How do I reinforce a sagging roof?

Ot will depend on which roof type you have, flate, apex, skillion.

If the roof is saging it means that the supports beneath is breaking down, has broken down or there isnt enough support iin the initial build. The most effective in any situation is to get in to the roof space and identiy what has caused the drop. You may then have the option of using an acro to getly lift the sagging area and then replace damaged supports, add in missing suppoprts remove and replace damage.