Swienty has a bigger footprint than wooden but is compatible with wooden: Compatible with wood .comes assembled. very strong.No leaks
Swienty footprint is 460 x 460 mm and matches the standard National footprint.
It's sold flat-pack because transport is priced on volume, and it locks together well with polyurethane glue. Mind you, the expense of glue, paint and labour must be considered.
Originally designed as top bee space, Swienty converted it later to bottom beespace by the addition of plastic runners. My guess is that Swienty realised long after the TBS moulds were made, that to sell well in the UK it would have to be BBS to match the prevailing wood format.
They didn't bother to go to the trouble and expense of changing the mould to adapt the box to BBS, so two wider base rims of 35mm each crush bees when replacing boxes. Not only that, but that width guarantees that the rims are propolised to the ends of the top bars below.
The dimension between opposite lug recesses is excessive and frames can offset easily leading to side bars propolised to end walls. No big deal? Wait till you waste time ungluing frames when they're covered in bees.
The top box rims were re-designed to curve to match a re-designed roof; the curves were found to be a weak point (no field trials, clearly) and Swienty returned to using the original square-cut roof. Did they re-mould the box rim to match? No, of course not. Is this junction thermally efficient? Doubt it.
The Ashforth feeder was also designed as TBS so it has a flat base; if you want to run it BBS then an 8mm rim of stripwood will have to be fitted (more parts and labour).
By the time the glue and bits and paint are paid for and labour used to fix this dog's dinner of a hive, you'd be far better off with Abelo, a one-piece unit that's ready painted and can have bees in it two minutes after the delivery driver has left.