^^ X2 ^^
The top portion of my main permanent table pops off for when we have games outside on the back deck. It's composed of a single sheet of 3/4" plywood for the base sheet, and two 3/4" thick pieces for the rail (it has a 3/4" raised rail). It weighs 90lbs:
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It has two spring loaded flip out handles (like the ones used on big speaker equipment) on the underside that make it really easy to move around:
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A very good rule of thumb is a 4x8 sheet of plywood weighs 25 lbs per 1/4" of thickness (so a 3/4" 4x8 sheet weighs ~75lbs). Assuming a 90"x42" table, the base sheet weighs ~55lbs and 5" wide rail (single sheet) would be ~18lbs, so the whole table would be about 75lbs. The table works great, it's obviously not as sturdy as a table with oak pedestals, but it's light years better than a cheapo
eBay table. Moving to 1/2" for the base sheet would get you to ~50lbs total but I wouldn't recommend it - not only would it flex more, but you'll have "bumping" issues with a table that light - i.e. if someone bumps it with their elbow or knee getting up or reaching for a beer, the whole table will shake and send chip stacks tumbling. It's amazing how much an extra 20-30 pounds helps to make the table feel more solid.
If if were me, I'd go with the single 3/4" base sheet and a single 3/4" (or maybe 1/2") rail piece. Then add two braces under the table with the main sheet cutoffs to help stiffen the table, kind of like this (this table had different style legs, you'll need to put the braces on the outside of the folding legs):
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Good sturdy folding legs (i.e. not the ones from home depot or harbor freight) will also go a long way in making the table stable. Good ones will run you $80-$120. I usually get mine from MetalKraft, and others have used the nice ones from Rockler with success (should be a thread somewhere around here with more info).