The Scottish Beekeepers Assoc. national hive plans:
http://www.scottishbeekeepers.org.uk/learning/html/plans.html
are a good place to start.
Basically, you just need to make a box with inside dimensions of 18 ¾ inches by 18 ¾ inches by at least 5 ¾ inches and put some bits of wood inside it to keep it from sitting directly on the crown board.
Here is one way of doing it:
Get a bit of 12mm ply. If you want it to last, get “WBP”. If you want it to last a little bit longer, but be much much heavier, get 18mm WBP and use the dimensions in brackets.
Cut a square 50cm x 50cm – this is the top of the roof.
[51.2cm x 51.2cm for 18mm ply].
Now cut four bits that are 48.8cm x 15cm – these are the sides.
[49.4cm x 15cm for 18mm ply].
Glue and nail / screw the sides to the roof and to each other. They fit inside the footprint of the roof and one edge of each side forms a corner of the roof (the picture might help explain what I mean).
Now get four bits of wood that are 45.7cm x 32mm x 18mm [same size for 18mm ply]. These are glued and nailed / screwed inside the top of the roof, so that when it sits on the crown board there is a 1¼ inch (32mm) head space between the crown board and the inside of the roof.
If you want some vent holes in the roof, drill 18mm holes through the side of the roof into this head space – but you’ll need some mesh to cover the inside of the holes.
If it is going to last any time outside, you will need to seal all the edges of the ply with PVA or similar, and I also paint them with a (kind to bees) preservative. You will probably also want to put a roof covering on. This could be roofing or shed felt, which will last a few years, or it could be a ‘tin’ roof. You can buy tin roofs from hive suppliers or from Bee Hive Bits (google them).
You may already have a tin roof cover from your existing roof. If you do, you may need to adjust the dimensions of the wooden parts to fit the tin roof (or adapt the tin roof to fit these dimensions).
