I flipped the boards just now, thanks. They're still in our garage with the brooder plates, but some are starting to sleep on the roosts all night. tonight is 39 degrees. It's been 80 during the day, but we had a low of around 10 a couple weeks ago.
It comes up in several places (see below for a few sources' quotes):
"Flat 2x4 roosts (wide side up) are generally superior to round roosts for chickens because they allow birds to sit flat-footed, keeping their toes covered by breast feathers to prevent frostbite in winter. While round roosts (1.5-3" diameter) can be used, flat, wider surfaces reduce pressure on the keel bone and improve overall comfort."
"Round perches work, but most chickens prefer flat 2×4 roosts—they’re warmer, safer on the feet, and better for heavier breeds"
"Width:
Chicken roosting bars should be around 4cm wide. Chickens don’t wrap their feet around a perch as wild birds do, they use their toes. They actually prefer to sleep flat-footed. This has the added benefit of keeping their feet protected from frostbite in the winter from below using the roost as protection and using their body as protection from above."