Forgot, dimensions are sixteen and five eighths by eight and a half inches. Use a mix of 50 and 25mm so it;s easier to adjust space or even when inspecting, as you can see, there is no dummy board space. Remove a rear 25mm and the other board can move to give you space.
I run my bees this way as they seem to prefer it and I find it useful and versatile, the boards are convenient and light. I've used abelo roofs for years as I found them much better than a wood/celotex combo. Insulated, do not leak, no constructing and all for basically the price of a second shallow roof.
Now just to chuck a spanner in the works, I have had to press some old stuff back into service and I have old wooden roofs with no insulation and the bees in these hives are just as happy as the rest.