How to get rid of cluster flies?

Assuming that cluster flies in the same in the USA as in England they are Pollenia rudis. These flies lay their eggs in the soil which then hatch out & and are parasitic inside earth worms. For this reason it is impossible to treat the source of the infestation easily. An insecticidal treatment of your roof void in the spring & autumn will help.

Getting a pest control company to put a insecticidal smoke bomb in the loft will kill most. They are only overwintering in the loft, contrary to other answers they do not breed in the loft. Have you had your dinner? Cluster flies lay eggs on the soil in the spring, these then hatch and the maggot stage lives inside the earth worm as a parasitic maggot. In the autumn they hatch out and pupate, the adult fly lives in the loft over winter and repeats this the next spring.

Ugh.... I have the same problem but inside my old house!

I found some very helpful nontoxic options for getting rid of these pests at www.Lehmans.com under "outdoor" and then "pest control".

I have also found that flies dislike the smell of vinegar but I'm not sure how this would work outside.