Truss Roof Framing Basics

A truss roof is essentially a series of triangles that provide structural support to a building's roof.

Each truss is made up of several components, including top chords, bottom chords, and web members.

The top chord is typically the highest point of the truss, while the bottom chord is the lowest point.

A truss can be either a simple truss or a more complex truss type, such as a Howe truss or a Fink truss.

The choice of truss type depends on the specific design requirements of the building.

A well-designed truss roof can provide excellent structural support and help to distribute loads evenly.

The spacing of trusses can vary depending on the design, but a common spacing is between 24 and 48 inches on center.

Suggestion: Roof Truss Design

There are thousands of different types of roof trusses out there, but we'll focus on the basics.

In the United States, most homeowners have one of three roof shapes: gambrel, hip, or gable. This is a good starting point for determining what type of truss you may have.

Hip roofs, for example, use hip roof trusses, while vaulted ceilings often utilize scissor trusses. If you have an attic with a living space, you probably have attic trusses.

Here are the three main factors to consider when determining what type of roof truss you may need:

Closed truss roofs have a distinct characteristic that sets them apart from their open counterparts. A closed truss roof is one where the framing is not visible.

There are two main ways to identify a closed truss roof.

A truss with a tie beam is one way to define a closed truss roof. This type of truss has a beam that runs along the top and connects the truss members together.

Alternatively, a roof with a ceiling that covers the framing is also considered a closed truss roof. This type of roof is often found in homes and buildings where a vaulted ceiling area is not desired.

Here are the key characteristics of a closed truss roof:

The arch-braced truss is a great option for those who want a more open look in their roof's interior. This design lacks a tie beam, creating a sense of openness.

A collar beam supported by a pair of arch braces stiffens the structure and helps transmit the weight of the roof down to the supporting wall. This is a crucial aspect of the arch-braced truss.

In a double arch-braced truss, a second pair of arched braces is added lower down, from the rafter to a block or inner sill. This design is often referred to as a wagon, cradle, barrel, or tunnel roof due to its cylindrical appearance.

Take a look at this: Gabled Roof Architecture

Scissors trusses are a traditional truss style with many variations, and they're perfect for small or large spans. They're not recommended for shallow roof pitches, however, as they require a minimum pitch of 8/12.

Scissors trusses have a unique design that resembles a pair of shears, with two defining features: a firmly connected joint where the bottom chords pass and rafter feet that land on the bottom chords. If the bottom chords join to the underside of the top chords, it's called a "scissor braced" rather than a scissor truss.

Scissors trusses are often used for open concept vaulted ceilings, attics, and A-frame style homes. They can be quite expensive, with costs ranging from $120 to $500 per truss for materials alone.

Here are some key characteristics of scissors trusses:

Pre-fabricated wood trusses offer advantages in building construction through machine-made accuracy. This precision can lead to a more efficient construction process.

Pre-fabricated wood trusses tend to use less timber compared to traditional methods.

However, this method doesn't account for site-specific design alterations that require customized truss design.

Additional reading: Shed Roof Wood

The post and beam method is a type of timber frame construction where horizontal beams support vertical posts.

Platform framing is another type of timber frame construction, often used for smaller buildings and homes.

The balloon frame is a type of timber frame construction where studs extend from the foundation to the roof.

The panelized timber frame is a type of construction where pre-fabricated panels are assembled on-site.

Worth a look: Bed Frame

Platform framing is often used for smaller buildings and homes due to its simplicity and cost-effectiveness.

The post and beam method is commonly used for larger buildings and homes, such as barns and log cabins.

The balloon frame is a type of timber frame construction that is often used for multi-story buildings.

The arch chord timber frame is a beautiful and unique design option for truss roofs. It's a variation on the classic king post style, but with a twist.

An arch chord king post truss features an arched lower chord, which adds a curvilinear element to the ceiling space. This design raises the structure slightly and gives some extra volume below.

The arch chord with queen posts timber frame truss is another variation, with twin vertical queen posts adding more vertical elements above the arched bottom chord. This design often requires concealed steel connectors to resist tension and spreading along the arch.

Worth a look: Modern Hip Roof Designs

The radius cut for the arch should be shallow in order to be cut out of solid sawn timber. This is important to keep in mind when planning your truss roof.

Using a grain matched glue laminated arch material can accommodate more semi-circular arches, but it's also considerably more expensive than solid sawn timber.

A hammer beam truss is a beautiful and more complicated design that creates dramatic vaulted spaces.

This design is often associated with religious structures, great halls, or great rooms, where it can make a stunning impression.

A hammer beam is a short beam projecting laterally from the inside of a wall, and serving as a tie beam.

To handle the spreading forces this configuration will develop, a combination of heavy timber and metal tie rods and turnbuckle is required.

Without a metal tie rod and turnbuckle, deep wall posts and/or concealed steel connectors are needed to handle the forces this configuration will develop.

A truss roof is a type of roof structure that consists of a web of support beams that criss-cross to distribute the weight of the wood. There are literally thousands of different types of roof trusses, so it can be overwhelming to choose the right one for your project.

Roof trusses come in various shapes and sizes, including hip, gable, and gambrel roofs. The type of truss you need will depend on the shape of your roof, with hip roofs utilizing hip roof trusses and gable roofs using gable trusses. Ceilings can also influence the type of truss used, with vaulted ceilings often requiring scissor trusses.

Trusses are built to provide strength and stability through compression and tension. They are often made in bulk, which can make them less expensive than traditional rafters. However, trusses can be more challenging to install, requiring multiple people and a forklift to handle the heavy lifting.

Related reading: Side Gabled Roof

Some popular types of trusses include the arch-braced truss, which gives a more open look to the interior of the roof, and the queen post truss with straining beam, which can span distances over 50 feet. Scissors trusses are another option, but they are not recommended for shallow roof pitches.

Here are some common types of trusses and their characteristics:

Purlins can also be used in conjunction with trusses to create a strong and cost-effective roof structure. Purlins are horizontal timbers that sit between the trusses, creating a secondary layer to the roof structure. They can be used to support the roof covering and can also be a great place to install LED lighting fixtures.

For more insights, see: Purlins on Roof

Cruck Frame Bents are a type of roof frame found primarily in the UK, and they're essentially the original A-frame.

A true Cruck Frame consists of a pair of timbers, known as blades, that serve as the principals of the roof and stretch to the apex, often from a level well down the side walls.

These blades can be straight or curved, and they're a defining characteristic of a Cruck Frame.

Roof trusses need to be designed by an engineer, and there are a few ways this can be done.

Software is often used to design trusses, as it's quick, affordable, and easy. This method is typically used for new construction builds. Truss companies utilize engineering software to get their designs right, which is why ordering trusses in this manner makes the most sense.

There are circumstances when an engineer needs to design a truss by hand, which entails fervent calculations and analysis. This method is time-consuming and is usually used for truss repairs or modifications.

Using roof trusses offers several advantages over traditional hand-framed rafters. They are stronger, easier to install, and more affordable.

One of the main benefits of trusses is their strength. Trusses are well-designed to distribute loads evenly throughout the truss system, providing strength and structural integrity to the roof.

Trusses also save labor costs significantly, which is a major advantage. According to the code, trusses are more affordable than hand-framed rafters in most situations.

In addition to being stronger and more affordable, trusses are also delivered to the job site "ready to install", making the installation process much quicker. This is in contrast to hand-framed rafters, which require highly skilled carpenters and a lot of time to install.

Here are some common types of trusses and their estimated costs:

The cost of trusses can vary depending on several factors, including truss size, truss type, material, and labor costs.

The hammerbeam roof was the culmination of the development of the arch-braced truss, allowing greater spaces to be spanned.

A hammerbeam truss can span a significant distance, as seen in the Westminster Hall in London, which spans just over 20 metres (66 ft). The hammerbeam roof of Westminster Hall is considered to be the best example of a hammer-beam truss in England, designed by Hugh Herland and installed between 1395 and 1399.

A hammer beam truss can have a single hammerbeam or multiple hammerbeams, depending on the design.

For your interest: Hammerbeam Ceiling

A false hammerbeam roof (truss) has two definitions: there is no hammer post on the hammer beam or the hammer beam joins into the hammer post instead of the hammer post landing on the hammer beam.

In case you were wondering, a hammer beam is a short beam projecting laterally from the inside of a wall, and serving as a tie beam.

Bracing is a crucial aspect of roof truss design, and it's been proven to improve lateral stability, especially when it comes to j chords. Permanent bracing designs can make a big difference.

Continuous lateral braces can be diagonally set in to prevent negative effects of lateral forces on the truss. The length of the chord will determine the number of brace locations.

The placement of permanent braces on the truss web can be affected by the length of the chord. This means that the number of brace locations will vary depending on the design.

Here's a breakdown of the effects of permanent bracing on the truss web:

Connections in wooden trusses are a vital aspect of their design and functionality. The earliest connections consisted of mortise-and-tenon joints, which were often crafted at the construction site with the posts.

These joints were prone to cracking due to the subsequent shrinkage of the unseasoned posts. This issue was further exacerbated by the fact that the mortise-and-tenon joints were located at the weakest point in the post, accelerating failure.

In an effort to mitigate these issues, holes were drilled slightly off-centre, allowing the peg to naturally pull the posts together with gravity. This design consideration anticipated structural behavior under loads.

Here are some key architectural elements, roofs, and timber framing concepts related to wooden trusses:

Mortise-and-tenon joints in older trusses were often crafted at the construction site, highlighting the importance of skilled craftsmanship in traditional building methods.

The arch chord design is a beautiful variation on the classic king post style. It adds a curvilinear element to the ceiling space with an arched lower chord.

The arch chord design raises the structure somewhat, giving some extra volume below. This can be achieved by using a shallow radius cut for the arch, which is essential to be cut out of solid sawn timber.

Some of our clients have pointed out how the curves in this timber frame truss soften the lines and convey a natural aesthetic. This is especially true when compared to the standard horizontal timber.

On larger spans, the arch chord design often requires concealed steel connectors to resist tension and spreading along the arch itself. This is a crucial consideration to ensure the structural integrity of the truss.

We like to use a spline tenon to joint the 2 half arches to the base of the king post in this design. This helps to create a strong and stable connection.

Using a grain matched glue laminated arch material can accommodate more semi-circular arches. However, this option is considerably more expensive than solid sawn timber.

A queen post truss can extend the span of a roof, making it a great option for large buildings.

The queen post truss has two principal rafters and two vertical queen posts, which can be replaced with iron rods to create a queen rod truss.

Combining a queen post truss with a king post truss creates a compound truss, also known as a Palladian truss, which was often used by the Renaissance architect Andrea Palladio.

The straining beam of a queen post truss can be used as the tie beam for a king post truss above, creating a unique and strong roof structure.

Each piece of lumber or steel in a roof truss is designed with a purpose and is necessary for the long-term durability of the truss system.

If one component is damaged, the entire roof truss system is weakened and should be inspected by a structural engineer immediately.

The apex of a roof truss is its highest peak, and it's often the "peak" of the roof.

The bottom chord is the foundation of the roof truss, resting upon which all other components are built.

The girder is the main roof truss in a roof framing system, and all other trusses are framed into it.

The heel joint is where the top and bottom chords meet in a pitched truss.

The overhang is the distance the top chord "hangs" over its load-bearing support system.

Purlins are smaller beams that run perpendicular to the top chords of each roof truss.

Webs connect the top and bottom chords in a triangular fashion, helping to prevent bending within the roof truss system.

Open-concept plans are incredibly popular right now.

Since trusses can span long distances without needing any posts or interior supports, they are perfect for these types of designs.

As we explore ways to make our design and planning more environmentally friendly, it's worth noting that trusses utilize less timber than other methods. This is because they require less material to construct, which in turn reduces waste.

One of the most significant benefits of trusses is that they produce less waste than other methods. This is because the unused timber and steel during production can be recycled or used for other projects.

Roof trusses don't need interior load bearing walls for support, they're designed to remove the need for them altogether.

Traditional hand framing required load bearing walls to transfer roof and snow loads to the footings.

Roof trusses can span up to 90 feet without any interior support.

Removing a load bearing wall without proper planning can have devastating results, such as home collapse or long-term structural damage.

It's essential to seek a professional, especially if rafters are involved, before pulling walls in your home.

Longer roof trusses may require additional support, such as steeper slopes or more internal webbing, to maintain stability over long spans.

Because of modern engineering software, it's extraordinarily rare to have a design error in a roof truss system.

However, issues can still arise, often during transportation or installation.

Any damage to a truss member can lead to structural integrity issues within a home.

The load path in its entirety is disturbed when a roof truss member is compromised.

A few things to look for:

These issues can cause the structural system to become unbalanced, leading to stress on certain areas of the home.

You may hear a loud bang or notice cracking, severe settling, or other structural issues within the home.

Roof truss failures can be caused by a variety of factors, and it's essential to understand these reasons to ensure your home's structural integrity.

High winds during installation can put excessive stress on roof trusses, leading to failure.

Transportation damage can also cause roof truss failures, especially if the trusses are not properly secured during transit.

Post installation alterations to a roof truss without structural engineer approval can compromise the structural integrity of the truss.

Gusset plate failure is another common cause of roof truss failures, and it's often due to improper installation or design.

Improperly repaired trusses can also lead to failures, and it's crucial to hire a professional for any repairs.

Extremely heavy snowfall can put too much weight on roof trusses, causing them to fail.

Overloading can also cause roof truss failures, especially in businesses like butcher shops where heavy items are hung from the ceiling.

Here are some common causes of roof truss failures:

Yes, roof trusses can be modified, but it's not a simple process. Truss modification involves altering a truss to fit a situation it wasn't originally designed for, which is different from truss repair that restores a truss to its original shape.

You need a structural engineer to design the modification because altering just one component can throw the whole system out of balance. This is crucial to prevent structural damage in a home.

Some common reasons to modify a roof truss include adding a bonus room over the garage, renovating the attic for storage, or turning an attic into a living space. These modifications can breathe new life into your home.

Worth a look: Eichler Roof Insulation

Here are some examples of why you might want to modify a roof truss:

Design is a crucial step in the construction process, and it's essential to understand the different ways it can be approached. There are a few ways to design roof trusses.

Roof trusses need to be designed by an engineer, and this can be done through various methods. Software can be used to design trusses, which is a quick, affordable, and easy way to get them right. This is the preferred method for new construction builds.

Designing trusses by hand is a more time-consuming process that involves fervent calculations and analysis. This method is typically used for truss repairs or modifications.

Roof trusses have a history that's worth noting when it comes to design and planning.

The first trusses were designed in the 1950's, marking the beginning of a new era in roof construction.

It wasn't until the 1960's-1970's that trusses really started to gain popularity, replacing traditional hand framing methods.

Hand framing was basically over by the 1980's, a testament to the efficiency and speed of truss construction.

To determine the pitch of your roof, you need to figure out the ratio of rise to run. This can be done with a simple equation - rise divided by run.

For example, if your rafter rises 6 inches over every 12 inches of run, your roof pitch will come out to 6:12.

The International Residential Code requires roof slopes no smaller than 2:12 for asphalt shingles, so you'll need to check your local building codes to ensure your roof pitch meets the requirements.

You can calculate your roof pitch in inches, but it's also a good idea to consult with a professional to ensure your design meets local building codes and regulations.

A roof pitch of 6:12 is a good starting point for most roofing materials, but you may need to adjust it depending on the specific requirements of your project.