Introduction
Selecting the best roof decking sheet is a critical decision when building or replacing a roof. The decking layer sits directly on top of the roof rafters or trusses and serves as the substrate upon which roofing materials like shingles or tiles get installed.
The ideal roof decking provides structural support, weather protection, and durability to handle all the elements.
After comparing materials like oriented strand board (OSB), metal, concrete, synthetics, and plywood, contractors agree that plywood is the best roof decking sheet.
Why Plywood is best roof decking sheet
As a professional roofer with over 5 years of experience, I get asked very often what the best roof decking sheet is. My answer is always the same.
High quality plywood is better than oriented strand board (OSB), metal, concrete, or synthetic underlayments for roof sheathing. This is because plywood has great structural strength, weather resistance, and is easy to install.
Plywood has changed a lot over time from a basic wood product to now having specialized grades made specifically for roofing. In this article, I share details on the best plywood grades to use and the right ways to install them.
This includes things like proper nail and screw spacing, adding supports for overhangs, and setting up attic ventilation. Follow these tips and plywood decking will give decades of durable performance under your roofing shingles or tiles.
Structural Strength of Plywood Decking
Plywood is indeed a best roof decking sheet when comes to structuaral strength. Plywood used for roof decks provides great structural rigidity and resilience. Plywood offers impressive structural performance as a roof decking material.
Its high uniform load capacity reaches 80 psf (Pounds Force per Square Foot) depending on the plywood grade and thickness specified.
Compared to oriented strand board (OSB) and traditional wood planks, plywood better resists common issues like cracking, warping, and rot when exposed to elements. The high strength-to-weight ratio also improves structural efficiency and make it the best roof decking sheet material.
Extensive testing demonstrates plywood’s durability, showing the ability to withstand over 2,000 pressure cycles. When properly fastened, plywood resists surface deterioration better than most other decking substrates.
The following table compares the relative load capacity of common plywood thicknesses used for roof sheathing:
| Plywood Thickness | Live Load Capacity |
| 15/32 inch | 50 psf |
| 1/2 inch | 60 psf |
| 5/8 inch | 80 psf |
Weather Protection Qualities
Properly treated and sealed plywood stands up exceptionally well to outdoor weathering:
These qualities enable plywood sheets to shield the roof structure and interior from rain, snow, and su. While inhibiting mold, fungal and termite growth which could compromise integrity.
Sustainability Factors of best roof decking sheet
As an engineered wood product derived from thinning overly dense forest sections, plywood has environmental advantages over solid sawn lumber:
Many plywood manufacturers have ISO 14001 environmental management certification. Look for eco-certified plywood containing recycled/recovered content for maximum sustainability.
Here is an updated in-depth blog post on why plywood is the ideal roof decking sheet, honing in on key details around proper installation:
Appropriate Plywood Grades
While plywood span ratings indicate load capacity, the bond durability and face durability also impact weather protection. These grades provide the optimal blend:
CDX Plywood
The standard grade for roof sheathing meets building code requirements for racking strength and exterior gluing.
Rated for Load Span E 24/16, meaning maximum 24” rafter centers when installed with the strong axis across supports. Face durability rate of 40/20.
CDX Zip System
Features integrated tape, foam insulative backing, and enhanced weather-resistive barrier with superior lap sealing. Span rating of 48/24 permits 48” rafter spacing.
Both grades use phenolic resin pressure-impregnation on the face and back plies for Class 3 vapor perm rating across the panel. This added durability improves moisture, fungal and termite resistance.
Correct Fastener Selection and Spacing
Proper fastening enables the plywood roof decking sheet to remain flat and structurally sound. Use ring shank nails or wood screws for maximum grip.
Follow perimeter spacing of 6 inches and field spacing of 12 inches between fasteners. Overdriving leads to damage so ensure flush installation without embedding.
| Fastener Type | Recommended Size | Notes |
| Ring Shank Nails | 10d threaded | Provides maximum grip |
| 8d | Adequate for 5/8” and 3/4” sheets | |
| Screws | #8 wood screws | Meets code, offers better grip than nails alone |
| Perimeter Spacing | 6 inches apart | Helps prevent buckling from expansion/contraction |
| Field Spacing | 12 inches apart | Wider spacing, allows for fewer fasteners used |
Overdriving nails or screws leads to compression damage so ensure fasteners sit flush without embedding deeply into the plies.
Blocking Supports for Overhang Areas
Properly supporting overhangs is critical for ensuring the roof decking sheet can handle the weight loads without failing over time.
The plywood panels can safely overhang the roof framing by limited amounts unless additional blocking supports get added.
- 1⁄2” plywood – 7” max
- 5/8” plywood – 9” max
- 3⁄4” or greater – 12” max
2x ledger boards properly attached as blocking or the roof’s wall structure helps supports overhangs exceeding those limits. This prevents fascia board damage from overhang weight loads.
Gaps between plywood roof Decking sheet
Creating small gaps between plywood roof decking sheet enables necessary airflow. But ridges vents along the roof peak work with soffit vents at the eaves to channel hot air out gable end vents.
Adding powered attic fans significantly improves airflow to exhaust heat and moisture from attic spaces beneath the roof decking.
These combined ventilation strategies prevent condensation related damage issues under the plywood over time.
Conclusions
In conclusion, this article has shown why plywood is the best roof decking sheet option. Plywood is superior to OSB, metal, concrete and synthetics.
This is due to its structural integrity and its weather resistance and sustainability. Plywood is also easy to install. Be sure to select plywood with durable glue lines. And exterior grade face plies like CDX or Zip System.
Also adhere to best practices. Such as proper perimeter and fastener spacing. And adequate overhang support through blocking.
Furthermore, enable proper attic ventilation. The plywood underlayment will provide critical performance for decades.
FAQ’s
What thickness of plywood is needed for roof decking?
For roof decking, use at least 5/8″ plywood; 3/4″ is better for durability. Some building codes now require 1″ thick roof sheathing.
How far apart should you space fasteners when installing roof plywood?
Use perimeter nail spacing of 6″ and field nail spacing of 12″. Ring shank or screw fasteners are best for grip.
What is the maximum overhang for plywood roof decking?
Adhere to these maximum overhangs – 1/2″ plywood: 7″, 5/8″ plywood: 9″, 3/4″ or thicker: 12″. Exceeding requires additional blocking support.
Can OSB be used instead of plywood for roof decking?
While cheaper, OSB does not resist moisture as effectively long-term, so plywood is preferred for enhanced durability as a roof deck.
What’s the benefit of Zip System over regular plywood?
Zip System plywood integrates weather-resistive barrier layers to better seal and insulate against moisture intrusion from seams, cracks and fasteners.