Underlayment is a general term used to describe a membrane installed between the steel sheets and the sheathing (plywood or OSB) or roof framing. There are a variety of materials used to manufacture underlayments with the most common being an asphalt impregnated organic fibre (roofing felts). The minimum weight of roofing felt should be equivalent to a #30 (30 pound). There are also premium synthetic products available that provide improved performance where required or desired.
The underlayment also provides a valuable second layer of protection against water getting into your home whether from wind-driven rain or from any condensation that may still occur on the back of the steel sheets. The only situation where an underlayment may not be necessary is an un-heated building (e.g. garage or storage shed) that does not contain any source of moisture (e.g. livestock or humid materials) or materials that could be damaged from possible moisture.
Click to download Fact Sheet 35: Residential Steel Roofing Installation Considerations to learn more about underlayment, strapping and building code requirements for roof installations.