How To Make Scissor Trusses (7 Pro Woodshop Tips)

Let’s dive into the world of scissor trusses! These aren’t your average roof supports; they add a unique architectural flair while often providing increased interior space. I’ve built my fair share of these over the years, learning the ropes through trial and error (and a few head-scratching moments). In this guide, I’ll share my top 7 woodshop tips for crafting scissor trusses that are structurally sound and visually stunning. We’ll cover everything from design considerations and material selection to cutting techniques and assembly secrets. So, grab your safety glasses, and let’s get started!

Understanding Scissor Trusses: A Woodworker’s Guide

Before we start cutting wood, it’s crucial to understand what makes a scissor truss unique. Unlike standard trusses with horizontal bottom chords, scissor trusses feature angled bottom chords that intersect somewhere along the span. This creates a vaulted ceiling effect, adding both aesthetic appeal and potential extra headroom.

What is a Scissor Truss?

A scissor truss is a type of truss with two bottom chords that intersect at a point somewhere between the supports. This creates a triangular shape that provides both structural support and an open, vaulted ceiling.

Benefits of Using Scissor Trusses

Aesthetic Appeal: The vaulted ceiling adds visual interest and a sense of spaciousness.
Increased Headroom: The raised ceiling can be particularly beneficial in rooms with limited vertical space.
Longer Spans: Scissor trusses can often span longer distances than traditional rafters, reducing the need for interior support walls.
Design Flexibility: They offer greater flexibility in roof design compared to standard trusses.

Common Applications

I’ve seen scissor trusses used in a variety of projects, including:

Residential homes with vaulted ceilings in living rooms or master bedrooms.
Garages or workshops where extra headroom is desired.
Commercial buildings like restaurants or retail spaces where an open, airy feel is preferred.
Barns or agricultural buildings where clear span is essential.

Design Considerations

Designing scissor trusses requires careful consideration of several factors:

Span: The distance the truss needs to cover.
Rise: The vertical height of the vault.
Roof Pitch: The angle of the roof.
Load Requirements: The weight the truss needs to support, including snow load, wind load, and dead load (the weight of the roofing materials).
Wood Species: The type of wood used for the truss members.
Connector Type: The type of fasteners used to join the truss members.

Remember, it’s always best to consult with a structural engineer or experienced truss designer to ensure your scissor truss design is safe and meets all local building codes. I learned this the hard way on one project where I underestimated the snow load, resulting in some costly repairs.

Takeaway: Scissor trusses offer unique design possibilities, but proper planning and professional guidance are essential.

Tip #1: Mastering the Design Phase

The design phase is where the magic truly begins, and it’s where I’ve seen many projects succeed or fail. A well-thought-out design is the backbone of a successful scissor truss build.

Calculating Angles and Dimensions

Accurate calculations are paramount. You’ll need to determine the angles of the top chords, bottom chords, and any internal bracing. Trigonometry is your friend here! I rely heavily on online calculators and CAD software to double-check my math.

Span: Measure the total distance the truss needs to cover.
Rise: Decide on the desired vertical height of the vault. This will greatly affect the interior feel.
Roof Pitch: Determine the desired slope of the roof. Common pitches are 4/12, 6/12, or 8/12.
Bottom Chord Angle: This angle is crucial for achieving the desired vault height. It can be calculated using trigonometry based on the span and rise.
Top Chord Angle: This is determined by the roof pitch.

Example:

Let’s say you have a span of 24 feet and want a rise of 4 feet. The bottom chord angle can be calculated using the following formula:

arctan(rise / (span / 2)) = arctan(4 / 12) = 18.43 degrees

Therefore, the bottom chord angle would be approximately 18.43 degrees. The top chord angle would be determined by the roof pitch, let’s say it’s a 6/12 pitch, which translates to an angle of approximately 26.57 degrees.

Software and Tools for Design

CAD Software (e.g., AutoCAD, SketchUp): These programs allow you to create detailed drawings of your scissor trusses, making it easier to visualize the final product and identify potential problems. I personally use SketchUp for its user-friendly interface and ability to create 3D models.
Truss Design Software (e.g., Mitek, Alpine): Specialized software designed specifically for truss design can automate calculations, generate shop drawings, and ensure structural integrity.
Online Truss Calculators: These tools can help you quickly calculate angles and dimensions based on your input parameters.
Pro Tip: Always double-check the output from any software or calculator with manual calculations to ensure accuracy. I once trusted a software program blindly and ended up with a truss that was several inches off!

Creating a Detailed Shop Drawing

A detailed shop drawing is essential for accurate cutting and assembly. It should include:

Overall dimensions of the truss.
Length and angle of each member.
Location of all joints and connectors.
Type and size of connectors to be used.
Material specifications (wood species, grade, etc.).

I recommend printing multiple copies of the shop drawing and keeping one handy at each workstation. This helps minimize errors and ensures everyone is on the same page.

Takeaway: Invest time in the design phase. Accurate calculations and detailed shop drawings are crucial for a successful scissor truss build.

Tip #2: Selecting the Right Materials

The strength and durability of your scissor trusses depend heavily on the quality of the materials you use. Choosing the right wood species and ensuring proper grading are essential.

Wood Species Considerations

Different wood species have different strength and stiffness properties. Common choices for trusses include:

Southern Yellow Pine: A strong and readily available softwood, often used for its cost-effectiveness.
Douglas Fir: Another strong softwood, known for its straight grain and dimensional stability.
Spruce-Pine-Fir (SPF): A group of softwood species often used for framing lumber.
Hardwoods (e.g., Oak, Maple): While less common, hardwoods can be used for trusses in specific applications where high strength and aesthetic appeal are desired. However, they are generally more expensive and harder to work with.

I typically opt for Douglas Fir for its superior strength and resistance to warping. However, Southern Yellow Pine can be a good choice if budget is a major concern.

Understanding Wood Grading

Lumber is graded based on its appearance and structural properties. The grading system varies depending on the species and region, but common grades include:

Select Structural: The highest grade, with minimal knots and defects.
#1 Common: A good grade for general construction, with some knots and defects allowed.
#2 Common: A lower grade, with more knots and defects.

For trusses, I always recommend using Select Structural or #1 Common lumber to ensure adequate strength and minimize the risk of failure. Avoid using #2 Common or lower grades, as they may contain too many defects.

Moisture Content

The moisture content of the wood is another critical factor. Ideally, the wood should be kiln-dried to a moisture content of 19% or less. This helps prevent warping, shrinkage, and cracking after the truss is installed.

I use a moisture meter to check the moisture content of the lumber before I start cutting. If the moisture content is too high, I let the wood acclimate to the shop environment for several days before using it.

Takeaway: Choose your wood wisely. Select a species and grade that are appropriate for the load requirements of your scissor trusses, and ensure the wood is properly dried.

Tip #3: Precise Cutting Techniques

Accuracy is key when cutting the members for your scissor trusses. Even small errors can accumulate and lead to significant problems during assembly.

Essential Tools for Accurate Cuts

Miter Saw: A miter saw is essential for making accurate angle cuts. Look for a saw with a large blade capacity and a precise angle adjustment mechanism. I prefer a 12-inch sliding compound miter saw for its versatility.
Table Saw: A table saw is useful for ripping lumber to the correct width and for making straight cuts.
Circular Saw: A circular saw can be used for making cuts on larger pieces of lumber or for cuts that are difficult to make with a miter saw or table saw.
Hand Saw: A hand saw can be useful for making small adjustments or for cutting intricate shapes.
Measuring Tape: A high-quality measuring tape is essential for accurate measurements.
Speed Square: A speed square is useful for marking angles and for ensuring that cuts are square.
Combination Square: A combination square is useful for marking depths and for checking the squareness of cuts.

Setting Up Your Miter Saw for Angle Cuts

Calibrate Your Saw: Before making any cuts, calibrate your miter saw to ensure that it is cutting accurately. Use a protractor or angle gauge to verify the angle settings.
Use a Stop Block: A stop block is a piece of wood that is clamped to the miter saw fence to ensure that all cuts are the same length. This is particularly useful when cutting multiple pieces of lumber to the same size.
Make a Test Cut: Before cutting your actual truss members, make a test cut on a piece of scrap lumber to verify the angle and length.

Cutting Strategies for Different Truss Members

Top Chords: Cut the top chords to the correct length and angle using a miter saw. Be sure to cut both ends of the top chords at the correct angle.
Bottom Chords: Cut the bottom chords to the correct length and angle using a miter saw. The angle at the peak where the two bottom chords meet is particularly critical.
Web Members: Cut the web members to the correct length and angle using a miter saw. These members often require compound angle cuts, so be sure to set the miter saw blade to the correct bevel angle as well.

Avoiding Common Cutting Mistakes

Cutting on the Wrong Side of the Line: Always double-check which side of the line you are cutting on. It’s easy to make a mistake, especially when cutting angles.
Not Clamping the Lumber Securely: Always clamp the lumber securely to the miter saw fence before making a cut. This will prevent the lumber from moving during the cut and ensure a clean, accurate cut.
Using a Dull Blade: A dull blade will produce rough, inaccurate cuts. Be sure to use a sharp blade and replace it when it becomes dull.

Takeaway: Precision cutting is paramount. Invest in quality tools, calibrate them regularly, and double-check your measurements before making any cuts.

Tip #4: Mastering Joinery Techniques

The joints are the weakest points in any truss. Strong, well-executed joints are essential for ensuring the structural integrity of your scissor trusses.

Common Joint Types for Scissor Trusses

Gusset Plates: Gusset plates are flat pieces of metal or plywood that are used to reinforce joints. They are typically attached to the truss members with nails, screws, or bolts. I prefer using plywood gusset plates for their ease of use and cost-effectiveness.
Metal Connectors: Metal connectors are specialized fasteners that are designed to transfer loads between truss members. They are typically used in conjunction with gusset plates.
Bolted Connections: Bolted connections are a strong and reliable way to join truss members. They are typically used in conjunction with gusset plates or metal connectors.
Screwed Connections: Screwed connections are a less expensive alternative to bolted connections. They are typically used for smaller trusses or for non-structural applications.

Choosing the Right Connector

The choice of connector depends on several factors, including:

Load Requirements: The amount of weight the joint needs to support.
Wood Species: The type of wood used for the truss members.
Joint Configuration: The angle and orientation of the truss members.
Cost: The cost of the connector.

I recommend consulting with a structural engineer or experienced truss designer to determine the appropriate connector for your specific application.

Proper Fastening Techniques

Nailing: When nailing gusset plates or metal connectors, be sure to use the correct size and type of nails. Drive the nails straight and evenly spaced.
Screwing: When screwing gusset plates or metal connectors, be sure to use the correct size and type of screws. Pre-drill pilot holes to prevent splitting the wood.
Bolting: When bolting truss members together, be sure to use the correct size and type of bolts. Use washers under the bolt heads and nuts to prevent crushing the wood. Tighten the bolts to the specified torque.

Ensuring Joint Alignment

Proper joint alignment is crucial for ensuring the structural integrity of the truss. Use clamps to hold the truss members in place while you are fastening the joints. Double-check the alignment of the joints before you release the clamps.

Takeaway: Strong joints are essential for a structurally sound scissor truss. Choose the right connector for the job and use proper fastening techniques.

Tip #5: Assembly Strategies for Efficiency

Assembling scissor trusses can be challenging, especially for larger spans. A well-organized workspace and a systematic approach can significantly improve efficiency.

Setting Up Your Assembly Area

Level Surface: Ensure your assembly area is level. This is crucial for maintaining the correct geometry of the truss. I use shims to level my workbench.
Adequate Space: Provide ample space to lay out all the truss members and maneuver around the assembly.
Good Lighting: Good lighting is essential for accurate assembly.
Clamps and Jigs: Have a variety of clamps and jigs on hand to hold the truss members in place while you are fastening the joints. I’ve found that pipe clamps and right-angle clamps are particularly useful.

Step-by-Step Assembly Process

Lay Out the Bottom Chords: Lay out the bottom chords on the assembly area and align them according to the shop drawing.
Attach the Gusset Plates at the Peak: Attach the gusset plates at the peak where the two bottom chords meet. Use clamps to hold the gusset plates in place while you are fastening them.
Attach the Top Chords: Attach the top chords to the bottom chords using gusset plates or metal connectors.
Install the Web Members: Install the web members according to the shop drawing. These members provide additional support and prevent the truss from buckling.
Inspect the Truss: Once the truss is assembled, inspect it carefully to ensure that all joints are properly fastened and that the truss is square and true.

Using Jigs and Fixtures

Jigs and fixtures can greatly simplify the assembly process and ensure accuracy. A simple jig can be made from plywood or lumber to hold the truss members in the correct position while you are fastening the joints.

I’ve built a jig for assembling scissor trusses that consists of a series of plywood blocks that are screwed to a large sheet of plywood. The blocks are positioned to hold the truss members in the correct position and angle.

Teamwork Makes the Dream Work

Assembling larger scissor trusses is often easier with a team of two or more people. One person can hold the truss members in place while the other person fastens the joints.

Takeaway: A well-organized assembly area and a systematic approach can significantly improve efficiency and accuracy. Consider using jigs and fixtures to simplify the assembly process.

Tip #6: Bracing and Reinforcement Strategies

Proper bracing and reinforcement are essential for preventing buckling and ensuring the long-term stability of your scissor trusses.

Importance of Web Members

Web members are the diagonal or vertical members that connect the top and bottom chords of the truss. They provide additional support and prevent the truss from buckling under load.

The number and placement of web members depend on the span of the truss and the load requirements. Consult with a structural engineer or experienced truss designer to determine the appropriate web member configuration for your specific application.

Lateral Bracing

Lateral bracing is used to prevent the truss from buckling sideways. It typically consists of horizontal or diagonal members that are attached to the top and bottom chords of the truss.

Lateral bracing is particularly important for long-span trusses or for trusses that are subjected to high wind loads.

Reinforcing Joints

The joints are the weakest points in the truss. Reinforcing the joints with gusset plates, metal connectors, or bolted connections can significantly increase the strength and durability of the truss.

I often use multiple gusset plates at each joint to provide additional reinforcement.

Addressing Potential Weak Points

Carefully inspect the truss for any potential weak points, such as knots, cracks, or splits in the wood. Reinforce these areas with additional gusset plates or by replacing the affected member.

Takeaway: Proper bracing and reinforcement are essential for preventing buckling and ensuring the long-term stability of your scissor trusses. Pay close attention to web member placement, lateral bracing, and joint reinforcement.

Tip #7: Safety First: Protecting Yourself

Woodworking can be dangerous if proper safety precautions are not followed. Always prioritize safety when working with power tools and handling lumber.

Essential Safety Gear

Safety Glasses: Always wear safety glasses to protect your eyes from flying debris.
Hearing Protection: Wear hearing protection when working with power tools to prevent hearing loss.
Dust Mask or Respirator: Wear a dust mask or respirator to protect your lungs from sawdust.
Gloves: Wear gloves to protect your hands from splinters and sharp edges.
Steel-Toed Boots: Wear steel-toed boots to protect your feet from falling objects.

Safe Tool Operation

Read the Manual: Always read the owner’s manual for each power tool before using it.
Inspect Tools Regularly: Inspect your tools regularly for damage or wear.
Use Sharp Blades: Use sharp blades on your saws to prevent kickback.
Keep Your Work Area Clean: Keep your work area clean and free of clutter.
Never Reach Over a Running Blade: Never reach over a running blade.
Unplug Tools Before Making Adjustments: Always unplug power tools before making adjustments or changing blades.

Lifting and Handling Lumber Safely

Lift with Your Legs: Lift lumber with your legs, not your back.
Get Help with Heavy Loads: Get help lifting heavy loads.
Wear Gloves: Wear gloves to protect your hands from splinters and sharp edges.
Use a Hand Truck or Dolly: Use a hand truck or dolly to move heavy loads.

Fire Safety

Keep a Fire Extinguisher Handy: Keep a fire extinguisher handy in your woodshop.
Store Flammable Materials Properly: Store flammable materials in a safe place.
Sweep Up Sawdust Regularly: Sweep up sawdust regularly to prevent it from accumulating.

Takeaway: Safety should always be your top priority. Wear appropriate safety gear, operate tools safely, and handle lumber with care.

Building scissor trusses can be a rewarding experience, adding a unique architectural element to your projects. By following these 7 pro woodshop tips, you can create structurally sound and visually stunning scissor trusses that will last for years to come. Remember to prioritize safety, take your time, and double-check your work. Happy woodworking!