What underlayment goes beneath a metal roof?
The underlayment depends on roof slope, deck, climate, code, metal type and manufacturer instructions. Common categories include synthetic underlayments and self-adhered membranes at vulnerable areas. High-temperature-rated products may be required beneath some metal assemblies.
What affects the answer?
- Manufacturer approval and local code
- Steep versus low slope
- Eaves, valleys and penetrations
- Potential high surface temperatures
- Vapour permeability and assembly design
- Temporary exposure during construction
What homeowners should understand
Underlayment is a secondary layer; it does not justify weak panel seams or flashing. Self-adhered membrane is often concentrated where ice or backed-up water may occur, but covering an entire deck can change drying behaviour. The roof and attic moisture strategy should be considered together.
Ask the contractor to name the product, coverage pattern and fastening. Generic terms such as ‘ice shield’ or ‘synthetic’ are not enough. Confirm compatibility with metal, sealants and any treated wood.
Questions to ask your roofing contractor
- What exact underlayment is specified?
- Where will self-adhered membrane be installed?
- Is it approved for temperature and assembly conditions?
The bottom line
Use the underlayment required by the tested roof system and local conditions. It is an important backup layer, not the primary roof.