Roofing materials are designed to keep water out of the structure beneath the roof. Metal roofs perform this task well, but in residential roofing, building codes usually require the installation of an additional moisture barrier.
In other building scenarios, no moisture barrier is necessary beneath the sheets of metal.
Residential Metal Roofs
A standard roof consists of rafters or trusses, which give the roof its shape and pitch, that are then covered by roof sheathing. The sheathing, which may be plywood or oriented strand board (OSB), expands when it gets wet.
The installation of a moisture barrier over the sheathing protects it during roof construction. The moisture barrier offers limited protection if condensation occurs beneath the metal roofing.
Sheds, Barns and Outbuildings
Metal roofing originally was reserved for the construction of barn and shed roofs because it was available only in unattractive, corrugated or flat sheets. This type of metal roofing is still popular for agricultural buildings and other outbuildings and is often installed over the rafters or horizontal strips called purlins.
These buildings are not built to be watertight or climate-controlled, so there is no reason to install sheathing or a moisture barrier between the rafters and the metal roofing.
The Moisture Barrier
On a home, the roofer will install 15- or 30-pound asphalt-infused felt paper over the sheathing to provide a moisture barrier. Because rain is the enemy of wood sheathing, the installation of the felt paper takes place right after the sheathing is installed.
Felt paper comes in 3-foot-wide rolls, and the roofer starts at the bottom edge of the roof and rolls out the paper, stapling it down. The paper features lap marks that indicate how much the next roll of paper must overlap the previous row.
Metal roofing rarely leaks unless the metal is cut to install a vent, and then that spot becomes a weak area. If the area around the vent leaks, the felt paper beneath the metal roofing will help shed the water down the outside of the roof, instead of allowing it to drip onto the sheathing.
It’s not a foolproof system because the felt paper is also cut during vent installation. In addition to protecting the sheathing from moisture, felt paper dampers some of the sounds associated with metal roofs popping as they expand and contract with temperature changes.