I'm not sure if this is the right forum but I thought someone here might know. I have to change the frame for a manhole cover in my house. I need to borrow some heavier duty tools to do it, in the meantime I was a thinking of just covering the hole with a piece of steel.

The opening is a 20"x20" square. I'd probably get a 24" x 24" piece of metal to cover. What thickness of steel would not deform if stepped on by an adult human?

 

24" x 24" will only leave 2" around the perimeter supporting the steel. It's enough to support weight but it doesn't leave much leeway if the plate gets bumped or slid out of position. I would consider adding a couple bolts or pins to hold it in place.

There will be some deflection no matter what you put there. 11 ga (1/8") is the thinnest I'd probably go. It will be strong enough but you'll feel it deflecting. 7 ga (3/16") will make it feel sturdy but if it were mine I'd do 1/4" thick which should feel solid when you walk or stand on it and would survive a car driving over it.