28. These multi-level systems allow birds to occupy much of the volume of the building by the provision of perches (usually timber) throughout the space. Small perforated platforms are incorporated in some designs. Feed, water and nests are provided at all or most levels. The ground level may be a fully perforated floor, or partly littered. The birds jump or fly from one level to another and across from perching areas to nest boxes. In well designed percheries, fouling of birds below is minimised by careful siting of the perches in relation to each other.
29. Stocking density is generally similar to that in aviaries. The total available surface includes the perches. The available equivalent "surface area" of a perch is generally calculated assuming that a hen on a 15cm length of perch occupies approximately 15cm in front of and behind that perch. An allocation of 15cm of perch length is therefore equivalent to 450cm² of surface area. Thus perches, if raised by at least 40cm to allow hens to pass underneath them, add to the available surface area of the house.