Underlayment for a Metal Roof

• ●Do not use asphalt-saturated felt. It wrinkles when it gets damp before the panels go on, it softens against a hot panel, and it tears at fastener shanks when the roof moves.
• ●A heavy synthetic is the default field sheet for a vented, solid-decked roof at normal slope — flat, walkable, and a fraction of the cost of peel-and-stick.
• ●High-temp self-adhered membrane goes where water sits or gets driven: eaves in a snow climate, valleys, headwalls, sidewalls, every penetration — and the whole roof if the slope is low, the roof is complicated, or the panel is through-fastened.
• ●“High-temperature” is not marketing. Standard self-adhered ice-barrier membrane is formulated for the temperatures under a shingle. A dark panel over a tight deck runs hotter, and an asphalt membrane that softens will bond itself to the underside of a panel that has to slide.

The complaint is not that felt is a bad water barrier on day one. It is that a metal roof subjects it to three stresses a shingle roof does not.

1. It telegraphs

Felt absorbs water. Roll it out Thursday, get a shower Thursday night, and by Friday morning it has cockled into ridges between the fasteners. Under a shingle nobody sees it. Under a flat standing seam pan — wide, uninterrupted, often reflective — every ridge reads as a shadow line down the roof. It is not a leak, but it is what the homeowner photographs.

2. It gives up at the fasteners

Felts have modest tear strength, and a metal roof puts cyclic shear right at the fastener. On an exposed-fastener panel the screw goes through panel, underlayment and deck, and it sees the panel expand and contract every day of its life; a standing seam clip fastener sees the same movement. Felt elongates the hole. A woven synthetic scrim resists it, and a self-adhered membrane gaskets around the shank.

3. It sticks to the panel

This is the failure that surprises people. Asphalt softens with heat, metal panels get hot — the darker the finish and the tighter the deck, the hotter — and a softened asphalt sheet in contact with a panel underside tacks to it. Now a panel designed to float is partly bonded to the deck. It still tries to move, so instead of sliding it buckles: oil canning and stressed seams. When it finally breaks free it takes the membrane surface with it.

A metal panel is a thin, conductive skin with a clear view of the night sky. On a still, clear night it radiates heat and can drop several degrees below the outdoor air temperature. If the air touching its underside is at or above the dew point at that temperature, water forms on the back of the panel. Not a leak — a phase change. It drips, stains a ceiling, rusts fasteners from the inside and rots a deck, all on a roof that has never once let rain through.

How much that matters depends on what is under the deck, which is why “what goes under metal” is really three different questions:

Over a vented attic with a sealed ceiling

The easy case. The attic sits near outdoor conditions and little moisture reaches the deck. A synthetic field sheet with self-adhered membrane at the vulnerable details is a defensible, widely used spec. What hurts you here is air leakage from the house into the attic — a ceiling problem, not a roofing one.

Over an unvented, insulated cathedral assembly

Now the deck is the drying plane, and a fully adhered membrane on top of it means the deck can only dry downward. That can be fine — plenty of unvented assemblies are built exactly that way — but only if the interior side is air-sealed and the insulation keeps the deck above the dew point. That is an assembly decision, not an underlayment decision.

Over open framing or purlins

Barns, shops, carports. There is no deck, so there is no underlayment in the normal sense, and condensation on the panel underside is not a risk — it is a certainty on the right night. The purpose-built answer is a fleece-backed anti-condensation layer factory-applied to the panel underside, which holds condensate until it can evaporate again. The other answer is to insulate the underside. Ignoring it is not an answer.

For a typical residential standing seam roof over a vented attic and solid sheathing, in a climate that sees ice:

• ●High-temp self-adhered membrane at all eaves, run far enough up-slope to satisfy the local ice barrier requirement — which is measured from the roof edge to a point inside the exterior wall line, so a low-slope roof with deep overhangs can need two courses.
• ●The same membrane full-width in valleys, up sidewalls and headwalls behind the flashing, and collared around every pipe, curb and skylight.
• ●Heavy synthetic in the field, lapped per the printed instruction, fastened with cap fasteners rather than staples, never left exposed past its published limit.
• ●A decision on condensation control made before anything is ordered — vented attic, unvented assembly with interior vapor control, or above-sheathing ventilation. Pick one on purpose.