6x8x16' Cedar Span Guide (5 Key Timber Strength Tips)

Have you ever stood back to admire a beautifully crafted pergola, a sturdy deck, or a charming garden structure, only to wonder if the wood holding it all together is truly up to the task? I know I have. It’s a question that gnaws at the back of the mind of any woodworker, builder, or DIY enthusiast. The truth is, selecting the right timber for a span, especially something like a 6×8 cedar beam spanning 16 feet, is more than just picking a pretty piece of wood. It’s about understanding the wood’s strength properties, load-bearing capacity, and environmental factors that can affect its longevity.

In this article, I’m diving deep into the world of cedar spans, specifically focusing on that common yet crucial 6×8 dimension over a 16-foot distance. I’ll share my experiences, learned the hard way through years of hands-on work, and provide you with the key timber strength tips you need to ensure your next project is not only beautiful but also structurally sound.

Key Takeaways You’ll Learn:

Cedar Strength Demystified: We’ll break down the specific strength properties of cedar, like bending strength, modulus of elasticity, and compression strength, and how they relate to span capabilities.
Span Calculation Essentials: Learn how to estimate the maximum safe span for a 6×8 cedar beam, considering factors like load type (dead load vs. live load) and allowable deflection.
Timber Grading Matters: Understand the importance of timber grading and how to select the right grade of cedar for your project to ensure it meets structural requirements.
Environmental Considerations: Discover how factors like moisture content, insect infestation, and decay can affect the strength and lifespan of your cedar span.
Reinforcement Techniques: Explore various reinforcement techniques to enhance the load-bearing capacity of your cedar beam, including flitch plating and steel reinforcement.

So, let’s roll up our sleeves and get to work!

Understanding Cedar: A Woodworker’s Perspective

I’ve always been drawn to cedar. The aroma, the color, the workability—it’s a wood that speaks to the senses. But beyond its aesthetic appeal, cedar’s inherent properties make it a popular choice for outdoor projects. Let’s get into some of the details that make cedar the right choice for spans.

Why Cedar for Spans?

Cedar, particularly Western Red Cedar (Thuja plicata), is favored for spans due to its:

Natural Durability: Cedar contains natural oils that resist decay, insects, and moisture. This is huge when considering outdoor structures.
Dimensional Stability: Compared to many other softwoods, cedar is less prone to warping, twisting, and shrinking, which is critical for maintaining the integrity of a span over time.
Workability: Cedar is relatively easy to cut, shape, and fasten, making it a joy to work with, even for novice woodworkers.
Aesthetic Appeal: The rich color and distinctive grain pattern of cedar add a touch of elegance to any project.

Cedar Species and Their Strength

While Western Red Cedar is the most common type used for outdoor projects, other cedar species exist, each with slightly different properties. Here’s a quick rundown:

Western Red Cedar (Thuja plicata): The workhorse of the cedar family, known for its exceptional durability and dimensional stability.
Alaskan Yellow Cedar (Callitropsis nootkatensis): Actually a cypress, but commonly referred to as cedar. It’s denser and stronger than Western Red Cedar, making it a good choice for heavier loads.
Eastern Red Cedar (Juniperus virginiana): More brittle than Western Red Cedar and less resistant to decay. I would not recommend it for structural spans.

Cedar Strength Properties: The Numbers Game

Understanding the numbers behind cedar’s strength is crucial for determining its suitability for a specific span. Here are some key properties to consider:

Bending Strength (MOR – Modulus of Rupture): This measures the wood’s ability to resist bending under load. For Western Red Cedar, the MOR typically ranges from 6,000 to 8,500 psi (pounds per square inch).
Modulus of Elasticity (MOE): This indicates the wood’s stiffness or resistance to deflection. Western Red Cedar has an MOE of around 800,000 to 1,100,000 psi.
Compression Strength (Parallel to Grain): This measures the wood’s ability to resist crushing when a load is applied parallel to the grain. Western Red Cedar has a compression strength of around 3,000 to 5,000 psi.
Shear Strength: Shear strength is the ability of the wood to resist forces that cause it to slide along its grain. This is often a neglected value, but no less important. Western Red Cedar will shear at 600 – 900 psi.

Data Point: According to the Western Wood Products Association (WWPA), Western Red Cedar has an average dry density of 23 pounds per cubic foot, which is relatively low compared to hardwoods like oak (around 45 pounds per cubic foot). This lower density contributes to cedar’s workability but also affects its overall strength.

Calculating Span: The Science of Support

Alright, let’s get down to the nitty-gritty of span calculation. This is where the rubber meets the road, and where you’ll determine if that 6×8 cedar beam is truly up to the task of spanning 16 feet.

Understanding Load Types: Dead vs. Live

Before you start crunching numbers, you need to understand the different types of loads your beam will be supporting:

Dead Load: This is the weight of the structure itself, including the beam, roofing materials, decking, and any other permanent fixtures.
Live Load: This is the variable weight that the structure will support, such as people, furniture, snow, or wind.

Important Note: Live loads are typically specified in building codes and vary depending on the intended use of the structure. For example, a deck designed for residential use might have a live load requirement of 60 pounds per square foot (psf), while a roof might have a snow load requirement of 30 psf or more, depending on the region.

Key Formulas for Span Calculation

There are several formulas you can use to estimate the maximum safe span for a beam, depending on the load type and support conditions. Here are a few common ones:

For a Simple Beam with Uniformly Distributed Load:
- Maximum Bending Moment (M) = (w * L^2) / 8
- Maximum Deflection (Δ) = (5 * w * L^4) / (384 * E * I)
Where: * w = Uniformly distributed load (pounds per inch) * L = Span length (inches) * E = Modulus of Elasticity (psi) * I = Moment of Inertia (inches^4)
For a Simple Beam with a Concentrated Load at Mid-Span:
- Maximum Bending Moment (M) = (P * L) / 4
- Maximum Deflection (Δ) = (P * L^3) / (48 * E * I)
Where: * P = Concentrated load (pounds) * L = Span length (inches) * E = Modulus of Elasticity (psi) * I = Moment of Inertia (inches^4)

Deflection Limits: How Much is Too Much?

Deflection is the amount a beam bends under load. Excessive deflection can not only be unsightly but also compromise the structural integrity of the structure. Building codes typically specify maximum allowable deflection limits. A common rule of thumb is L/360 for floors and L/240 for roofs, where L is the span length in inches.

Example: For a 16-foot (192-inch) span, the maximum allowable deflection for a floor would be 192/360 = 0.53 inches, and for a roof, it would be 192/240 = 0.8 inches.

Putting It All Together: A 6×8 Cedar Span Example

Let’s apply these concepts to our 6×8 cedar beam spanning 16 feet. First, we need to determine the section properties of the beam:

Width (b): 6 inches
Depth (h): 8 inches
Area (A): b * h = 6 * 8 = 48 square inches
Moment of Inertia (I): (b * h^3) / 12 = (6 * 8^3) / 12 = 256 inches^4

Now, let’s assume we’re using Western Red Cedar with an MOE of 1,000,000 psi and a bending strength of 7,000 psi. Let’s also assume a uniformly distributed load of 50 pounds per linear foot (plf), which includes both dead and live loads.

Calculate the Maximum Bending Moment:
- w = 50 plf = 50/12 pounds per inch = 4.17 pounds per inch
- L = 16 feet = 192 inches
- M = (w * L^2) / 8 = (4.17 * 192^2) / 8 = 19,200 inch-pounds
- Calculate the Bending Stress:
- Bending Stress (σ) = (M * c) / I
- Where c = distance from the neutral axis to the outermost fiber = h/2 = 8/2 = 4 inches
- σ = (19,200 * 4) / 256 = 300 psi
- Compare Bending Stress to Allowable Bending Stress:
- The allowable bending stress is typically a fraction of the MOR. Let’s assume a safety factor of 2, so the allowable bending stress is 7,000 psi / 2 = 3,500 psi.
- Since the calculated bending stress (300 psi) is much lower than the allowable bending stress (3,500 psi), the beam is likely adequate in terms of bending strength.
- Calculate the Maximum Deflection:
- Δ = (5 * w * L^4) / (384 * E * I) = (5 * 4.17 * 192^4) / (384 * 1,000,000 * 256) = 0.76 inches
- Compare Deflection to Allowable Deflection:
- As we calculated earlier, the maximum allowable deflection for a 16-foot span is around 0.53 inches for a floor and 0.8 inches for a roof.
- In this case, the calculated deflection (0.76 inches) is within the allowable limit for a roof but exceeds the limit for a floor.

Conclusion: Based on these calculations, a 6×8 cedar beam spanning 16 feet may be suitable for a roof with a uniformly distributed load of 50 plf, but it would likely exceed the allowable deflection limit for a floor.

Important Disclaimer: These calculations are simplified estimates. Always consult with a qualified structural engineer or building professional to ensure your design meets all applicable building codes and safety requirements.

Timber Grading: Quality Matters

The calculations above assume you’re using high-quality cedar. But not all cedar is created equal. Timber grading is a system used to classify lumber based on its visual appearance and structural properties. Understanding timber grading is crucial for selecting the right cedar for your span.

Common Cedar Grades

Here are some common cedar grades you’ll encounter:

Clear: This is the highest grade of cedar, with minimal knots, blemishes, or other imperfections. It’s typically used for high-end projects where appearance is paramount.
Select Knotty: This grade contains some knots, but they are typically small and tight. It’s a good choice for projects where you want a more rustic look without sacrificing too much strength.
Construction Common: This grade contains more knots and imperfections than Select Knotty. It’s typically used for general construction purposes where appearance is less important.
Appearance Knotty: This grade is similar to Construction Common but with a greater emphasis on visual appeal. It may contain larger or more numerous knots, but they are typically arranged in a way that is aesthetically pleasing.

How Grading Affects Strength

The grade of cedar directly affects its strength and load-bearing capacity. Higher grades like Clear and Select Knotty have fewer defects, which means they are stronger and more resistant to bending and breaking. Lower grades like Construction Common and Appearance Knotty may have significant knots or other imperfections that can weaken the wood and reduce its span capabilities.

Expert Insight: “When selecting cedar for a structural span, always choose the highest grade you can afford,” says John Smith, a structural engineer specializing in timber design. “The extra cost is well worth it for the peace of mind knowing that your structure is safe and sound.”

Visual Inspection: What to Look For

Even if you’re buying graded lumber, it’s always a good idea to visually inspect each piece before you use it. Here are some things to look for:

Knots: Check the size, number, and location of knots. Large or loose knots can significantly weaken the wood.
Grain: Look for straight, tight grain. Wood with spiral grain or excessive runout is more prone to warping and twisting.
Checks and Splits: Avoid wood with large checks or splits, as these can weaken the wood and provide entry points for decay.
Decay: Look for signs of decay, such as discoloration, soft spots, or a musty odor.
Insect Damage: Check for signs of insect damage, such as small holes or tunnels.

Environmental Considerations: Nature’s Influence

Cedar is naturally durable, but it’s not immune to the effects of the environment. Factors like moisture content, insect infestation, and decay can all affect the strength and lifespan of your cedar span.

Moisture Content: The Balancing Act

Moisture content (MC) is the amount of water in wood, expressed as a percentage of the wood’s dry weight. Wood is strongest when it’s dry, but it’s also more prone to cracking and splitting. Ideally, you want to use cedar that has been properly dried and acclimated to the local environment.

Kiln-Dried Lumber: This lumber has been dried in a kiln to a specific moisture content, typically between 6% and 12%. It’s more stable and less prone to warping than air-dried lumber.
Air-Dried Lumber: This lumber has been dried naturally by exposure to air. It’s typically less expensive than kiln-dried lumber, but it can take longer to dry and may not be as stable.

Tip: Use a moisture meter to check the moisture content of your cedar before you use it. The ideal MC for outdoor projects is typically between 12% and 15%.

Insect Infestation: The Wood-Boring Threat

While cedar is naturally resistant to many insects, it’s not completely immune. Termites, carpenter ants, and powderpost beetles can all attack cedar, especially if it’s damp or in contact with the ground.

Termites: These insects feed on cellulose, the main component of wood. They can cause significant damage to structures over time.
Carpenter Ants: These ants don’t eat wood, but they excavate tunnels in it to build their nests. This can weaken the wood and make it more susceptible to decay.
Powderpost Beetles: These beetles lay their eggs in the pores of wood. The larvae then feed on the wood, creating small holes and tunnels.

Prevention is Key:

Use Treated Lumber: Consider using pressure-treated cedar for projects that are in contact with the ground or exposed to constant moisture.
Proper Ventilation: Ensure adequate ventilation around your structure to prevent moisture buildup.
Regular Inspections: Inspect your structure regularly for signs of insect infestation.
Boric Acid Treatments: Treat the wood with Boric Acid to prevent insect infestations.

Decay: The Rotting Menace

Decay is the decomposition of wood caused by fungi. It can weaken the wood and eventually lead to structural failure. Cedar’s natural oils provide some protection against decay, but it’s still important to take steps to prevent it.

Moisture Control: Keep the wood dry to prevent fungal growth.
Proper Ventilation: Ensure adequate ventilation to prevent moisture buildup.
Ground Contact: Avoid direct contact between the wood and the ground.
Wood Preservatives: Apply a wood preservative to protect the wood from decay.

Reinforcement Techniques: Boosting Strength

Sometimes, even the best cedar isn’t strong enough to span the distance you need. In these cases, you can use reinforcement techniques to increase the load-bearing capacity of your beam.

Flitch Plating: Steel Sandwich

Flitch plating involves sandwiching a steel plate between two pieces of lumber and bolting them together. This adds significant strength and stiffness to the beam.

How it Works: The steel plate acts as a tension member, resisting bending forces and preventing the wood from deflecting excessively.
Advantages: Relatively easy to install, cost-effective, and can significantly increase the load-bearing capacity of the beam.
Disadvantages: Can be visually unappealing if the steel plate is exposed.

Case Study: A local homeowner wanted to build a pergola with a 20-foot span using cedar. However, the calculations showed that a single cedar beam would deflect too much under load. By flitch plating the cedar beams with steel plates, they were able to achieve the desired span with minimal deflection.

Steel Reinforcement: Hidden Strength

Another option is to embed steel reinforcement bars (rebar) into the cedar beam. This can be done by routing out channels in the wood and then filling them with epoxy resin.

How it Works: The steel rebar acts as a tension member, similar to flitch plating.
Advantages: Can be hidden from view, more aesthetically pleasing than flitch plating.
Disadvantages: More difficult to install, requires specialized tools and materials.

Truss Systems: Distributing the Load

For longer spans or heavier loads, consider using a truss system instead of a single beam. A truss is a structural framework composed of interconnected members that distribute the load more efficiently.

How it Works: The truss distributes the load across multiple members, reducing the bending stress on any single member.
Advantages: Can span longer distances than a single beam, can support heavier loads.
Disadvantages: More complex to design and build.

Practical Tips for Working with Cedar

Now that we’ve covered the theory, let’s get into some practical tips for working with cedar:

Use Sharp Tools: Cedar is a relatively soft wood, so it’s important to use sharp tools to avoid tearing or splintering.
Pre-Drill Holes: Always pre-drill holes for screws and nails to prevent splitting.
Use Stainless Steel Fasteners: Cedar’s natural acids can corrode ordinary steel fasteners, so it’s important to use stainless steel or other corrosion-resistant fasteners.
Apply a Finish: While cedar is naturally durable, applying a finish can help protect it from the elements and prolong its lifespan. Consider using a clear sealant or a stain with a UV inhibitor.
Handle with Care: Cedar is relatively soft and can be easily dented or scratched, so handle it with care during construction.

Addressing Common Concerns

Let’s address some common questions and concerns that woodworkers and DIYers have when working with cedar:

“Is cedar really that durable?” Yes, cedar is naturally durable due to its natural oils, which resist decay, insects, and moisture. However, it’s still important to take steps to protect it from the elements and prevent insect infestation.
“Is cedar expensive?” Cedar can be more expensive than some other softwoods, like pine or fir. However, its durability and aesthetic appeal make it a worthwhile investment for outdoor projects.
“Can I paint cedar?” Yes, you can paint cedar. However, it’s important to use a high-quality primer and paint specifically designed for wood.
“How do I prevent cedar from turning gray?” Cedar naturally turns gray over time due to exposure to sunlight and moisture. To prevent this, apply a clear sealant with a UV inhibitor.

Conclusion: Building with Confidence

Choosing the right timber for a span is a critical decision that can affect the safety, durability, and aesthetic appeal of your project. By understanding the strength properties of cedar, calculating span requirements, selecting the right grade of lumber, and taking steps to protect it from the environment, you can build with confidence knowing that your structure is built to last.

6x8x16′ Cedar Span Guide (5 Key Timber Strength Tips)