Decking Materials and Their Thicknesses for Residential Roofs

Rafters are commonly used to frame roofs. A Katy roofing contractor will also employ rafters connected by internal braces, which can be installed as a single component called trusses. The rafters extend from a roof’s peak to its eaves. In most cases, some kind of sheathing or decking, such as plywood or oriented strand board (OSB), is needed to cover the roof’s rafters before the application of finishing roof materials. These are panels that are made from layers of bonded wood. The plywood that is used for a roof’s decking is referred to as sheathing. It is marked with a grading stamp that indicates its performance factors, such as load limits and span ratings. The sheathing will bear the brunt of your roof’s protection. It has several duties.

  • The roofing material is nailed to the sheathing
  • It keeps water, snow and ice out of your house
  • It provides supports over the rafters or trusses
  • It help guard your home against earthquake and wind shear damage

Layers of Wood Create Plywood

Plywood is composed of three or more thin layers of wood that are glued together. Extra strength is gained because the grain is alternated. Plywood is flexible, inexpensive, durable and easy to cut with a circular saw or other handheld cutting tool. Katy residential roofing companies generally use 4-by-8 foot sheets in thicknesses from 1/4 inch to 1 inch, but longer panels are available if required. Standard sheets cover 32 square feet. Plywood that is thicker than 1/4 inch is usually made up of up to five layers of wood.

How to Determine the Right Thickness

When deciding how thick the plywood for Katy roof replacement should be, factors such as slope, rafter spacing and how heavy the finishing material will be are critical. External factors, like the amount of snow or ice that can be expected, also play a part in choosing the thickness of the sheathing. This is called the design load.

If the rafters are 16 inches apart and have minimal roof loads, then 3/8-inch sheathing will suffice. Flatter roofs must endure a greater load and steeper roofs have a lighter load per square foot. All plywood used for exterior sheathing should be marked “Exposure 1” or “Exterior.” This means it has been rated for exterior use and will withstand exposure to moisture until the finishing material has been put in place.

When rafters are greater than 20 inches apart, the recommended thickness for the decking is 1/2 or 5/8 of an inch. The most common rafter spacing is 24 inches, which requires 5/8-inch plywood. If the roof will have a heavy load, 3/4-inch thickness may be required.

Oriented Strand Board (OSB)

Decades ago, plywood and particle board sheathing were common. However, today, OSB and 1/2-inch plywood are used almost exclusively. OSB is composed of rectangular strands of wood that are glued and pressed together. It costs less than plywood, but it is still quite strong. Plywood is used more frequently, but OSB will probably become the preferred sheathing material in the future.

OSB panels have an orientation, which is shown by arrows that are printed on it to indicate the direction of the strength of the board. The grain is usually in the length axis. Longer sections are also available that are meant to be used vertically, so the grain runs across the width. Grain should always run perpendicular to the rafters or trusses.

Problems With Sheathing

Some common problems that can expose sheathing to moisture include the following.

  • Improper attic ventilation
  • Leaking flashing
  • Ice dams in the gutters

Look for water stains or dampness in the attic. You can also check the sheathing with an awl or ice pick to see if it is sound.

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