How do I Make collisions constantly subtract from variables?

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Simple test above but My goal is that for every few ticks or seconds the zombies on the barracade to lower or add to barracades variable But the problem they will most likely remain contact on the barricade.

So they will stop adding/– from barracades hp variable.Because once then stop adding/- because they are still colliding with sed object.

Is there any easy way to fix this? other than making it to where zombie collided with object spawn object ( that damages the barracde ) in some weird hassling way ?

thanks (yes I know my spelling -.- )

You could do an every x seconds, is-overlapping wall, subtract from variable.

Chris is right, Collisions only trigger once when colliding where as 'Is overlapping' will continue to trigger for the whole duration.