Conservatory flies

[ottosump]

Hello group, what with the nice weather we've been having recently, we like to leave the conservatory door open, however flies get in, and they can't get out, they just buzz around in the higher roof window Panes , constantly banging into them until they drop dead from exhaustion. We have x2 opening windows in the roof but these are fairly well down from the apex of the roof, they also have thick chunky frames around them which the flies tend not to navigate, it may sound hard to believe but I bet someone else is experiencing the same thing. What I'd like is a small vent right at the top of the roof, any suggestions.
regards
Otto

PS I will not use any type of killing product, i.e. zapper, fly paper

[mxyzptlk]

Why not get a mesh door fitting to prevent them entering.

[4getmenot]

ah prevention before cure

[ottosump]

We have the stringy dangley things hanging in the door frame, a mesh would be inconvenient as the cat likes to climb up it..