Somebody on the US based BeeList was asking for a design for a ‘screened bottom board’; what we in the UK would call an open mesh floor for helping cope with Varroa. So here’s my description that I sent as a reply: I’ve just made a deep screened board for my apprentice’s new National hive as those supplied by the appliance companies are unsatisfactory in design. The National hive is 18″ square, so you will need to adapt the description to your own hive type.
Obtain a sheet of 1/8″ mesh the same size as the footprint of your hive. Cut 4 pieces of wood 2″ deep and of appropriate length, when joined, to make a rectangle to fit. Join them up. I used glue and long nails. Fit the screen on top, stapling or pinning it in place. Cut a rim to fit on top 1/4″ deep. Glue and nail three of the sides on top of the screen around the flanks and rear of the board. Cut 2 shorter strips for the front, leaving a gap in the middle for an entrance, the width being up to your judgement, bearing in mind that they
won’t need to use it for ventillation, only for passage and defence.
The deep screen sits on the ordinary solid floorboard which is reversed so the entrance is to the rear. Cut to fit by sliding in a sheet of white corrugated plastic (from a roadside advertising board perhaps) and draw roughly 4″ squares on it to simplify varroa counting and examination/ locating other floor debris. You can learn a lot about what’s happening upstairs from what falls down and where.
Reasons for the details above: 1/8″ square mesh is better than the diamond patterned ‘expanded metal’ as the latter has too wide ledges upon which matter can sit; however, the expanded metal will withstand a blow torch better if this becomes necessary. 1/4″ gives a beespace below the bottom bars. More than that will encourage the bees to build extra drone comb below the bars and this can be a nuisance when lifting tight frames out. 1/4 inch at the entrance will keep nearly all mammal predators out and may make it more difficult for other animals. The 2″ drop is as recommended by Jeff Pettis who noted that healthy mites that fall through a mesh all get back upstairs again if the drop is only half an inch, whereas with a 2″ drop none gets back, with a range in between. As your hive probably came with a solid floor you might as well use it as described. If you don’t intend to study what drops, you could dispense with it and leave a longer drop to the ground, assuming your hive is in a stand; however, the solid floor beneath might give the bees better temperature control for wintering.