What Types of Plywood Are Used for Roofing?
Roofers have a number of structural panels to select from when selecting an appropriate roof sheathing, including plywood, OSB, and zip wood. So, what are the differences in roof plywood?
Plywood gets its strength from multiple layers of wood bonded together. The wood grain alternates from layer to layer, adding significantly to the strength of the plywood sheet. A special grade of plywood called “CDX” is a popular choice for roof sheathing. No, the “X” in CDX plywood does not stand for “exterior.” Rather, it references the kind of glue used to bond the layers of veneer together. CDX plywood can withstand temporary exposure to rain and other moisture sources. For both roofers and homeowners, this characteristic of CDX plywood provides essential peace of mind. With CDX plywood, the three letters represent:
- C: The grade of the front of the plywood sheet
- D: The grade of the back of the plywood sheet
- X: The type of glue used to bond the layers
“A” grade plywood is devoid of knots and other imperfections, and it is the highest plywood grade. A C-grade surface is perfectly acceptable for use under shingles and roofing felt. The D-grade surface is perfectly acceptable on the underside of the plywood sheet. Overall, when it comes to roof sheathing, CDX plywood represents an ideal balance between price and functionality.
Half- or three-fourths-inch Plywood for Roof Sheathing?: Traditionally, half-inch CDX plywood was the common choice with 24-inch rafter spacing. However, heavier roof loads may require the use of three-fourths-inch CDX plywood for roof sheathing. Roofs with less pitch often carry heavier loads, including snow and ice accumulations in the winter. If plywood of inadequate thickness was used on your roof, a complete roof replacement is more likely.
Plywood vs. OSB (oriented strand board): Ironically, OSB takes longer to get saturated when wet, but it also takes substantially longer to dry. The edges of OSB may noticeably swell when exposed to moisture. Although OSB manufacturers address this with a special edge sealant, edges cut on-site remain vulnerable to this quality of OSB. The swollen edges may create “ghost lines” that show right through the shingles.
ZIP wood is an engineered product known for its strength and durability. A key to ZIP wood is the use of a water-resistant barrier that still breathes. This vapor permeable barrier eliminates the need for roofing felt. All five layers of ZIP system insulated sheathing are moisture-resistant, making it easy to work with in places like Montgomery County known for such variable weather.
The simplified two-step installation reduces labor costs by as much as 40% when compared to traditional plywood sheathing and felt installations. Roofers simply install the panels and tape the seams. The final result is a continuous, rigid air barrier that greatly reduces air leakage. Homeowners enjoy greater energy efficiency, lower utility bills, and a reduced carbon footprint.
Because a fire can spread from townhome to townhome, building codes often require the use of fire-retardant (FRT) plywood for roof sheathing, depending upon the design of the structure. Fire exposure to FRT plywood initiates a process of acid hydrolysis that increases fire resistance at a critical time.
However, In the 1980s, an estimated 1 million housing units were built with potentially defective FRT plywood. Although today’s sheathing does not present the same challenges, FRT plywood from several decades ago may degrade prematurely. Roof replacements are often the solution in such cases.
At BRAX Roofing, we work to stay on top of the latest developments in energy-saving, long-lasting roofing materials. Please contact us today, and we’ll promptly arrange for a visit by one of our friendly, knowledgeable estimators. We’ll patiently answer your questions as you come to a roofing decision that is in your best interests.
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