In respect of Garth Cambray's comment on the bees liking the extra space in
the lid so that mats may be a bad idea, perhaps this is the reason of
ensuring there is a margin of space so bees can move into the lid.  I assume
they sometimes use the lid to draw cooler air through the hive as sometimes
I lift the lid and find a mass of bees on top of the hive mat.  These are
likely to be highly productive hives.  I visit hives at least once a month
between August and the end of May and when a honey flow is on the hive may
have added burr comb above the frames if there is no mat, making it more
difficult to open and more difficult to move the bees off the frames.  With
a hive mat, even with a box overflowing with bees it is easy to lift off the
mat and place it near the entrance.  You can then start working the hive
with minimum attention from the bees.
 
Betty McAdam
HOG BAY APIARY
Penneshaw, Kangaroo Island
j.h. & e. mcadam<[log in to unmask]
http://kigateway.eastend.com.au/hogbay/hogbay1.htm