It is a very popular misconception that METAL in the vicinity of a
ladder-line or open-wire line causes bad loss in the line.

It doesn't! Simply because metals have a very low resistance. Line
conductors are made of it.

There IS an effect however, the magnitude of which depends on whether the
foreign metal is in small pieces, as with staples or clamps, or continously
distributed as when an insulated line is lying across a galvanised
corrugated iron sheet, touching only at intervals.

Each of the two wires has a stray low-loss capacitance to the foreign metal.
The pair of stray capacitances are in series which approximately halves the
effect. The effect is a decrease in Zo.

Now line attenuation in dB per 100' is directly proportional to R / Zo where
R is line conductor resistance. So the small extra capacitance does in fact
result in a negligible increase in line loss. But even less loss is induced
in the foreign metal especially if it is bulky or has a large surface area
and is not resonant.

The small increase in capacitance is equivalent to an un-noticeable shift in
tuner settings.

Where loss DOES occur is when the close-by foreign material is lossy such as
the surface of damp garden soil. Loss falls off again when the material is
an insulator such as dry sand although the increase in capacitance still
occurs.

Maximum loss occurs when the foreign material has a resistivity which
fortuitously "matches" into the line impedance.

But the existing rule still applies - avoid suspending twin-line nearer to
ANY material less than 3 or 4 times conductor spacing over appreciable long
distances. Even though copper sheet may be OK.
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Reg, G4FGQ