Hi, Twinsen!
You are living in single story house. If you have 40 meters half wave dipole in your roof, than you have about 20 meters of roof in one direction - it is a big field (almost enough to play football
! Please, tell me:
What is real size and form of your roof, is it flat or high?
Could you maintain vertical mast in center of your roof?
Is your house made of reinforced concrete?
Have you some points to fix wire outside of your house?
I think, dipole lying flat in a roof can not work well. At least it can not transmit well. I know a curios fact that polar stations in Antarctica simply stretch a dipoles on the ground (They have an ice under feet and this deep ice is a nice dielectric). I think you are living not in antartic and your roof is not perfect dielectric. And all antennas with open end (dipoles) need good height (at least 1/4 of a wave length) . Antennas with closed wire (quads, loops) do not need big height.
About magnetic loop. I've made magnetic loop at 1983. It was aluminium circle (hula hoop). I made a coil inside a pipe with a number of turns with fluoroplastic isolated wire. It was tuned to resonance by air isolated capacitor. The main purpose of this antenna was to clean QRMs from jammer located few kilometers near by me. This hula hoop antenna allows me to listen a lot of propaganda from abroad of USSR
. This antenna has near round diagram with very deep hole in side direction. The hole was near -25 -40 dB to main direction signal. I tried to use it in HAM radio band in 40 meters to clean from "Radio Tirana". But it was not enough efficient to listen weak amateur signals from DXs.
73!