sedan man thanks for offering advice but it's not a concrete roof. there's several wooden beams running across from the side where the existing door is to the opposite side and these are covered with asbestos board and felted.

if i had taken a slightly wider shot, you would have seen that the concrete on the top of the end wall is just a capping for a couple of courses of bricks that are higher than the rest of the sloping roof that slopes away from that end. you can just about make it out by looking at both pics

so above the french doors will be a few courses of bricks but that's all. i figured that they would pretty much support themselves and the door frame by itself would be enough.

lee, thanks again for the advice. i'll take a couple up the builders merchants and find a match