What type of screwgun are you using to remove screws?
Doubt it matters, if they're snapping they've probably corroded under the washer and, combined with hardwood that can expand/contract, prolly little hope to consistently remove all and reuse sheets.
I'd look for a way to cover what's there.
What is the existing panel corrugation pattern?
There are structural components available that are notched for the highs or ribs, of existing sheets. Tallest I've used was 4", filling the cavity with 4" back fiberglass itch, sheeting over, but these purpose of those projects was to ensure water tightness, not increase R-value. (4" - R13)
Offered in dimensional increments that afford the option to pitch, if the overall run is short enough.
What's important is a solid structural connection to the existing (assumed) solid framing and there's other means to achieve......but need to know what the existing pattern is.
Could "retro" a frame out of light gauge structural, which is similar to stick building, but with "studs" that are inches to a few feet tall.
Or a more conventional built up roof which, I'd imagine, common in your area, based on uninsulated, flat roof you have.
Built up roofs are common in all types of construction, but essentially, consist of a thick, heavy board insulation that's attached to (commonly) steel decking, then overlaid with elastomeric polymer, tar, or other rolled/sprayed waterproofer.
Cheapest method I can think of to waterproof the surface is elastomeric polymer and is about the easiest to apply, if DIYing.
There's a process to clean and prep the substrate that is critical to adhere to, but it can be sprayed (special HVLP gun), rolled, mopped on.....again, as stop gap measure.
I've seen a fair share of similar, low pitch, reroofs but weather-tightness has always been the problem to resolve and, due to the commercial construction sector serviced, there's typically consistency in the existing structural/panel type.
That, coupled with the varying degrees of finished product quality (kind of like the differences between restorations here on Mud. There's lesser expensive, but they show and depends upon the individual's needs, budget, goals) can say the cheaper end of the spectrum should be around $2.25-2.50 PSF ( elastomeric polymer), with the upper end nearing $8-10PSF, depending on specifics to existing.
DIYing a resheet would put materials around $2PSF, including r-13 itch.
If you have pics and are interested in specific methods and pricing, post them up and will offer more precise opinion and costs.
