How to stop damp above doors

If it is an external door with a cavity wall, a flexible tray damp would be used to lead any moisture away from the inside walls with weep vents for the water to exit on the external leaf.

Put a damp tray 450mm wide over the lintel. Put weep holes in every two bricks (450mm) so that any moisture can drip out. The damp must run past the lintel at each end at least 150mm and be turned up in to the nearest perp on the brickwork. In this perp (perpendicular joint) there should also be weep holes. The damp tray that runs out of the back of the brickwork should be lapped up in to the block work at least 150mm but usually it is easier to lap it up on top of the next course of block work 225mm (three courses of brick). This would mean any damp/water cannot travel in to the interior wall and cannot travel along the brickwork as the damp is lapped up, and will drip out of the weep holes in the brickwork.

I would agree with the above post, installing a damp proof cavity tray onto an L lintel ideally, from above the internal skin, including weep vents to allow the water to escape.

You should form a cavity tray using a polythene damp proof membrane. It should be used above the door on the lintel and should extend past either side by at least 150mm. The damp proof membrane should lap from the external wall(brickwork/stonework) to the internal wall (blockwork) by at least 150mm and be lapped up to prevent water ingress. Weep holes are required at 450mm intervals to allow for water to escape the cavity and not breach through the top of the door. Keep cavities clear and minimal mortar from forming on the cavity tray in order for the weep holes to work effectively.