Restoration Guide: Roof Sheathing

Editor's Note: This is article 3 of 12 in the Roofs Chapter 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.

3. SHEATHING

Section 1--Overview

Roof sheathing--the material nailed to your trusses or rafters to which your roofing is attached--is your roof's workhorse.

Sheathing is:

• The anchor of the system that keeps water out
• The base onto which you nail your roofing
• The major part of the brace system for your trusses/rafters
• A key piece of the stiffening system that protects your house from earthquakes and blasts of wind

Sheathing in old houses was typically 1 inch dimensional lumber, either butted against one another or spaced up to 4 inches and placed perpendicular to the rafters/trusses. It was common into the 1980s for cedar-shake roofs to have 1x4 boards spaced at three and a half inches. Old houses with asphalt shingles usually had either tongue-and-groove boards or straight-sided boards abutting.

In the 1950s, plywood sheathing and particleboard became common, and, today, most standard-construction roofs use either half-inch plywood or particleboard's big brother, oriented-strand board (OSB).

1.1: What Is OSB?

Developed in the late 1970s, OSB is made of rectangular wood strands glued and pressed together. It's cheaper than plywood, yet still very strong--rating agencies don't differentiate between OSB and plywood for strength. But, while plywood is still more widely used, OSB likely will overtake it in the near future as the favored sheathing.

As the name implies, OSB has an orientation--it will have arrows printed on it showing its strength axis (grain). With 4x8 roof sheathing, it will lay lengthwise perpendicular to the rafters/trusses--the grain is on the length axis. OSB also is made in 9 foot lengths for wall sheathing and is designed for vertical installation, with the grain across the width. The grain always runs perpendicular to the rafters/trusses or studs. Building inspectors disapprove of OSB sheathing that runs the grain parallel to the underlying support.

1.2: Exterior-Grade Sheathing

When shopping for plywood for your home renovation, you may hear the term CDX for plywood sheathing.

CDX is a common term, meaning:

• the quality of one side is rated "C."
• the other side's quality is rated "D," which could stand for "dreadfully ratty."
• the "X" stands for exterior.

But, this isn't an official grade for plywood. When shopping for supplies for your home renovation, the important rating is "Exposure 1," meaning it's rated for exterior use and can withstand prolonged moisture until the final roofing material has been laid.

OSB also should be rated "Exposure 1."

Often, a 4x8 sheet of plywood or OSB is marked "sized for spacing," meaning the sheet is one-eighth of an inch short of 8 feet to account for the recommended one-eighth of an inch gap between ends to allow for heat expansion. The ends of both plywood and OSB should be gapped, though it's more critical with OSB.

1.3: Fire-Retardant Sheathing

Fire-retardant-treated (FRT) plywood sheathing is available for your remodeling project, though it's seldom used on single-family homes. Chemicals used in FRT plywood produced in the 1980s had problems with degradation, leading to the replacement of many roofs. It's unlikely your old home's roof has this substandard sheathing, but you should be aware of the possibility on your present roof or when remodeling your home.

Section 2--Sheathing Problems

If a house is properly constructed, the sheathing should be pretty much trouble-free. Through wind, sleet, rain, cold, and heat, it should sit comfortably under your roofing, just serving its multi-functional duty.

But, we don't live in a perfect world, and sometimes sheathing can be damaged.

Here are some common problems:

• Leaking flashing
• Ice-damming at the edge of the roof that allows moisture to get to the sheathing
• Moisture from inside the house which causes problems if the attic isn't properly vented

You can inspect sheathing from the attic, where you may find dampness and water stains. Tapping the sheathing should tell you whether it's sound or not, as will probing with an awl or other pointed instrument.

2.1: Sheathing Repair

Small glitches have relatively easy remedies. For instance, a branch through the roof can be largely ignored-- just fix the finish roofing material, and don't worry about a little hole in the sheathing. After all, sheathing has plenty of holes cut in it already for vents.

Patches of mold can be treated with fungicide or bleach, if rot hasn't yet set in.

But, if the damage is more widespread, like rot, you really are stuck with tearing off the roof and replacing the bad sheathing.

If your old house has 1 inch dimensional sheathing (1x4 or 1x6 or greater), you can use plywood or OSB for the replacement, even if you're only partially replacing your roof. The old sheathing boards are likely three-quarters of an inch. So, you can either get three-quarters of an inch plywood or OSB, or go with the cheaper half-inch size, and fir-up the rafter/truss the extra quarter inch to match the old boards.

Make sure you follow the proper nailing pattern required in your area. And, as always, sheathing joints should be overlapped.

2.2: Unusual Cases

If you're dealing with a roof that has rafters/trusses spaced more than the standard 2 foot (common in "Pole and Beam" construction), you must use something other than the standard half-inch plywood or OSB for sheathing.

Rafters/trusses in such construction can be spaced as widely as 8' and require much more substantial sheathing. Common materials include 2x4 or 2x6 tongue-and-groove decking or specialty fiberboard sheathing.