[jeff_taylor]

Both steel and aluminum are sold by "gage" in the US. Steel gage and aluminum gage of the same number refer to different thickness. Here's some common sizes:

gauge_________steel_______aluminum
10.....................0.135" .............0.100"
11.....................0.120" .............0.090"
12.....................0.110" .............0.080"
14.....................0.075" .............0.063"
16.....................0.060" .............0.050"
18.....................0.048" .............0.040"
20.....................0.036" .............0.032"
22.....................0.030" .............0.025"
24.....................0.024" .............0.020"

As you can see, there is quite a difference in material thickness between steel and aluminum of the same gage. Galvanized steel sheet is about 0.004" thicker for a given steel gage number. It is best to always specify the gage and decimal thickness of material to make sure you get what you need.