Building extension onto gable wall

[daveelsy]

How do you prevent dampness passing from the external wall (above the roof and outside the new extension) to what becomes an internal wall (inside the new extension).
When the external gable wall gets wet the damp will tend to migrate down through the wall into the interior of the new construction possibly damaging plasterwork and decorations.
When constructing a new house a cavity tray is usually built into the gable to stop that.............however what do you do with an existing (in my case, rendered) gable wall.
Do you just take a chance & hope it doesn't travel that far?

[welsh brickie]

You can seal it.Just hack off the render and fit the rafters tight to the brickwork.foam any gaps fit your tiles/slates and your lead flashing seal the flashing with mastic.And fit bellbead to the wall on the flashing and replaster to match.Make sure the render on the gable is in good condition first though if it isnt hack it off and re render.