Install Sheathing and Weather Barrier
Sheathing transforms your frame into a solid structure, while weather barriers keep water out and let moisture escape. This step makes your addition "dried in"—protected from weather so interior work can proceed regardless of conditions outside.
Quick Summary
Time needed
2-4 days
Cost
$3,000-$8,000
Professional help
Framing crew
Why This Step Matters
Sheathing provides structural rigidity (racking resistance) and a nailing base for siding. Weather barriers are your primary defense against bulk water and wind-driven rain while allowing interior moisture to escape. Together, they create the "building envelope" that separates inside from outside.
Critical Milestone
Once the structure is "dried in" with sheathing, weather barrier, and temporary roofing, you can work inside regardless of weather. Many contractors push hard to reach this point before rainy or cold seasons.
OSB vs. Plywood Sheathing
Both OSB (Oriented Strand Board) and plywood are code-approved for structural sheathing. Here's how they compare:
OSB (Oriented Strand Board)
- 20-30% less expensive than plywood
- Consistent quality, no voids
- Available in larger sheets (up to 24')
- Swells at edges if wet (slow to dry)
- Holds moisture longer
Plywood (CDX or Structural)
- Better moisture resistance
- Dries faster when wet
- Holds nails better
- More expensive
- May have voids in inner plies
Our recommendation: For walls, OSB is fine if you install weather barrier promptly. For roof decking, plywood is worth the extra cost because roofs get wet during construction and plywood handles it better.
How to Install Sheathing and Weather Barriers
Install Wall Sheathing
Start with wall sheathing while you have scaffolding access. Install panels vertically with the long dimension spanning studs.
- Thickness: 7/16" or 1/2" for walls
- Gap: 1/8" between panels for expansion
- Nailing: 8d nails, 6" at edges, 12" in field
- Stagger joints: Don't align seams between rows
Pro tip: Run sheathing past window and door openings, then cut out with a router. This is faster and more accurate than pre-cutting around openings.
Install Roof Sheathing
Roof sheathing needs to be thicker than wall sheathing to handle loads and foot traffic during installation.
- Thickness: 1/2" minimum, 5/8" or 3/4" preferred
- H-clips: Metal clips between panels at unsupported edges
- Start at eave: Work from bottom up, stagger joints
- Overhang: Extend past fascia per plans (typically 1-1.5")
Install Ice and Water Shield
This self-adhering membrane provides waterproof protection at critical areas prone to ice dams and water infiltration.
- Eaves: Install at least 24" past the exterior wall line
- Valleys: Full length, minimum 36" wide
- Around penetrations: Skylights, vents, chimneys
- Tie-in area: Where new roof meets existing
Cold climate requirement: Code requires ice and water shield from eave edge to at least 24" inside the exterior wall. In very cold climates, extend this further or cover the entire roof.
Install Roof Underlayment
Over the ice and water shield, install synthetic underlayment or traditional felt paper over the remaining roof deck.
- Synthetic: More durable, better traction, lays flat
- Felt (15# or 30#): Traditional, less expensive
- Overlap: 4" horizontal, 6" at ends
Install House Wrap (WRB)
House wrap (Water-Resistive Barrier or WRB) goes over wall sheathing. It blocks bulk water while allowing water vapor to escape.
- Start at bottom: Overlap upper courses over lower (like shingles)
- Overlap: 6" horizontal, 12" at vertical seams
- Tape seams: Use manufacturer's tape at all joints
- Cap nails: Use plastic cap nails, not staples
Common products: Tyvek HomeWrap, Typar, ZIP System (sheathing with built-in WRB), Henry Blueskin VP100. Each has specific installation requirements—follow manufacturer instructions.
Flash Window and Door Openings
Proper flashing at openings is critical. Water follows the path of least resistance, and windows are prime leak locations.
- • Install sill pan flashing first (self-adhering membrane)
- • Run membrane up jambs 6" minimum
- • Integrate with house wrap in shingle fashion
- • Head flashing goes under house wrap, not over
Common mistake: Cutting house wrap in an "X" pattern at windows. This creates leak paths. Instead, cut a "modified I" pattern and fold flaps properly for drainage.
Weather Barrier Products Comparison
| Product Type | Best For | Cost |
|---|---|---|
| Tyvek/Typar House Wrap | Standard wall applications | $0.10-0.15/SF |
| ZIP System | Integrated sheathing + WRB | $0.80-1.00/SF |
| Self-Adhered Membrane | Critical areas, openings | $0.50-0.75/SF |
| Synthetic Roof Underlayment | Roof deck (non-critical areas) | $0.08-0.15/SF |
| Ice & Water Shield | Eaves, valleys, tie-ins | $0.50-0.80/SF |
Frequently Asked Questions
How long can OSB sheathing be exposed before installing house wrap?
OSB manufacturers typically allow 30-90 days of exposure, but less is better. OSB that gets wet and stays wet can swell and degrade. Install house wrap within a week if possible, and cover with felt paper if rain is expected before you can install house wrap.
Do I need house wrap if I'm using ZIP System sheathing?
No—ZIP System panels have a built-in water-resistive barrier. You just need to tape all seams with ZIP System tape. This makes it faster to install but more expensive per square foot. The total cost often ends up similar.
What's the difference between house wrap and vapor barrier?
House wrap (WRB) blocks liquid water but lets water vapor pass through. Vapor barriers (like poly sheeting) block both. House wrap goes on the outside; vapor barriers go on the warm side of insulation (inside in cold climates). Never use poly sheeting as exterior house wrap—it traps moisture.
Can I skip the felt paper and just use ice and water shield on the whole roof?
Technically yes, but it's very expensive and can actually cause problems. Ice and water shield doesn't breathe, so moisture can get trapped under it. Use it at eaves, valleys, and penetrations as designed, with breathable underlayment elsewhere.
Ready for the Next Step?
With sheathing and weather barriers complete, your addition is now "dried in." The next step is creating the structural connection between your new addition and the existing house.