OSB, or oriented strand board, is a popular and affordable alternative to plywood for home improvement projects. Many homeowners wonder if they can install OSB directly over their existing plywood subfloor or sheathing. The short answer is yes, you can install OSB over plywood in most cases.
What is OSB?
OSB stands for oriented strand board. It is an engineered wood product made from wood strands or flakes that are compressed together with resins to form sturdy panels. The wood strands are oriented lengthwise to provide strength and stiffness in one direction of the panel.
OSB is used for subflooring, sheathing, flooring underlayment, roof decking, and various other construction applications. It is an affordable substitute for traditional plywood panels.
Advantages of OSB
There are several reasons why OSB has become a popular choice:
- Cost – OSB is generally cheaper than comparable plywood panels.
- Strength – OSB can be as strong or stronger than plywood when properly installed.
- Availability – OSB is widely available at home improvement stores.
- Consistency – The manufacturing process of OSB results in consistent, reliable panels.
Can You Install OSB Over Plywood Subfloor?
In most cases, it is perfectly fine to install OSB over an existing plywood subfloor. However, there are a few things to consider first:
Check Condition of Existing Subfloor
Make sure the current plywood subfloor is in good condition with no delamination, severe cracks, or excessive movement. Repair any major problem areas before installing OSB over top.
Add Proper Underlayment
It is recommended to place a layer of asphalt-impregnated felt paper or rosin paper between the OSB and plywood. This equalizes moisture and prevents squeaking.
Use Proper Fasteners and Glue
Use construction-grade adhesive and the correct length screws or nails to securely fasten the OSB to the underlying plywood subfloor.
Stagger Panel Seams Properly
When fastening down OSB sheets, be sure to stagger the seams so that they do not line up with seams in the plywood. This provides extra structural rigidity.
Assuming the existing plywood subfloor is flat and stable, and you take measures to properly install and secure the OSB, there is no reason it cannot be placed directly over plywood.
Can You Install OSB Over Plywood Wall Sheathing?
OSB can also be installed over plywood that is being used as wall sheathing. The same general guidelines apply:
- Verify the plywood is in good condition with no deterioration or excessive moisture.
- Place building felt between the OSB and plywood.
- Use the appropriate length fasteners spaced 6-12 inches apart at the perimeter and 12 inches in the field of the panel.
- Stagger seams between OSB sheets.
Pay attention to proper flashing techniques around windows, doors, and the roofline when applying OSB over wall sheathing.
Advantages of Using OSB Over Plywood
Assuming the existing plywood is structurally sound, what are the advantages of going over it with OSB?
Better Thermal Performance
OSB generally provides better thermal insulating properties compared to plywood. The extra layer of sheathing improves energy efficiency.
Added Strength
When installed properly, the addition of structural panels like OSB over plywood can noticeably stiffen and reinforce floors and walls.
Improved Moisture Protection
OSB forms a new weather barrier to enhance protection from moisture and performance problems like mold growth.
Easier to Install Finishes
The smooth, uniform surface and edges of OSB make it easier to install flooring, siding, trim, and other finishes.
Potential Drawbacks of Adding OSB Over Plywood
While it can be beneficial in many cases, there are also some potential downsides of overlaying OSB over existing plywood:
- Increased Cost – OSB and additional hardware/adhesives add cost.
- Height Changes – Thresholds and finishes may need to be adjusted for height.
- Weight Gain – Greater dead load could impact structural requirements.
- Time Consuming – Prepping and installing OSB takes more time and labor.
Best Practices for Installing OSB Over Plywood
To ensure a quality installation, follow these best practices if applying OSB over plywood:
- Inspect – Check for moisture damage, rot, mold, or other issues with the existing plywood before installing OSB.
- Prepare – Fill any cracks or holes as needed; sweep and remove debris.
- Underlayment – Always use roofing felt or similar membrane between OSB and plywood.
- Stagger – Offset OSB seams from plywood seams for stability.
- Screw – Use proper fastener length and spacing for robust attachment.
- Flash – Be sure to integrate flashing around openings and penetrations through the OSB.
- Seal – Caulk or seal sheathing board edges to limit moisture entry.
Consulting with a building professional is advised to ensure the overlay project is done properly.
Conclusion
Installing oriented strand board over existing plywood subflooring or sheathing is certainly possible in many home renovation or new construction scenarios. With proper preparation and installation technique, the OSB and plywood used together can form a robust shell for floors and walls.
Always inspect the current plywood thoroughly, include underlayments, use suitable fasteners and spacing, and be diligent with flashing and sealing the new OSB panels. This will provide optimal structural strength, moisture protection, and energy efficiency when overlaying OSB over plywood substrates.
