Wood Roof Truss Design Guide for Homes

A well-designed wood roof truss is essential for a safe and durable home. The truss's strength and stability depend on the type of wood used, with pressure-treated wood being a popular choice due to its resistance to rot and insect damage.

A typical wood roof truss consists of rafters, a ridge beam, and purlins. The rafters are usually 2x6 or 2x8 lumber, spaced 16 or 24 inches on center, depending on the load and span of the truss.

The type of wood used for the truss also affects its durability, with hardwoods like oak and maple being more resistant to wear and tear than softwoods like pine and spruce.

Discover more: What Type of Roof Do I Have

Choosing the right type of roof truss is crucial for a sturdy and long-lasting structure. Measure your structure to determine the size of your truss, as trusses can be built in various shapes and sizes to accommodate different structures.

A king post truss is a good option for small, lightweight structures, featuring a single vertical joist that runs through the center of the triangular frame. King post trusses are the most standard kind of truss, spanning 5 to 8 meters and using the fewest truss members.

Fink trusses are another option, ideal for bolstering a floor, roof, or deck, with internal joists arranged in a "W" shape for even weight distribution. Fink trusses were originally invented for bridges, but are now used to increase the strength of interior structures.

Here are some common types of roof trusses:

Queen post trusses are similar to king post trusses, with simple and sturdy designs. They have a pair of queen posts connected by a sturdy straining beam.

The two vertical queen posts make queen post trusses a bit pricier than king post trusses. This design allows the structure to span greater areas.

Queen post trusses are suitable for larger projects, spanning areas of up to 12 meters.

Selecting the right type of truss is crucial to ensure your structure is sturdy and safe. To start, you need to measure your structure to determine how big your truss needs to be.

Measure the overall length and height of the structure being reinforced. This will help you plot the exact specifications of your truss.

A simple king post truss is a great option for small, lightweight structures. It features a single vertical joist that runs through the center of the triangular frame.

King post trusses are useful for structures that don’t require as much support as those bearing more weight. I've seen them used in small sheds and gazebos with great success.

A Fink truss is ideal for bolstering a floor, roof, or deck. The internal joists are arranged in a “W” shape, ensuring even weight distribution from one end to the other.

Fink trusses were originally invented for bridges, but now they're commonly used in interior structures to increase strength. They're a popular choice among contractors.

If you need to provide unshakable support from below, consider a Howe truss. It closely resembles the King Post and Fink styles, but with additional angled beams arranged around the central joist at regular intervals.

Howe trusses are often built onto the underside of the structure they're intended for. They're a great option for structures that require extra support.

Here's a quick rundown of the truss types mentioned:

King Post Truss: Simple, single vertical joist, ideal for small structures.Fink Truss: Internal joists in a “W” shape, great for floors, roofs, and decks.Howe Truss: Additional angled beams for extra support, often built underneath structures.

Scissor trusses offer a unique solution for creating dramatic vaulted ceilings in homes. They feature sloped bottom chords that give the underlying rooms a sense of height and grandeur.

Builders can install scissor trusses in specific areas of a home where high ceilings are desired, while using other types of trusses in areas where vaulted ceilings are unwanted. This flexibility is a major advantage.

Scissor trusses can span up to 22 meters, making them a viable option for larger homes or buildings.

Worth a look: Vaulted Roof Truss Design

Roof loads play a crucial role in wood roof truss design. The type of roof load depends on the location and use of the building.

Wind loads are a significant factor in roof design, especially for open buildings like garages. The wind load for components and cladding is calculated using the SkyCiv Load Generator, taking into account factors such as building length, width, and mean roof height.

The ASCE 7-16 standard is used to determine the wind load, which is then applied to the roof trusses as positive and negative pressures. The positive pressure acts towards and perpendicular to the roof surface, while the negative pressure acts away and perpendicular to the roof surface.

The wind load is calculated as 0.651 kPa (3.33m) = 2.168 kN/m for the positive case and -0.453 kPa (3.33m) = -1.508 kN/m for the negative case.

Dead loads, on the other hand, are the weight of the materials in the structure and any items permanently placed on the structure. The dead load for roof trusses includes the weight of roof sheets and accessories, applied at the top chord, and the ceiling, applied at the bottom chord.

Additional reading: Parallel Chord Truss Roof

The self-weight of the roof truss will be checked later in the design process. For now, we'll consider the superimposed dead load, which is calculated as 0.5 kN/m for the top chord and 0.833 kN/m for the bottom chord.

Live loads are non-permanent loads, such as snow, wind, rain, or seismic forces, and are usually uniform in their application. The live load for roofs is 0.96 kPa, which is applied to the roof trusses as 3.197 kN/m.

Here's a summary of the roof loads:

Understanding the different types of roof loads and their values is essential for designing a safe and durable wood roof truss.

A wood roof truss design consists of several key components that work together to provide structural support and stability to a building.

The top chord of a roof truss is typically the longest and most visible component, often made from 2x6 or 2x8 lumber.

The bottom chord, on the other hand, is usually shorter and less visible, made from 2x4 or 2x6 lumber.

The web members, which are the vertical and diagonal pieces that connect the top and bottom chords, are usually made from 1x4 or 1x6 lumber.

These web members help to distribute the weight of the roof evenly and provide additional support to the truss.

King post trusses are the most standard kind of truss, spanning 5 to 8 meters.

They use a relatively simplistic design with the fewest truss members: a central vertical post, one bottom chord, two top chords, and a pair of webbing chords.

King post trusses are less expensive compared to many other trusses due to their simple design and few materials.

However, these trusses cannot span especially long spaces, making them more suited for smaller projects.

Rafters are often used for smaller roof spans or widths, typically less than 16 ft, due to greater economy.

Each rafter spans from the bearing wall to the ridge board at the roof peak, half the distance between the two walls. They're sized per the building code for various factors like span, spacing, load, slope, wood species, and grade.

Rafters are typically spaced at 16" o.c. or 24" o.c, which is completely prescriptive in the code and usually requires no additional engineering. This spacing can vary depending on the specific building code requirements.

A collar tie is commonly used in the upper third of the roof height below the ridge board to secure the rafter pairs against uplift, spaced at a minimum 4 ft. o.c. This helps to prevent the rafters from splaying out under load.

The ceiling joist acts as a tension tie to keep the rafters from splaying out under load and also creates a ceiling surface.

Consider reading: Roof Rafter vs Truss

Compressive stress in the grain direction is a critical factor to consider when designing roof trusses. Wood members in tension are generally safe, but those in compression are prone to buckling.

The longer and more slender a wood member is, the less compressive force it takes to buckle. This is why truss top chords are often designed with a focus on minimizing their length and width.

The compressive strength of lumber is measured by its Fc value, which indicates how much force it can withstand before buckling.

When designing a wood roof truss, it's essential to consider the installation process. Individual trusses should always be carried vertically to avoid lateral strain and damage to joints and members.

A good temporary bracing system is crucial during installation. A good publication for guidance in the design of a temporary bracing system is the publication Temporary Bracing of Metal Plate Connected Wood Trusses, DSB-89, published by the Truss Plate Institute (TPI).

Tag lines should always be used to control movement of trusses during lifting and placement. This helps prevent accidents and ensures a smooth installation process.

For your interest: Wood for Garden Shed Roof

When designing and installing trusses, spacing is a crucial aspect to consider. Trusses should be given center-to-center spacing.

If trusses are spaced greater than 24 inches center-to-center, it's necessary to indicate the purlin spacing and method of attachment to the trusses.

Discover more: Roof Truss Spacing for Metal Roofing

Handling, Installing, Bracing is a crucial part of the design and installation process for metal plate connected wood trusses. The truss installer is responsible for safe handling, installation and both temporary and permanent bracing of the trusses after they reach the job site.

A good guide for these areas of responsibility is Handling, Installing and Bracing Metal Plate Connected Wood Trusses - HIB-01 published by the Truss Plate Institute (TPI). The publication is also available in a six page fold-out summary form for use as a job site reference.

Trusses should be supported at joints and the peak while being raised to avoid excessive lateral strain. This is especially important for large trusses that require a crane or forklift for installation.

Recommended read: Wood Shed with Metal Roof

Tag lines should always be used to control movement of trusses during lifting and placement. This helps prevent damage to the trusses and ensures a safe installation process.

Temporary bracing is a critical aspect of the installation process. A good publication for guidance in the design of a temporary bracing system is the publication Temporary Bracing of Metal Plate Connected Wood Trusses, DSB-89, published by the Truss Plate Institute (TPI).

Do not cut or notch any truss member without permission of the truss designer. This is a critical safety precaution that helps prevent accidents and ensures the structural integrity of the trusses.

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Bending is a common issue that can occur between supports when lumber is subjected to loads. Bending strength is measured by the Fb value of the lumber.

Lumber can bend significantly under load, which can affect the overall structure's integrity. Bending occurs due to the uneven distribution of forces on the lumber.

The Fb value is a critical factor in determining the lumber's ability to withstand bending forces. A higher Fb value indicates greater bending strength.

To minimize bending, it's essential to ensure proper support and spacing of lumber. Bending can be reduced by using lumber with higher Fb values or by adjusting the load distribution.

To start designing and installing your project, you'll need a few essential tools and materials.

You'll need 2 in (5.1 cm) x 4 in (10 cm) wooden lumber to build the foundation of your project.

A measuring tape is a must-have for taking precise measurements and ensuring everything fits together correctly.

Plywood or galvanized steel gusset plates are necessary for added support and stability.

Construction adhesive is used to secure the various components together, so make sure you have a reliable supply.

Safety goggles and work gloves are a must for protecting yourself from potential hazards and injuries.

A jigsaw is a versatile tool for cutting curves and irregular shapes in your wooden lumber.

A pencil and graphing paper can be helpful for sketching out your design and making any necessary adjustments.

When designing a wood roof truss, it's essential to consider the span and load requirements. A typical residential roof truss can span up to 40 feet without intermediate support.

The type of wood used for the truss is also crucial, as it affects the overall strength and durability of the structure. Southern yellow pine is a popular choice due to its high strength-to-weight ratio.

To ensure a stable and secure roof, the truss design must account for the weight of the roof's covering materials, such as shingles or tiles. A well-designed truss can support up to 10 pounds per square foot of roofing material.

When building a house, it's essential to consider the design of the attic. Many trusses have webbing that can limit attic space.

Builders can use attic trusses to solve this potential issue, which can provide more room for storage or a living space. These trusses can span up to 25 meters.

The vertical posts of attic trusses are further spaced to provide more attic area. The wider the house, the greater the attic space will be.

A steeper roof pitch will also result in a higher attic ceiling. This can be beneficial for people who want to use their attic as a living space.

Gable trusses are a type of roof structure that serves as end caps for roofs. They are constructed on each end of the roof's framework to help support roof sheathing.

Gable trusses typically have one bottom chord, a pair of top chords, and several vertical posts. They are often used in conjunction with other types of trusses.

Gable trusses generally cost 25% to 50% more than typical truss designs. This increased cost should be taken into consideration when planning a roof project.

Worth a look: Framing a Cross Gable Roof

Ordering is a crucial aspect of design that can make or break the user experience. When users are presented with too many options, they can become overwhelmed and frustrated.

A good rule of thumb is to limit the number of options to 5-7. This is because research has shown that users can effectively process and compare 5-7 options, but any more than that can lead to decision paralysis.

In some design situations, you may need to make adjustments to your design values. Other adjustments to design values may be necessary, such as increasing the value of lumber in extreme fiber bending "Fb" when there are three or more trusses spaced not more than 24 inches on center and are joined by load-distributing elements.

In these situations, it's worth considering the impact of deflection on your design. Deflection may be a critical factor in special single-member applications, or where deformation must be limited.

To mitigate deflection, you may need to reduce the modulus of elasticity (E) value, which can help limit deformation. This is especially important in situations where deflection is critical or must be minimized.

You need to size the bottom chord 0.25 inches longer than the floor to ensure a proper fit.

To start, note the exact length of the structure's floor and add 0.25 inches to it. This will account for possible inconsistencies and ensure that the truss fits perfectly.

Incorporating overhanging top chords can add extra visual flair to your truss. The top chords can protrude beyond the bottom chord anywhere from 1–3 feet (0.30–0.91 m).

You can also create a detailed sketch of your truss design on graphing paper or use architectural design software to generate a diagram based on your measurements.

It's essential to submit your building plans to your local licensing authority for approval. Most cities have agencies responsible for overseeing building codes and regulations.

Here are the key steps to designing your truss: