Why metal roofing should not be installed directly on plywood

By Jesse Sanchez. 

Proper underlayment helps protect the roof deck from condensation, friction and water intrusion. 

A metal roof is designed as a complete system and the layers beneath the panels are just as important as the finished surface. According to Mountaintop Metal Roofing, metal roofing should not be installed directly over bare plywood because doing so removes a critical layer of protection between the roof deck and the metal panels. Roofing underlayment, whether felt or synthetic, serves as a moisture barrier over the plywood decking. This is especially important in damp climates such as Washington and Oregon, where changing temperatures can cause condensation to form on the underside of metal panels. Without underlayment, that moisture can come into direct contact with the plywood, potentially
contributing to mold, warping and wood rot. 

Underlayment also helps address the natural movement of metal. As temperatures change, metal panels expand and contract. When panels sit directly against rough plywood, that movement can create friction that wears against the back of the metal. Mountaintop Metal Roofing says a smooth slip sheet can also be installed over the underlayment to help the metal move without damaging the waterproof layer beneath it. 

The underlayment provides another line of defense against water intrusion. If wind-driven rain gets beneath a seam or flashing, the barrier can help direct water away from the roof deck and toward the gutters. 

In some cases, metal roofing can be installed over an existing layer of asphalt shingles if the shingles are flat and the underlying structure is free of rot. Mountaintop Metal Roofing describes this approach as a “re-cover,” which can reduce tear-off labor and keep existing shingles out of landfills. 

Before installation, the company says crews should inspect the plywood decking, replace damaged wood and verify that the deck can securely hold the roofing fasteners. 

Learn more about Mountaintop Metal Roofing and the installation practices it recommends for residential metal roofing!