- Q.I am
considering installing steel roofing over 2x4 pressure-treated
purlins. Can the chemicals used to treat lumber attack steel
responds: I would not recommend using pressure-treated
purlins under steel roofing without taking special precautions.
A metal can corrode when minuscule amounts of electricity
travel through an electrolyte connecting dissimilar metals.
Water is an electrolyte. Since condensation is very likely to
collect on the underside of metal roofing, you have an
electrical conduit waiting to be plugged in.
Most pressure-treated wood is treated with chromated copper
arsenate. The metal present in the treatment chemical (copper)
is dissimilar to the metal roofing (steel). I would be
concerned that galvanic corrosion could degrade the metal
roofing or fasteners.
Galvanic corrosion occurs when electrons move away from an
anode toward a cathode. This electron transfer causes the anode
to degrade, while the cathode stays intact. If you remove one
of the three components (the anode, cathode or electrolyte),
you can prevent corrosion.
The easiest way to avoid this potential problem is to use
untreated purlins. If you insist on using pressure-treated
purlins — for example, for termite protection — you
could paint the purlins or the back of the roofing to break the
electrical connection. Perhaps installing a plastic spacer or a
strip of self-adhering roofing membrane between the metal and
pressure-treated purlins would be a more practical way to
separate the dissimilar metals.