Quick Answers
To calculate how much plywood you need for your roof, you’ll need to follow these steps:
- Measure the total square footage of your roof
- Determine the amount of roof surface overlap for the plywood sheets
- Calculate the adjusted square footage based on the overlap
- Divide the adjusted square footage by the coverage area of a plywood sheet
This will give you the total number of plywood sheets needed. Be sure to add 5-10% extra for scrap and waste.
Measuring Your Roof
The first step is to accurately measure the total square footage of your roof surface. To do this:
- Measure the length and width of each roof section in feet
- Multiply the length x width to get the square footage of each section
- Add together the square footages of all sections
Be sure to account for complex roof shapes and surfaces. You may need to break the roof into several sections to get accurate measurements.
For example, let’s say you have a simple rectangular roof that is 40 feet long by 25 feet wide. The total square footage would be:
40 x 25 = 1,000 square feet
If the roof has a more complex shape or multiple sections, measure and calculate each section separately then add them together.
Accurately measuring all roof surfaces is crucial to calculating the plywood needed.
Determining Plywood Overlap
Plywood sheets are installed on the roof with an overlap between sheets. This helps shed water and prevents leaks.
The standard overlap is 4 inches. However, building codes may require different overlap in your area. Be sure to check local codes and account for this in your calculations.
The overlap will reduce the effective coverage area of each plywood sheet. This needs to be factored in when determining how many sheets you need.
For example, with a 4 inch overlap:
- A 4ft x 8ft sheet has an initial area of 32 sq ft
- With a 4 inch overhang on all sides, the effective area is reduced to 28.44 sq ft
Always check the coverage area of the specific plywood sheets you plan to use, accounting for overlap.
Calculating Adjusted Square Footage
Once you know the total roof square footage and the effective coverage area of the plywood sheets, you can calculate the adjusted square footage required.
To do this:
- Take the total square footage of the roof
- Subtract the overlap area on all sides of each sheet from the coverage area
- Divide the total square footage by the adjusted coverage area per sheet
Continuing the example above with 1,000 sq ft total and 28.44 sq ft coverage per sheet:
1000 sq ft (total) / 28.44 sq ft (adjusted coverage) = 35.18 sheets
Always round up to the nearest whole number since you can’t use a partial sheet!
Adding Extra Plywood
It’s smart to add 5-10% extra plywood to account for:
- Scrap from cutting sheets
- Potential damage during transport or installation
- Mistakes in cuts or measurements
- Odd sized spaces that require partial sheets
For a roof requiring 35 sheets, adding 10% would mean getting 39 sheets (35 x 1.10 = 38.5 rounded up).
Having extra plywood on hand ensures you don’t run short during the roofing project. Any unused full sheets can be returned or used for other projects.
Plywood Sheet Recommendations
For roofing, 5/8 inch or 3/4 inch plywood is typically used. The thicker plywood provides extra strength and stiffness.
Pressure treated plywood is a good option as it is resistant to rot and moisture damage. This helps it last longer under roofing.
Plywood sheets are commonly 4×8 feet, but other sizes are available if needed. Be sure to check on availability before purchasing.
Talk to your local building supplier about plywood grades and specifications recommended for roof sheathing in your area.
Using Other Roof Sheathing Materials
While plywood is a common roof decking material, there are other sheathing options:
- Oriented Strand Board (OSB) – Structural panels made of compressed wood flakes. Cheaper than plywood.
- Waferboard – Made from compressed wood particles. Not as strong as plywood.
- Fiberboard – Dense compressed fibers. Used for flat or low-pitched roofs.
- Plank boards – Solid wood planks installed perpendicular to rafters.
Each material has different strengths and weaknesses. Plywood is generally considered the optimal solution for most roofing applications.
If using an alternate roof sheathing, adjust the calculations accordingly based on the size and coverage of those sheets.
Storing Plywood Safely
Plywood sheets are heavy, awkward loads that require proper storage:
- Store plywood flat and evenly supported on a level surface
- Stack the sheets no more than a few high, spaced with supports across the width
- Keep the stacks on blocking to prevent sagging
- Cover top of stack to protect from weather
- Store inside a garage, shed or under a tarp to prevent moisture damage
Damaged or warped plywood sheets will need to be replaced, so take time to safely store the material.
Only move as much plywood to the roof as you can install that day. This prevents weather exposure and avoids overloaded roof framing.
Safely Lifting Plywood Sheets
Plywood sheets can be heavy, awkward loads. Use proper lifting techniques to avoid injury:
- Get helpers – plywood sheets often require two or more people to maneuver
- Plan the lift – coordinate and communicate roles for each person
- Use handling equipment – suction cups, lifting straps and plywood carts make the job easier and safer
- Lift with the legs, keeping the back straight and load close to the body
- Avoid twisting motions – turn using foot pivoting
- Make sure the path is clear before lifting and carrying the load
Rushing with plywood or overexerting can easily lead to pulled muscles, back injuries or accidents. Take it slowly with a partner whenever possible.
Cutting Plywood Safely
Field cutting plywood to fit your roof will be required in most cases. Follow these safe practices:
- Use a circular saw with a sharp plywood blade for straight cuts
- Support the plywood fully on sawhorses or stacks of material
- Keep fingers and body parts clear of the saw blade path
- Wear eye, ear and dust protection
- Let the saw do the cutting, don’t force the blade
- Use a straightedge guide for long straight cuts
- Clean up dust and cutoffs promptly to prevent slipping hazards
Avoid cutting plywood while on rooftop ladders. The awkward position increases risk of slips, hand injuries and electric saw dangers.
When possible, take measurements and cut sheets on the ground, then lift into position.
Installing Plywood Decking
Follow best practices when installing roof plywood:
- Snap chalk lines so sheets align correctly
- Space sheets 1/8″ apart for expansion
- Stagger joints between rows, don’t line up seams
- Place edges along rafters for support
- Allow proper overhang for drainage
- Nail every 6 inches along edges and every 12 inches in interior areas
- Drive fasteners flush to prevent issues with subsequent roofing
- Immediately replace any sheets damaged during installation
Taking the time to install clean, tight plywood decking leads to a better roofing finish and helps prevent future problems.
Don’t install more decking than can be covered with roofing that day. Plywood left exposed can warp and deteriorate.
Conclusion
Determining the right amount of plywood for your roof takes careful measurement, planning and safety considerations. But with the techniques outlined above, you can accurately calculate material needs and efficiently build a code-compliant roof deck.
Be sure to:
- Accurately measure all roof sections
- Account for plywood overlap reduction
- Add extra material for mistakes and waste
- Safely lift, carry and cut sheets
- Install decking to manufacturers specifications
With the right plywood and installation process, you can feel confident your roof will stand up to the elements for years to come. Let us know if you have any other questions!
