Minecraft roof with redstone open?

So to understand it I want to build a mob trap and it should work so that the zombies are all flushed into a room and then I turn off the water and open the roof with a switch that will burn all the zombies. Then I turn the water back on and get the items. Problem?! I have no idea how to make the roof, I can do a maximum of 2x2

I could offer you an 8x8 door. If you put it all in a lying position now, then you have a reasonably big roof.

But why not a mob farm in which the mobs are flushed with water into a manhole, drop down and arrive at the bottom with half a heart? Then you can bang it with a wooden sword and even get the XP?

Ahm to 1 I'm not going to build it takes much too long xd 2 because of the drop down yes I could do that but I prefer to do more compact and together

Just take magmablocks on the ground where the mobs are dying. Among them are hopperminekarts and connected hoppers among those directly.

Or just leave her 30? Block fall down.

Edit: and remember that with daylight not burn all mobs. Eg dying does not spin during the day. Creeper I know just net ^^