen if the wood is dry initially and then gains moisture (perhaps from the concrete, which can take 6 months to leave, or from the natural high humidity in a basement) increases the moisture, and then the bark side will swell more.
For this reason, we need to avoid flatsawn pieces sawn from close to the pith of the log, as they move more than pieces further away. For this reason, lumber where moisture changes can be large, quartersawn grain, which moves half as much as flatsawn, is preferred.
We cannot seal wood with a finish. A finish merely slows the rate of movement.
how many pieces are in each tread? most factory treads are 5 pieces. crosscut , rip, rip and flip, rip again,etc
The treads were maple and they would be made up from 3 to 5 pieces which would be flipped when glued up .
Is the air under the stairs trapped without any circulation? If so, then indeed we have different humidity opportunities top and bottom.
I am curious as to the type of warp...cup, twist, bow, side bend?
During the early 1980's I ran a cabinet shop for a general contractor. Whenever door thresholds were done, his system was to deeply kerf the underside of the thresholds, apply finish to only the top surface, and then he would use silicone adhesive to secure them to a raw ground level concrete floor.
Upon being told to produce them in this manner, I can't begin to convey the skepticism that I felt about this method, and thought that this would be a perpetual warping warranty issue.
In the three years that I was there, there was never a callback on the thresholds. I can't speak to the wood science that this method either reinforces or defies; only to the fact that it worked.
Would it be possible to remove the treads and kerf the bottoms, stopping short of the exposed breadboard ends?. You might try this before taking on more drastic measures, as this might be the path of least financial and labor involvement.
My two cents, FWIW. Good luck.
The air underneath the stairs is in an area without any air circulation . It is completely sealed off. What I have is some cupping. And not very much at that. I would say on that flight the 2 bottom treads are cupped the worst, about 1/8" over 11 1/2" ,maybe slightly more. They get progressively better as we make our way up the stairs.
If this is over a new concrete slab, I'm sure it is picking up moisture from there.
I'd see if there is a way to put some kind of grill in the top and bottom risers.
Even better, if you could pick up some power somewhere to allow putting a little fan in there to get some air circulation. Even a little pancake fan like used in computers or other electrical equipment would turn over the air often enough to prevent this.
I think hat as we learn more about what you have, Keith has the right solution.