Which did you or would you use? Are tongue and groove a lot more stable? I don't like that you have to undo everything above it if you have a repair to make, but I don't want warped boards poking out every which way. There is an 11' stretch in the stall and I wasn't sure if regular boards would warp too much with that.
I should add that if money were no option I'd just use tongue and groove and be happy. But.... as the barn budget is severely dwindling, I'm trying to save money and guess I'm not sure if T&G is WORTH it if boards would be okay. Using boards would be quite a bit cheaper, $3-$4 a board cheaper.
Depending on where you live and what kind of horses you have another option is pipe for stalls. I'm in Kansas where summer is still more brutal than winter and I have easy going stock type horses so we put in pipe stalls. Had a local pipe welder make the stall fronts (w/ built in blanket bars) and dividers (with a place for hay racks) to fit and they are perfect. Plan B is adding 3/4" plywood to the pipes should I ever own a horse that needs a true box stall. Total cost for four, 10' wide stall fronts and three 12' long dividers- $855.
I don't automatically recommend tongue and groove due to cost and lower strength (part of the board is machined away to make the tongue and the groove). The interlocking feature does add some strength due to rigidity of the entire assembly, but that can be attained with stiffeners as well. We do use them on stall fronts for appearance.
My preference is 2x8's installed flush with one or more stiffeners depending on length. You should use the widest boards economically feasible for both strength and labor/time issues, and my calcs show that's almost always 2x8's. I'd use 2x10's but they tend to be more expensive on a $/ft2 basis.
We use plain dimension lumber in all locations except for ground contact. Bottom and/or bottom two boards are always treated, and we bury the bottom board 1/2 into the stall base to prevent leg unders. Horses love to chew this material, so we strongly recommend metal edge protectors for all exposed surfaces.
Mine are regular dimensional lumber, pressure treated on the bottom board but otherwise just regular pine from Lowe's. (2x8) I didn't buy any boards that were already warped, and the regular stuff was nice and dry so no warping. The pressure-treated stuff needs to either dry with weights on it on a flat surface (or it will warp) or be screwed into place right away--preferably the former.
I did use stiffeners, and have had no boards moving or shifting after 3 years. Edge protectors everywhere a horse can reach.
I have t&g in my stalls, been there for 4 1/2 years now. My stalls are 12x12, and I used full 12' length boards. I do have a center metal stiffener as well. Now granted, the horses are rarely inside for more than a couple of hours in the morning, but occasionally (a few times a year) they are in overnight. I did have one horse spend 3 weeks 24x7 in there, then a couple of months 12 and 12. My horses are big - 17h WB, 16.1+h TB, and 15.2-ish but a hefty boy TB/Perch. They bump into things, one spent some time kicking because he was pissed he was the only one who got caught to put into his stall, and it's all held up really well.
it WILL be a huge pita to repair lower boards. Huge. But...
I did get the boards at 10% over cost from the guy who put up the barn shell, so that helped make the decision at that point.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET
I used 2 x 10's and have had no problem with warping. T&G was out of the question due to the price. So far, they look great and haven't really been chewed on, but my stalls all open out to the pasture, so they aren't forced to stand there.