Plywood Over Plank Roof is a common retrofit solution for older homes with plank sheathing. This article explains when to install plywood over planks, how to prepare the substrate, fastening patterns, ventilation, flashing, and cost considerations. It focuses on practical guidance and code-aware practices for U.S. homeowners and contractors. Key Takeaways: plywood can modernize roofs but requires correct fastening, underlayment, and attention to ventilation to avoid moisture and structural issues.
| Topic | Quick Summary |
|---|---|
| When To Use | When plank sheathing is sound but non-structural, to provide nailable substrate for shingles |
| Main Risks | Trapped moisture, plywood delamination if planks are wet or decayed |
| Typical Cost | $1.50–$4.50 per sq ft for plywood sheathing installed over planks (material + labor varies) |
| Code Considerations | Local codes may limit layering; verify roof deck thickness and fastening schedule |
What Is A Plywood Over Plank Roof?
A plywood over plank roof involves installing plywood panels directly over existing wood plank roof sheathing to create a continuous, nailable surface for roofing underlayment and shingles. This approach is used when the plank substrate is still structurally adequate but not suitable for modern roofing fasteners or to upgrade insulation and ventilation arrangements. It is a retrofit method that can extend roof life when done correctly.
When Is Installing Plywood Over Planks Appropriate?
Installing plywood over planks is appropriate when the existing plank sheathing is sound, free of rot, and securely fastened to rafters or trusses. It is commonly chosen when replacing roofing materials with asphalt shingles or when a smoother, more uniform deck is needed for synthetic underlayments. It is not appropriate when planks are soft, excessively cupped, or show signs of moisture damage.
Assessing The Existing Plank Sheathing
Before adding plywood, a thorough inspection should be conducted to check for decay, insect damage, loose nails, and excessive deflection. Probing with an awl or screwdriver at multiple locations helps identify soft spots. Check attic for signs of leaks or mold and confirm rafter spacing and plank fastening. Any compromised planks should be replaced before plywood installation.
Building Code And Permit Considerations
Local building codes vary; some jurisdictions limit the number of roof layers and specify minimum deck thickness. International Residential Code (IRC) and local amendments may require inspection or permit for re-decking. Contractors should verify code requirements for fire rating and wind uplift resistance. Always obtain necessary permits and follow local code for fastening schedules and fastener type.
Choosing Plywood Type And Thickness
Exterior-grade plywood or OSB is typically used for roof decks. Common choices are 1/2-inch or 5/8-inch plywood for residential roofs; thickness depends on rafter spacing and span. Where rafters are 24 inches on center, 5/8-inch is often recommended to reduce deflection. The deck must meet span tables in the code and manufacturer recommendations for the chosen roofing material.
Preparing The Plank Surface
Preparation includes removing loose nails or projecting fasteners, cleaning debris, and ensuring planks are dry. If planks are cupped or uneven, they should be planed or selectively replaced. Installing a vapor-permeable underlayment at the interface is sometimes recommended to reduce trapped moisture risk, depending on climate. Proper surface preparation reduces the risk of plywood warping or separation later.
Fastening Patterns And Fastener Selection
Fasteners must penetrate through the plywood and into the rafters or into the plank then into framing, depending on plank thickness. Use ring-shank or screw-type roof decking nails or screws specified by code to improve withdrawal resistance. Fastener spacing often follows 6 inches in edges and 12 inches in field for plywood, but local codes and manufacturer instructions govern final spacing. Correct fasteners and spacing are critical for wind uplift resistance and long-term performance.
Underlayment, Ice & Water Shield, And Roofing Materials
After plywood installation, lay a proper underlayment: synthetic or felt underlayment, plus ice and water shield at eaves and valleys in cold climates. Many manufacturers require specific underlayments to maintain shingle warranties. Consider a synthetic underlayment for better tear resistance and breathability. Proper underlayment and flashing installation prevents leaks and protects the deck.
Ventilation And Moisture Management
One major risk with layering is trapped moisture between plank and plywood decks. Adequate attic ventilation—balanced intake and exhaust—is essential to manage moisture and temperature. In cold climates, ensure proper air sealing and insulation to reduce condensation. If ventilation is inadequate, remedial measures should be taken before re-decking. Ventilation upgrades are often required to protect the new plywood deck and roofing system.
Flashing And Transition Details
Flashing around chimneys, skylights, vents, and wall intersections must be reworked for the new deck thickness. Step flashing, counter flashing, and boot flashings should be sized to accommodate the added plywood and new underlayment. Failure to update flashing details can create leak points. All roof penetrations and transitions should be re-flashed to match the new deck height.
Common Mistakes And How To Avoid Them
- Skipping inspection: Never install plywood over unknown rot—inspect and repair first.
- Using wrong fasteners: Avoid smooth nails; use ring-shank nails or screws rated for decking.
- Ignoring ventilation: Assess and correct ventilation before re-decking.
- Improper flashing: Update all flashing details to the new deck thickness.
Costs And Timeframe
Costs depend on plywood grade, thickness, removal of old roofing, roof complexity, and regional labor rates. Typical installed costs range from $1.50 to $4.50 per square foot for plywood over planks, excluding new shingles and underlayment. For an average 2,000 sq ft roof, expect material and labor for re-decking to fall in the low thousands, with full roof replacement substantially higher. Projects commonly take 1–3 days for re-decking on a typical house roof. Obtain multiple bids and itemized estimates to compare apples-to-apples pricing.
When To Replace The Plank Deck Instead
If planks show extensive rot, insect damage, or are loose with many missing fasteners, replacing the deck is often safer and more cost-effective long term. Also consider re-decking when rafter spacing or structural issues require upgraded deck thickness. Replacing the plank deck is the right choice when structural integrity is compromised.
Warranty And Insurance Implications
Layering plywood over an existing deck may affect manufacturer shingle warranties; many shingle manufacturers allow re-decking but require specific underlayment and fastening. Insurance companies may require documentation that work meets local code and was permitted. Keep records of inspections, permits, and contractor warranties. Confirm warranty and insurance impacts with providers before starting work.
DIY Vs. Professional Installation
While a skilled DIYer can install plywood over planks, the work involves safety risks, code compliance, and technical details like flashing and ventilation that favor hiring a licensed roofer. A professional ensures correct fastening patterns, flashing integration, and adherence to local building code. For complex roofs or limited experience, hiring a contractor reduces risk and ensures quality.
Environmental And Sustainability Considerations
Using new plywood adds embodied energy and materials; consider salvaging existing plywood or choosing certified sustainable plywood. Proper disposal or recycling of old roofing materials reduces landfill waste. Additionally, upgrading roof deck can improve attic insulation and ventilation, lowering energy use. Choose responsibly sourced materials and recycle roofing waste where possible.
Maintenance Tips After Installation
After installation, inspect flashing and roof penetrations annually and after major storms. Keep gutters clean to prevent water backup at eaves and valleys. Maintain attic ventilation and check for signs of condensation or mold. Promptly repair any damaged shingles or flashing to protect the underlying plywood. Routine maintenance prolongs the life of the roof system and protects the new deck.
Frequently Asked Questions
Does Plywood Need To Be Glued To Planks?
Gluing is generally unnecessary and not recommended as a routine step; mechanical fastening to framing is the accepted method. Adhesives can trap moisture and complicate future repairs. Fastening with approved nails or screws to the framing is standard practice.
Can Plywood Be Installed Over Wet Planks?
Never install plywood over wet or damp planks. Moisture will accelerate plywood delamination and mold growth. Allow the substrate to dry fully and remediate any sources of moisture before proceeding. Installation over wet material risks premature failure.
How Thick Should The Combined Deck Be?
Combined thickness should meet local code for roof deck span. Typically, a total deck thickness equivalent to at least 3/4-inch to 1-inch is targeted depending on rafter spacing; consult span tables. Verify with local code and structural requirements before finalizing thickness.
Resources And Further Reading
Consult the International Residential Code (IRC), local building department guides, and manufacturer installation instructions for shingles and underlayment. Industry organizations such as the National Roofing Contractors Association (NRCA) provide technical bulletins on re-decking practices. Use authoritative codes and manufacturer guides to ensure compliant installation.
If additional specifics are needed—such as fastening schedules for a particular plywood thickness or regional considerations—request details and local code references so recommendations can be tailored precisely.