Use material with transparency to mask lensflare

Hi,

here's a picture of my scene

I'd like to use the transparency of the roof to mask the lens flare ...

How do i do that ?

the material on the roof is Transparent/Cutout/Diffuse
adding a mesh collider to that object mask the flare everywhere ...
evenutally, i guess i could end up creating a mesh in maya from the alpha and use it as a mesh collider, but i'm sure ther's a better solution

Thank you !