I'm planning on building a 10 meter ground plane antenna out of PVC pipe and #12 wire radials drooping about 40 degrees. This will be mounted on my roof on a tripod and mast, with the radials also serving as guy wires. The ARRL antenna book mentions that a ground plane antenna should be mounted at least one half wavelength above "ground". I know that this sounds like a stupid question but I have to ask: for this example do I consider the roof, which is more than 5 meters above ground level as "ground" or do I need to be at least 5 meters above the roof. I would like to secure the radials, (guy wires) to my roof, but with the antenna at 5 meters high my droop angle will be off and then I can't guarantee a 50 ohm match. The house is wood frame, no foil backed insulation and a non metallic roof. Thanks for any advice.. Lenny