Some designs and uses
These are a selection of designs of open mesh floors (called "screened bottom boards" in the U.S.) that have been generated by different individuals for ventilation or monitoring varroa mite fall.
The list at the top left identifies some of the types by the name of the inventor or proponent of the design. In the U.K. OMFs were not included in the British Standards (B.S.), so there are no standard designs, with appliance manufacturers making to different designs. This doesn't make much difference to the user, but it has meant that some, especially those in some "budget" ranges are poorly designed and made.
Floors need to be sturdy and able to take the weight of a good crop of honey, so to cut corners and make them weak will not produce a long lasting product.
OMFs are not a new idea, but have been around for about a century and a half, their main reason for use being to promote good wintering under harsh conditions.
OMFs have several advantages, they provide ventilation of the hive and may remove the need for special ventilated top screens when moving colonies in summer, although a screen may be advisable for a long journey. Their popularity has increased since the arrival of varroa.
They reduce opportunities for robbing to develop, because smaller entrance apertures can be used, that are easier for the bees to defend.
When they were first advocated in the U.K. as an aid in the fight against varroa, many beekeepers were frightened to use them because they thought the bees would winter badly, even though they had been used successfully for many years in quite cold climates. In the first winter I used them I put half my colonies on OMFs and I left half on solids. All those on OMFs had very little chalk brood in the spring, yet those on solid floors had a heavy infection. They also wintered well, with no mouldy combs, so I am in absolutely no doubt about their value.
They are convenient for monitoring natural mite drop or that induced by treatment.
In my experience swarms are reluctant to enter bait hives with an OMFs, presumably because they think it's open, so can't defend it. For the same reason I have found many swarms that have been hived on OMFs have tried to abscond.