a rental period in between two activities

[ketanco]

Hello,
In my schedule, for a facade repair, I have an activity for erecting a scaffold first. And then the repairs will start at a far future date. In between the scaffold will just sit there for a long time.

I have it as follows:

1)Build Scaffold

2)Scaffold rental period for the duration that scaffold remains in place



3)Do repairs repairs at facade (starts at a far future date, so has a constraint of start no earlier than 1/1/2015 for instance)

4)DIsmantle scaffold (as soon as 3 is done, this will take place)

My question is about activity #2 here. It is not an activity that involves any action, but it is to show a rental period for scaffold and therefore its cost. My problem is, I want to calculate its duration automatically, between finish of activity 1 and the finishing of activity 4. in other words it can only start after 1 is done, and can only finish after 4 is done. but its duration is to be calculated automatically dependent on when the other activities happen. how can i do it?