Welcome to the forum, May-Brith. I don't think that it will make a difference, if the router plate is to be located in the center of the top, at the low point. As long as the plate is even or slightly higher than the table top.
Hope this helps.. Good luck with it. Woodnut65
May-Brith, it is common for this type of material to sag a bit. When you build a frame underneath it there will be the needed support to keep it flat. I suggest you cut the table top so it is 30" long, this is plenty.
The error maybe "particle board " they do make 1 3/16" and laminated it and use it for office furniture in many application but most office furniture does not need to be real flat like a router table needs to be.
Now when it comes to MDF, it is flat and with some laminate that you can put on in a heart beat you will end up with a useable tool .
I can't reduce the length now, the legs are already on, they are made of the same plate as the top. The only way I can fix it is to saw through the top of the inner legs (there are four legs in all), and put something under them. I hope I can avoid it.
Did I use a wrong word ("particle board")? Maybe "pressed wallboard " is more correct. I live nearby a company that makes kitchen worktops, and they sell plates with defects very cheap. MDF is very expensive in my country.
May-Brith from Norway
PS. I know my English grammar is poor, I hope everybody can bear with it.
May-Brith you are doing fine. Pressed wallboard is also called OSB (Oriented Strand Board.) Particle board is glued up from ground up wood, very small chips. The next finer process would produce MDF, and the finest gives us hardboard which is sometimes called the brand name of Masonite. I think your best choice for a table top is baltic birch plywood with plastic laminate top and bottom. This is high quality and has no void spaces in it. The laminate keeps moisture out and gives you a smooth surface to slide your material on. Your OSB will flex more with the weight of your router and is not a good choice.
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