I've got a vertical space problem too. I want the automatic door on the side of my hoop tractor. The first 18 inches above the bottom frame is vertical and just wire, and above that, a second 3 inch frame where the bend of the hoop begins, so it would be difficult to have a vertical guillotine type door.
The Pullet Door (the one at ChickenDoors.com) solves that problem because it opens sideways on a hinge, like a door in your house. Pullet Door doesn't recommend turning the door so it opens like a gun port, or reversing it so it opens inwards, because they'd rather have a predator trying to pry it open, which is probably impossible, and if the predator tries to push the door open, they'd be pushing the door against the metal frame, truly impossible to get in.
It sounds like your door would be under cover on both sides, that's great, I love for equipment to not to be exposed to sun, ice, snow, and rain, equipment functions better and lasts longer that way.
I'm a big believer in wide eaves that overhang a long ways to protect the structure of a building, and all the openings, so I'm going to figure out some way to get a little roof over the pullet door, that'll keep rain and snow off the opening and the door, when the door's fully open.
For what it's worth, I talked to Mark at Pullet Door, I think he's the owner, and he said their small door, 9.5 inches wide and 10.5 inches tall, will work fine for most chickens. Most people buy the regular door though, which is 11 inches wide and 15 inches tall, and he's got an even bigger door for turkeys.
Right now, I've got my tractor door (people sized), chained open to only 6 inches wide while the chickens are out free ranging during the day, they return to the tractor several times during the day to lay, eat, and drink, and even the larger rooster gets through OK.
When I install the new Pullet Door, I'm considering screwing a piece of wood on the inside of the frame to choke the door entrance down to 6 inches wide, making it hard for a large predator, like a dog, to come in the tractor during the day. My tractor doesn't have a coop in it, it's wide open, so the Pullet door will give complete access to the inside of the tractor.
I'm not sure though, maybe that's overkill, because if there's an attack during the day, the chickens will probably be away from the tractor and will seek whatever cover is close. I'm just not sure if the inconvenience to the chickens of the smaller entrance (6 inches by 10.5 inches) is worth the slight increase in protection from a larger predator getting into the tractor during the day, or whether it's better to give the chickens the extra elbow room of the 9.5 inch by 10.5 inch unobstructed door.
We have hunting dogs around here, and in my minds eye, I can just see a pack of off course hunting dogs in the chicken tractor, while a hen is in there by herself during the day, in the nesting box, trying to lay an egg, the hens do go back one by one during the day to lay in the tractor.
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