In renegade quantity of hp repaired over 1s was a function f=exp(C*x) where x is the damage done by player.
At some point it repaired itself faster than player could deal some damage (from the outside).
Now i guess if you're talking about exactly the same thing, it's not really a good idea.
However if you will make HP over time a constant value, so that building slowly repairs itself even without engineers it may be okay.
What if make this function something like f=exp(C*t) or f=C*t where t is time outside combat so that building will repair it faster the more it stays outside the combat.
When someone attacks it, the healing stops and everything relies only on engineers.