You need to seal up the gaps where the mites breed - if you can move the chooks out for a bit while it dries thoroughly, give the whole house lots of coats of gloss paint to seal up the cracks - this is what the lady at the chook place advises.

You will need temporary accomodation for then though, as the fumes from paint as it dries will harm them, and i think it keeps giving off fumes for a while after it has dried to touch- I've read you should leave it a week before allowing chooks to use the painted house again.

I don't know what sort of roof you've got on there, but roofing felt should be avoided as the mites breed in the gap between it & the wood.

The best of luck with your problem - it's very common, must be so annoying.