Does It Hurt the Roof Sheathing to Get Rained On?

Roof Sheathing

• Roof sheathing comes in 4-by-8-foot sheets of either 1/2-inch-thick plywood or oriented strand board (OSB). Both types of sheathing contain laminated and compressed bits, or strands, of wood. Both types of sheathing are subject to damage from water. When saturated, the sheathing fibers swell and lose their ability to provide a structurally sound surface for installing the shingles. When this occurs, you must replace the compromised sheathing.

Sheathing Protection

• As soon as the sheathing is in place, the roofing contractor will install a protective moisture barrier. Typically, this is 15- or 30-pound asphalt-infused felt paper that comes in 3-foot-wide rolls. Once the felt paper covers the sheathing, the roof will withstand some rain, but it’s still important to get the shingles on as soon as possible. If the wind catches the edge of the felt paper and tears it, water can leak in and damage part of the sheathing.

Installing the Sheathing

• Because sheathing expands with moisture, the roofing contractor will leave 1/8 inch gaps between the individual sheets where they meet on the center of a rafter. If the sheathing does get wet, this little expansion gap will accommodate slight swelling without buckling. Likewise, the contractor will cut the edge of the sheathing so it is 1/8 inch shy of where it fits on the barge rafters, which are the outside rafters of the roof.

Best Practice

• The best roof construction practice is to store sheathing out of the weather until you’re ready to install it, and don’t install it if rain is in the immediate forecast. Once the sheathing is on, immediately install the moisture barrier, and then install the shingles as soon as possible after the moisture barrier is in place.