An alternative to crown board (inner cover)
A hive mat is an alternative to a crown board (inner cover) and is commonly used in Langstroth hives in conjunction with a "migratory lid". A migratory lid (or outer cover) fits squarely on top of the rim of the uppermost box, rather than telescoping over it as does a standard roof, and the lack of overhang allows hives to be packed more closely for transport. They commonly have rims around 40mm deep to provide a little space underneath to accommodate shallow feeders, varroa treatments or similar. The lids are held in place with straps, or perhaps a handy brick when standing in the apiary.
This pattern of outer cover is frequent in Australia, New Zealand and the USA where hives are frequently transported, or "migrated", between nectar sources or pollination contracts. Commercial beekeepers in Australia commonly migrate their hives from forest to forest following seasonal flowering events.
The hive mat itself is a piece of thick plastic, such as PVC "lino" offcuts or damp-proof course (DPC), cut to be slightly smaller all round than the inner dimensions of a standard hive box. This will leave a gap next to the hive walls when the hive mat is rested on the top bars. The size of the gap is not critical but 2-3 centimetres is usual.
The purpose of the mat is two-fold. Firstly, in the same way as an inner cover it dissuades bees from moving up into the cavity under the lid and building comb. Secondly, it provides a drip-proof cover and a degree of insulation for the brood nest or winter cluster while allowing for a free flow of air. As the air is rising around the hive edges next to the walls, the brood nest should not be subjected to a chimney effect. An incidental advantage is that shallow feed containers can be stood on the hive mat in the roof cavity, or fondant or sugar can be placed directly on the mat.
The main disadvantage of the hive mat is that bees will sometimes propolise the edge of the outer cover, particularly if it does not fit well against the upper rim of the top box. This does not present any problem with a migratory lid because it can easily be lifted with a hive-tool. Some patterns of telescoping lid may prove harder to dislodge. One hive I came across, which belonged to another beekeeper and had not been inspected for at least five years and had no inner cover at all, required a hammer and chisel to open and was not in good shape afterwards. Beekeepers who prefer to block all upwards airflow will also not appreciate this system.
Advantages include drier hives in winter and better airflow for ripening nectar in comparison with a standard inner cover. I also find fewer interlopers such as spiders, slugs, earwigs and woodlice.
Having used hive mats for many years in Australia, I have continued to use them in parallel with hives with conventional inner covers in the UK. Most of my outer covers are of the migratory type. I have had no problems so far using hive mats with telescoping lids but will in future stick to conventional inner covers with these. I like to make equipment and continue to use (ventilated) migratory lids because they are simpler and cheaper to construct, as well as being lighter. Hive mats too appear to give equally good results in the UK, with drier hive walls and less mould developing on combs. There is no reason why hive mats could not be used with other patterns of hive.