At a former job we HAD to receive WWV (10 or 15MHZ).
It was winter and icy, so all I was willing to do was "toss"
a wire on our flat roof. We were in the old university gym,
with a large, flat roof. About 100'X200'. I ran the wire
lengthwise. The roof was wood, covered by several
layuers of tar, paper and gravel. The antenna worked
well enough to "get by" but the first day the roof was clear,
I errected a better antenna, by elevating the wire on 10'
EMT sections. It was a difference like night and day.
The only change was the elevation. So, while an antenna
will work laying on the roof, it will work much better up in the
air over that roof. The electric shop was very unhappy to
have to install 2 ~60' wire to 8' ground rods.
Whne we took the antenna down 12 years later, internet
time services made WWV old hat, I scavenged the wire,
ground were and rods and the EMT and fittings. I was told to
remove that "old stuff" and "throw it away". It went into my
car. I used (and still use) the ground rods and heavy copper
ground wire, and still have the antenna wire rolled up. I hope
to errect it after I retire and "move to the country". We bought
the good, heavy hard cooper wire and they paid big bucks to
get it there the next day.

Terry