The modified British National Hive is built with bottom bee space but there is a lot of extra space (usually around 30mm) below the brood frames above the floor. Some bees build comb on the bottom of the brood frames, often filling it with drone brood or using it to hide queen cells.
The advantages of a slatted rack are:
- It stops the bees building comb on the bottom of the brood frames as it restores bee space below the frames. This is especially useful for colonies on double brood as it makes it easy to swap frames around with scraping the extra comb off the bottom of the frames.
- It is said to keep the hive cooler in summer and warmer in winter.
- It reduces draughts, especially around the bottoms of the brood frames.
- It provides more room for the bees to hang out, reducing congestion in a large colony and perhaps reducing the tendency to swarm.
- It moves the brood nest a bit farther from the entrance and encourages the queen to lay right to the bottom of the frame.
At the front of the slatted rack there is a shelf about 100mm wide which is said to reduce draughts through the entrance, although I don't know if it makes much difference if there is a mesh floor. I made the front, back and sides of the rack from western red cedar (some of it recycled from a restoration of the greenhouse) and the shelf and slats from pine.
This is the underside. I don't think the depth of the slats is critical but mine are 44mm, so slightly less than the 50mm of the frame.
Because the National hive has bottom bee space, one very important modification I made was to put the slats and shelf flush with the top of the rack (in the Langstroth version they are recessed to provide the bee space).
You can find instructions for making a Langstroth slatted rack here, all dimensions in old-fashioned inches.
The rack stays in position all year round. Here is my first one, nearly a year later.
I have now made three more and will fit them to my other three hives at the first inspection this spring.