I published a post a little over a year ago about the slatted racks I had made for my hives. A slatted rack is essentially a shallow eke fitted with slats which is placed above the mesh floor (US: screened bottom board) and below the brood box. Slatted racks are in widespread use in the US and are commercially available for 8, 9, and 10 frame Langstroth hives. I am not aware that they are available in the UK for National hives so I made my own. They have now been in place in my hives for a year and the recent merger of two hives to deal with a drone-laying queen allowed me to look more closely at one rack. This is how it looks after one year.
The advantages of a slatted rack include:
1. It restores bee space below the brood frames, something lacking when the brood box is immediately above the floor.
2. It provides extra space for bees to hang out and reduces overcrowding, and perhaps discourages swarming.
3. It reduces draughts and keeps the hive warmer in winter.
4. It assists ventilation and keeps the hive cooler in summer.
5. It encourages the queen to lay right to the bottom of the brood frames.
6. It may also discourage the bees from chewing through the lowest part of the foundation.
For me the main advantage is the complete absence of burr comb under the brood frames which makes swapping frames around in a double brood hive much easier. You can see here there is no comb on the bottom of the frames.
Langstroth hives have top bee space so commercially available slatted racks have the bee space built into the top. Modified National hives, on the other hand, have bottom bee space so I built my slatted racks with the tops of the slats flush with the frame.
It is important that each slat is directly underneath each frame. As the National hive takes 11 frames and a dummy board I offset the slats slightly to accommodate this.
The bees seem happy with them, and I am, so I'll continue to use slatted racks on my hives year round.