Sounds as if they were lurking in the roofing felt.
We burnt a hen shed to the ground once - very satisfying

But the mites will always come back and find somewhere to hide.
We maintain ours now with frequently going over with the weed wand - like a blowtorch on a stick but less likely to set the whole place on fire, then we creosote a few times a year, and dust both the hens and the house with diatomaceous earth and wood ashes. As far as I can tell the DE is just a well marketed form of chalk or limestone and I wasn't expecting it to work, but freshly applied frequently it does seem to work.
You need to break the cycle of the mites. When we have had an infestation we have done the weedwand routine for several days in a row to get the newly hatched ones. It's a palaver but pays off in the long run.
We have 3 garden shed type hen houses but not as many hens as we used to have, so they can all fit into one house. What I have noticed is that they will rotate which house they use from time to time, so if they have moved out then we give that shed a thorough cleaning and creosote it. Once they are ready they will move on to the next one. Although they are bound to take a few mites with them, it does seem to keep the numbers down.
Our hens also have access to wood ash dust baths as well as dry soil, so they can deal with any parasites themselves to some extent.