I'm beginning to think warping Homasote has become an urban legend.
For the typical home owner having their typically horizontal layout in a typical home with typical HVAC control throughout,...it's not a problem.
However, for typical retailers of Homasote...it can be a BIG problem. Before Menard's came to town, the only source around here was a classic lumberyard that has been around for 100 years. And that availability was only by special order, no cancellation, no partial returns.
The reason explained to me was that they didn't have a climate-controlled storage facility for this type of material (paper product) which for its residential/commercial construction market had rather low demand and inventory turnover. Warpage and swelling was, indeed, the problem.
Well, Menard's stores these sheet materials...Homasote, plywood, Masonite, MDF, etc...inside their main facility, fully climate-controlled. No problem. And, as I would suspect, having a founder with model railroading as his hobby...as we all know and appreciate!!!...was careful to ensure this popular layout construction product was part of their inventory.
BTW, before Menard's came onto the scene, I needed a sheet of Homasote for a project. The aforementioned lumberyard would not special order a single sheet. So I approached a couple of the 'Big Box' home improvement stores of that era. Since Homasote's usual market is in home/business construction, I went to the Contractor's Desk at the nearest store.
Me: "Do you guys happen to carry Homasote?...I just need a single sheet."
Counter Intelligence: "What's Homasote?"
Re: holding layout thingies down on the surface (assumed to be near-horizontal) of Homasote without penetrating to structural elements beneath... Screws of any sort are the LAST thing I'd consider for fastening to that usual 1/2" thick 'cardboard'. My first go-to is gravity. It does a good job in our geographic area of holding things down. OTOH, guarding against vibrations, wayward elbows, de-railing high-speed trains, wandering/curious felines, etc.,etc...requires some help. I've used plastic tie-wraps, magnets, Velcro, wax blobs, rubber cement, pushpins, T-pins, straight pins, thumbtacks, duct tape, ....but not bubble gum.
FWIW, and TEHO.
KD