1.   The existing ceiling joists must also be fire-protected with an overlay of glass fibre insulation, and new joists installed.

2.   If nailing is too hard to do, drill pilot holes through the new joist, but smaller than the diameter of the nail.

3.   Make sure the new joists are the full length of the old.

4.   The most effective solution is to saw off the rotted ends of the deck joists and nail a new joist in place.

5.   This requires removing the ceiling, and if the new joists are deeper than the old, the new ceiling will be lower.