Despite the plethora of info out there, I haven't been able to find things regarding the following.
1. Dealing with garage slope. Obviously, you use very thin pieces of wood to help raise the platform so that it's level... But how do you determine the gradient of the floor? The physicist in me says to time how long it takes a marble to roll a certain difference, then make some assumptions and calculate the angle of the floor... But I have a feeling that's not how most people do it. Also, these pieces of wood, used to raise the platform, will get rid of the platform's solid contact with the ground. Isn't that a problem? It seems like it would create some flexing throughout the platform, eventually leading to fractures.
2. Polyurethane. What is this stuff? More importantly, what differentiates it from other stuff? I've used stain (color, no protection), waterproofer (protection, no color), and sealant (color and protection) before, but have no experience with polyurethane. Does it provide protection against moisture? Do you normally coat all of the boards with it, or just the top board? Do you cover all surfaces of the top board, or just the top of that? Would it be wise to coat the rest of the pieces with a waterproofer to prevent damage from humidity in the Spring and Summer?
3. Extensions for power racks. I've got a small rack with a base that measures 48" deep and 48.5" wide. I figure I'm just going to lay it on top of a small 4x4 plywood extension. Would you recommend attaching it to the main platform, or is that not necessary? If so, I believe that I'll just keep the top board of that platform as a 4x8, and have that extend halfway beneath the rack, with a 4x2 addition to get the full length. There's no where to bolt the rack down, if that makes a difference. (I intend to store my sandbags along the back support brace of the rack to help stabilize it.)
4. 2x4 bases. I've seen a few racks built with 2x4's used as the bottom layer of wood. What does this accomplish? Better force dispersion? Reduced noise? Durability? It's an interesting idea, and I'll consider it if there's some strong reasoning behind it, but otherwise, I'll stick with plywood, simply for the cost benefits, and for having something shorter and less likely to be a safety hazard.
Thanks in advance.