Roof Deck Preparation, Underlayment, and Ice Dam Protection
Deck and underlayment choices decide whether the shingles lie flat, drain, and survive heat and ice. This domain tests solid sheathing, deck tolerance, high-temperature underlayment, ice-dam protection at eaves and valleys, and the separation layers some metal systems require.
Core rules
- Metal shingles need a solid, structurally adequate deck, usually plywood or OSB rated for the rafter spacing. Repair soft spots, replace delaminated sheathing, and fasten to code before covering.
- Flatten the plane. Check with a straightedge; abrupt joints, raised fastener heads, and sheathing gaps telegraph through thin shingles. Stagger end joints and leave the code-required edge gap.
- Use an underlayment approved for metal: high-temperature self-adhered membrane or a specified synthetic. Asphalt felt can soften and tear under hot metal.
- Ice-dam protection belongs at eaves, valleys, and penetrations. Install it from the eave to at least 24 in. inside the warm wall line, and follow the manufacturer when it requires a longer run.
- Some painted or stone-coated products need a slip sheet or a specific underlayment to prevent abrasion and backside corrosion. Do not substitute rosin paper.
Worked example
A re-roof has 3/8-in. sheathing at 24 in. o.c. with visible deflection. Adding underlayment will not fix the plane. The correct fix is to redeck with sheathing rated for the span, fasten per code, then install the ice barrier and approved underlayment. The reasoning: metal shingles are thin and reflect every deck defect; deflection also changes fastener embedment.
Traps
Skipping ice barrier because metal does not leak; using 15-lb felt under a dark metal roof; installing over skip sheathing without the manufacturer's approval; failing to protect the underlayment at the eave with drip edge.