This page is credited in full to Dave Cushman who created it. His voice is expressed in black colour text and any additions or comments in blue belong to myself. Credit: Dave Cushman’s website.
We all “KNOW” that bee space is between 4.5 mm and 8 mm and it is also widely reported as being between 6 mm and 9 mm. However it is not a “variable” quantity, it is either 5.3 mm + or – 0.5 mm or it is 9.0 mm + 0.0 mm – 1.0 mm. In other words there are two distinct bands of possible bee space and these occur because in some situations the bees will work individually, but in other situations they need to be able to work back to back.
A gap of:- less than 4 mm… is too small for any but deformed worker bees to pass through. Any spaces, cracks or crevices of this or smaller dimension will be filled with propolis or sometimes a mixture of wax and propolis and on yet other occasions pollen may be mixed in with the filling (I suspect that this is for reasons of porosity or possibly the transmission of light, but I am not certain).
A gap of:- 4.3 mm is a standard European spacing for wires in a Queen Excluder.
A gap of:- 5 mm if used between the wires of a square mesh will make an excellent pollen stripper as the workers can get through, but a significant portion of pollen will be stripped from their legs.
A gap of:- 5.2 – 5.4 mm is a spacing that can be used to exclude or differentiate Drones, as Workers and Queens will freely pass, but Drones cannot.
A gap of:- 6 mm Is the smallest gap that bees will leave between adjacent comb surfaces (outside of the usual clustering area) the bees can defend this more easily and they can work individually within this dimension. The smaller gap around the periphery of the nest, also renders the nest less susceptible to draughts, and may help in maintaining humidity.
A gap of:- 7 mm not used by the bees themselves, but some people regard it as a valid bee space to use in some parts of beekeeping equipment. If this spacing occurs between the side faces of frame top bars they are the least likely to suffer from accretions of wax. Frames spaced at 35 mm pitch (normal Hoffman spacing) that have top bars 28 mm in width give rise to this 7 mm gap.
A gap of:- 8 mm is a popular bee space among those that design their own equipment as it falls midway between the 1/4″ and 3/8″ figures so often quoted in old books. I used to be keen on this dimension myself, but I have come to regard it as ‘neither one thing or the other’ and now I favour 9 mm or in some circumstances 6 mm.
A gap of:- 9 mm is the usual space the bees will leave between adjacent areas of capped brood this allows two layers of bees to work back to back, usually in an oval pattern roughly in the centre of a frame.
A gap of:- more than 9 mm and we are into brace comb territory!
A Gap of 10 mm is practical from a design point of view. with the B.S. deep brood frame at 215 mm (some are 216 mm) and the Shallow Frame at 140 mm The boxes are then 225 mm and 150 mm respectively this gives 1 mm above the frames and 9 mm below (or the reverse if you are like me, top bee space oriented). This may seem large, but it only is this way with fresh equipment. The grain in the frame side bars is vertical and practically no shrinkage will occur in this direction. The box sides however are grain oriented horizontal and the shrinkage will occur in the vertical height of the box. So in use the space is often much less than the initial 10 mm.
In all things there are exceptions… when it comes to the gap between the frame bottom bars in the bottom box and the floor surface underneath it, this is usually 28 mm or 31 mm in UK hives, but it does not suffer brace or burr comb unduly, as the bees consider it a similar situation to a wild nest in a cave.
Interesting and thought provoking, and I too am top bee space orientated.