Underlayment for Metal Roofs: What Goes Under the Panels

Step by Step

1. 1Step 1: Inspect and prepare the deck

   Any bump in the deck telegraphs through metal, especially through thin panels, and any nail head that is proud will wear a hole in the underlayment as the panel moves. Hammer down or remove loose nails, plane down uneven sheet joints and replace damaged plywood.

   Sweep the deck clean and dry. Self-adhered membranes will not stick to dusty, damp or frosty sheathing. In cold weather, prime the deck if the membrane manufacturer requires it, and follow its minimum application temperature, which is commonly around 5 degrees Celsius for ice and water shield.

2. 2Step 2: Install self-adhered ice and water shield at the eaves

   Start at the eave with the membrane edge flush to the drip edge or overhanging it slightly, press it down firmly and continue up the roof. The BC Building Code's eave protection rule is written for shingle, shake and tile roofs (at least 900 mm up the slope and 300 mm inside the inner face of the wall), and it is a sensible minimum under metal too. On a typical 12 to 16 inch overhang that means two courses of 36 inch membrane. In snow country such as Hope and up the canyon it is common to go higher. Follow the panel manufacturer and your local building department. For full detail, see how to install ice and water shield.

   Choose a high-temperature membrane for metal. Standard ice and water shield is rated for shingle roofs and may soften under dark metal in summer. Products rated for metal list a service temperature of around 120 degrees Celsius. Check the product data sheet.

   Tip: Lay self-adhered membrane in the warm part of the day, peel the release film back a foot at a time and press from the centre outward to avoid wrinkles.

3. 3Step 3: Protect valleys, penetrations and roof to wall edges

   Valleys get a 36 inch wide strip centred on the valley, with each side lapping up the slopes. Around pipes and skylights, cut the membrane to the opening and press it tight into the corner so it seals around the penetration. At a sidewall, run the membrane up the wall by 4 to 6 inches and out onto the deck.

   These are the places where metal roofs leak first, typically because a trim or flashing allows water past the panel. The membrane gives a second chance. See metal roof valleys and metal roof wall flashing for the trim details that go over it.

4. 4Step 4: Roll out the main underlayment from eave to ridge, lapping shingle-style

   Snap a chalk line so the first course is square to the eave. Lap horizontally in a shingle fashion so water flows over every seam and never into it. End laps are commonly 6 to 12 inches and are staggered between courses. Overlap the ice and water shield at the eave, with the main layer lapping over the top of the self-adhered course.

   Use only the fasteners the product calls for, normally plastic cap nails or cap staples, at the spacing on the roll. Do not use thin staples that pull through. Cover the roof promptly after uncovering it, or keep it within the UV exposure limit on the label, which may be as short as a few weeks for some synthetics.

   Watch out: Never use standard 15 or 30 pound asphalt felt under a metal roof unless the panel manufacturer expressly approves it. Felt can dry out, bond to the panel and fail long before the panels do.

5. 5Step 5: Lap membrane over the ridge correctly

   Run the upper courses up past the ridge by a few inches and lap them over each other, or lay a single continuous piece centred on the ridge. On a vented metal ridge, hold the underlayment back from the ridge opening so air can move, or cut it to the width of the vent slot as the ridge detail requires. See ridge and hip caps.

   At hips, cut the courses square and lap them with a cover strip of self-adhered membrane over the hip line for extra protection.

6. 6Step 6: Add a slip sheet or separation layer if the panel calls for it

   Certain panels, especially clipped standing seam and snap-lock panels, slide against the underlayment as they heat and cool. A slip sheet, typically rosin paper or a proprietary slip layer, sits between the membrane and the panels to allow this movement and to reduce sticking. A self-adhered membrane directly under the panel can grip it and cause noise or oil canning, so the slip sheet matters more in a hot climate.

   Some high-temperature synthetics are built with a slick surface and need no separate slip sheet. The panel manufacturer's installation manual will state which is required. See thermal movement in metal roofs for why.

7. 7Step 7: Plan condensation control and ventilation

   Metal has no vapour permeability and its underside is cold at night, so warm humid air from the house can condense under the panels and wet the deck. The defences are an airtight ceiling below, a ventilated attic, and in some assemblies a ventilated airspace created by battens or strapping under the panels, with open intake at the eave and an exhaust at the ridge.

   Metal roofs installed over an existing shingle roof with strapping should have continuous airflow through the strapped cavity. Closure strips that seal the eave and ridge against birds and bugs must still allow vent paths where the design depends on them. Match the closures to the design, not the other way around.

Synthetic, Self-Adhered or Felt: How to Choose

Fraser Valley Notes

Common Mistakes

• Using standard felt or shingle-rated underlayment under dark metal, where heat softens it and it sticks to the panel.
• Skipping the self-adhered layer at eaves and valleys, where wind-driven rain and ice dams push water up under the panel.
• Leaving the underlayment exposed beyond its UV rating while waiting for panels to arrive.
• Using the wrong fasteners, such as plain staples that pull through the underlayment in wind before the panels are on.
• Sealing the eave and ridge with closures and leaving no ventilation path on a vented design, trapping moisture.
• Installing without a slip sheet where required, which can cause abrasion, sticking and noise.