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Restoration Guide: Walls Sheathing

Rob Sabo

Editor's Note: This is article 4 of 18 in Chapter 2: Exterior Walls of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.

4. SHEATHING

Section 1--Overview

Exterior wall sheathing provides structural stability, rigidity and shear resistance to wood framing. Installed behind wall coverings such as siding or stucco and a vapor retarder, sheathing also comprises a third line of defense for moisture protection and provides a modicum of insulation.

Carpenters who built old houses typically used 1-by-3-inch or 1-by-4-inch boards for sheathing, but since the 1960s plywood has been the most common sheathing material. Recently, oriented strand board (OSB) has been used in place of plywood due to lower production costs--in some areas of the country it's nearly half the cost of plywood. Both materials are rated equally for sheer strength by the Engineered Wood Association, but many carpenters prefer plywood over OSB for a variety of reasons. The edges of plywood panels better resist chipping on tighter nailing patterns, and they hold up better in moist conditions than does OSB, whose edges can swell up to 15 percent when wet.

During a home renovation, the sheathing should still be usable if moisture has not been trapped behind the siding. Moisture can be trapped between wall materials in a number of ways:

  • Improperly flashed/caulked joints
  • Inadequate flashing at roof lines
  • Water driven by excessive wind
  • Moisture penetration through mortar joints
  • Lack of a vapor retarder

Section 2--Repairing or Replacing Existing Sheathing

If the home restoration project has older board-style sheathing, it is likely that age has taken too great a toll on the material for it to be reused with any structural integrity. However, if the majority of wood sheathing is structurally intact, it's possible to replace small sections of damaged material. If existing plywood has rotted, it offers no shear resistance and must be replaced with either new plywood or OSB sheathing.

2.1: OSB Sheathing Benefits

Oriented strand board offers excellent shear resistance and stability. It's cheaper than plywood because it's made from wood waste that's been ground into thin strands, then mixed with wax and resins, and hot-pressed together. It's gained acceptance in the construction industry and with preservationists because it has significantly reduced demands placed on old growth timber.

Another big plus with OSB: It can be special ordered up to 24 feet tall and 12 feet wide.

2.2: Plywood Benefits

Plywood has different grades depending on how many repairs were made with putty to fill defects such as knot holes. Structural-grade plywood is considered the highest-rated sheathing material on the market due to its excellent structural and weathering characteristics. It can be used for nailing any type of wall cladding material, including fiber cement siding, stucco, and brick veneer. Although plywood has lost a great deal of market share to OSB in the past decade due to its higher cost, it's still preferred by many general contractors and sometimes is specified by engineers in areas with high seismic activity or moist climates.

Section 3--Environmentally Friendly Sheathing Panels

3.1: Fiberboard Sheathing

Fiberboard is made from recycled newspaper, wood fiber, and other types of cellulose products that are bound together. It's a widely used building product in many areas, but it's not typically found in western states. High-density fiberboard can be used for sheathing, and it provides sufficient backing for stucco and wood siding. One drawback of fiberboard sheathing is that it can't be used as a nailing base for vinyl or aluminum siding--the siding must be nailed to wall studs.

3.2: Gypsum Sheathing

Gypsum sheathing is a wax-treated wallboard with a core of gypsum and a water-resistant paper covering on its face. It is typically sold in 4-by-8-inch or 2-by-8-inch sheets. It's typically used for brick veneer, stucco, and Exterior Insulated Finishing Systems.

3.3: Glass Mat-faced Gypsum Sheathing

This material combines glass matting embedded in a water-resistant, silicone-treated core of gypsum and generally outperforms standard gypsum sheathing. Another benefit: Unlike regular gypsum sheathing, which must be covered by wall cladding within four weeks of installation, glass-matted gypsum sheathing can be left exposed for up to six months. It's disadvantage: It's nearly twice as costly as regular gypsum sheathing.

3.4: Paperboard Sheathing

Paperboard sheathing is popular with many of the nation's largest home builders. It's code approved for structural sheathing, yet it is significantly less expensive than other sheathing types. However, it's not as strong as OSB or plywood sheathing, and it's harder to work with on a small home restoration project because it's extremely thin. It also can't be used as a nail base for wall siding.

3.5: Fiber-cement Sheathing

Fiber-cement siding is preferred by many builders underneath Exterior Insulated Finishing Systems because it's extremely durable, non-combustible, and rigid. However, it's also among the costliest wall sheathing materials.

For most home restoration projects, plywood or OSB sheathing is more than adequate to replace older board-style shear paneling. Technological advances and a push for greener methods in the construction industry have led to the inception of different types of environmentally friendly sheathing materials, but most of these materials have higher price points than standard sheathing.

Sources
http://ww.hud.gov,
OSB vs. Plywood • http://www.pathnet.orghttp://www.pathnet.org/sp.asp?id=17336 • Partnership for Advancing Technology in Housing


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