Understanding Cathedral Roof Truss Design and Construction

Cathedral roof trusses are a staple in traditional architecture, and their design plays a crucial role in supporting the weight of the roof. They are typically triangular in shape, with a peak at the center and a slope on both sides.

The height of a cathedral roof truss is often determined by the number of levels in the building. For example, a two-story building may require a truss with a height of around 20 feet.

A well-designed cathedral roof truss can provide excellent structural support, while also allowing for a generous amount of headroom. This is particularly important in buildings where people need to move around freely.

King Post and Queen Post trusses are commonly used in small to medium-sized buildings, such as homes and small offices.

Engineers must also consider the ease of installation when choosing a truss type, with prefabricated trusses being easier to install due to fewer onsite connections.

Prefabricated trusses can take anywhere from 1-6 months to procure, which may be too long for some projects.

A roof is made up of several key components that work together to provide protection and durability.

The underlayment, also known as the roofing felt, is a layer of material that goes underneath the shingles to prevent water from seeping into the roof deck.

Shingles are the most visible part of the roof, and they come in a variety of materials such as asphalt, wood, and metal.

The drip edge is a small piece of metal or vinyl that's installed along the edges of the roof to prevent water from seeping underneath the shingles.

Flashing is used around chimneys, vents, and skylights to prevent water from entering these areas.

The roof deck is the layer of wood that the shingles are attached to, and it's usually made of plywood or oriented strand board (OSB).

A fresh viewpoint: Brown Steel Roof

When building a scissor truss, the choice of material is crucial for its stability and longevity. Wood is the most common material used due to its cost-effectiveness and workability.

Wood is a popular choice because it's relatively inexpensive and easy to work with. You can find various types of wood suitable for scissor truss construction.

Steel trusses are preferred when more strength is needed, as they resist warping and deterioration better than wood.

Steel trusses are particularly useful in areas prone to high winds or heavy loads. They're also a good option for buildings that will be used for extended periods.

Aluminum trusses are another metal option, offering a lightweight and corrosion-resistant alternative to steel and wood.

Related reading: Wood Roof Truss Design

Cutting is a crucial aspect of Scissor Truss construction, as it requires precision to achieve the desired shape and structure.

The angled bottom chords of a Scissor Truss cross each other in the shape of an "X" at the center, which can be challenging to cut accurately.

Typically, Scissor Trusses are used for spans up to 12 meters, and their design and construction are more expensive than simpler trusses.

On a similar theme: What Is Shape of Roof

The sloping bottom chords of a Scissor Truss provide additional height and openness to the space below, making it ideal for open planned living or commercial spaces with a vaulted or cathedral ceiling.

In contrast, simpler trusses like King and Queen Post trusses have horizontal bottom chords that do not cross each other.

Design standards are essential when designing roof trusses. The National Design Specification for Wood Construction (NDS) is the most common standard in the United States.

This standard provides guidelines for the design and construction of wood structures, including roof trusses.

In Australia, two standards are commonly used: the Australian Standard for Timber Structures (AS 1720) and the Australian Standard for Installation of nailplated timber rooftrusses (AS 4440).

These standards cover guidelines for the design and construction of timber structures and specifications for the bracing, connection, and installation of nailplated timber trusses.

ClearCalcs Truss Analysis Wizard conveniently includes these standards to streamline the design process.

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Deciding which type of truss to use for your roof design is crucial and requires careful consideration of various factors.

Engineers must consider factors such as the shape and span of the roof, height restrictions, architectural requirements, ceiling type, material availability, and load applied when deciding on a truss type.

King Post and Queen Post trusses are commonly used in small to medium-sized buildings, while Scissor, Attic, Fink, and Mono trusses are used in larger and more complex buildings and are typically more expensive.

Hip, Gable, and Flat trusses are used to meet architectural requirements for the shape of the roof.

Prefabricated trusses can be a cost-effective option, but procurement times can often be 1-6 months.

Stick-building trusses on-site can be a time-consuming process, but they can be a cost-effective solution for smaller projects or projects with unique design requirements.

Here are some common truss types and their typical applications:

Scissor trusses are less efficient in terms of load bearing capacity compared to other truss designs. This is due to the lack of vertical supports in the design that can resist downward force directly.

While they distribute load effectively, scissor trusses are not ideal for structures that require heavy loads. Overloading a scissor truss or using one for a purpose it was not designed for could lead to failure.

The unique design of scissor trusses is great for distributing load, but it's not as strong as other designs when it comes to bearing weight. This is something to consider when choosing a truss for your project.

Recommended read: Load Bearing Roof Truss

For a parallel chord truss, the truss depth can be determined by the span and snow load. A 40psf snow load flat parallel chord truss spanning 27ft requires an 18″ depth.

The top chord for such a truss would be a 2X6, while the bottom chord would be a 2X4. This combination is suitable for a total horizontal span of 27ft.

A parallel chord truss vaulted roof with an 8/12 slope and a snow load of 42psf would likely require a deeper truss than the 18″ depth mentioned earlier.

For another approach, see: Parallel Chord Scissor Truss

Dead Load is a critical factor in the design and construction of roof trusses. It's the weight of the materials themselves, such as roof sheets and accessories, and the ceiling.

The self-weight of these materials is applied at different points, depending on their location. Roof sheets and accessories are applied at the top chord, while the ceiling is applied at the bottom chord.

The weight of the roof sheets and accessories is 0.15 kPa, while the ceiling weighs 0.25 kPa.

To calculate the superimposed dead load, we multiply these weights by the spacing of the trusses. For roof trusses with a spacing of 3.33m from center to center, the superimposed dead load is calculated as follows:

These values will be used to check the self-weight of the trusses and inform the design process.

Live Load is a crucial factor in roof design, and it's essential to get it right. The ASCE 7-16 standard sets the live load for roofs at 0.96 kPa.

This translates to a load of 3.197 kN/m for roof trusses, calculated by multiplying the live load by the horizontal projection of the area, which is 3.33m in this case.

When applying this load to the top chord of the roof truss, it's assumed to act on the horizontal projection of the area, so we simply multiply the load by the member length and apply it to the top chord nodes.

The live load is a critical consideration in roof design, and understanding its impact is vital for creating a safe and durable structure.

Wind Load is a crucial factor in designing and constructing buildings, especially in areas prone to strong winds. The wind load on a structure can be calculated using wind pressure calculations for components and cladding, as specified in Chapter 30 of ASCE 7-16.

The location of the structure is key in determining the wind load, with the example given being a garage located at 8050 SW Beaverton Hillsdale Hwy, Portland, OR 97225, USA.

The risk category of the building is also important, with the example given being a garage, classified as Risk Category I.

The building dimensions, including length, width, and mean roof height, are also necessary for calculating the wind load.

The roof angle is another critical factor, with the example given being 29.745°.

The topography of the surrounding area can also affect the wind load, but in this case, the effect is negligible.

To calculate the wind load, we need to set the Type of Structure to ASCE 7-16 – Buildings – Components and Cladding, and the Enclosure Classification to Open Buildings.

The Wind Blockage is also important, with the example given being "Clear or Empty Under" since the cars below would not block more than 50% of the wind area below.

The effective wind area for the roof truss is calculated by multiplying the spacing by the length, which is 23.31 m².

However, according to Section 26.2 of ASCE 7-16, the effective wind area should not be less than one-third of the span length, which is also 23.31 m².

Here are the design wind pressures for each zone:

Since the wind pressures for zones 1, 2, and 3 are all the same, the zoning will not matter.

For roof load on the truss, we will have two cases – the positive (or max) case and negative (or min) case.

The positive case is calculated by multiplying the wind pressure by the length, resulting in 2.168 kN/m.

The negative case is calculated by multiplying the wind pressure by the length, resulting in -1.508 kN/m.

The unbalanced case requires considering loading on one side (p1) equal to 0 and the other (p2) equal to 0.42 kPa.

Installing scissor trusses can be more complex and challenging than installing other types of trusses due to their angled design.

This complexity requires a level of skill, experience, and accuracy, which potentially increases construction time and costs.

As mentioned earlier, installing scissor trusses can be a time-consuming process, which may lead to additional expenses.

A higher level of precision is necessary when working with scissor trusses, which can be a challenge even for experienced builders.

Design Restrictions can be a challenge when working with scissor trusses. They have a distinctive appearance that may not suit all architectural styles.

You'll need to consider the overall look and feel of the structure to ensure it aligns well with the use of scissor trusses. This might mean opting for a different design element to balance out the look.

The unique appearance of scissor trusses can be a great asset in the right setting, but it's essential to think carefully about how they'll fit into your overall design.

Cathedral roof truss design is a beautiful and functional choice for buildings that want to make a dramatic statement. Scissor trusses, in particular, are a popular choice for this type of design.

They can provide a cathedral-style ceiling without the need for arched beams, as seen in cathedrals, barns, and homes looking for a dramatic effect. This design element can add a touch of elegance to any structure.

The unique design of scissor trusses lends an elegant aesthetic to the structure while providing functional benefits, like additional headspace. This is particularly popular in domestic settings, typically in living spaces, kitchen and dining areas where a sense of openness is desirable.

Scissor trusses are versatile and can be used in a variety of structures, including homes, churches, concert venues, and buildings with vaulted or cathedral ceilings.

Maintenance of a cathedral roof truss is relatively straightforward, requiring regular visual inspections after significant weather events.

Cracks, sagging, or water damage are signs of stress or structural changes that need to be addressed promptly.

Consulting a professional is essential to assess the damage or potential repairs if any issues are discovered.

With proper installation, material quality, and maintenance, a scissor truss can last an exceptionally long time, with some structures withstanding decades or even centuries.

Their resilience can be attributed to the truss design, which excellently distributes load, reducing structural stress.

The space created by the scissor truss design often allows for better insulation and ventilation in the roof, reducing the problem of dampness and moisture build-up.

Snow Load can be a significant factor in determining the durability of a structure, especially in areas with heavy snowfall.

For a garage in an open space, the terrain category should be set to "B" and the exposure condition of the roof to "Fully Exposed".

The thermal condition should be set to "Unheated and open air structures" since it's a garage in the open space.

The sloped roof condition should be set to "Slippery" as the roofing material to be used is G.I. sheet.

In the case of a symmetric gable roof, the unbalanced snow load is significantly higher than the balanced snow load.

The balanced roof snow load is 0.23 kPa, which translates to 0.766 kN/m for the truss.

However, for the unbalanced case, the snow load on the purlins and roof trusses is 0 kN/m for one part and 1.399 kN/m for the other part.

To convert the snow load from a horizontal projection to an inclined load acting on the top chord of the roof truss, we need to divide the load by the cosine of the angle of the roof.

In this case, the angle of the roof is 29.745°, resulting in an inclined load of 1.611 kN/m for the unbalanced part.

Consider reading: Garage with Gambrel Roof

Regular visual inspection is key to maintaining your scissor truss, especially after significant weather events. This helps you catch signs of stress or structural changes early on.

Cracks, sagging, or water damage are all red flags that require attention. If you notice any of these issues, it's essential to consult a professional to assess the damage or potential repairs.

With proper maintenance, a scissor truss can last an exceptionally long time. Many structures with these trusses have withstood decades, even centuries in some cases.

Their resilience can be attributed to the truss design, which excellently distributes load and reduces structural stress.

Insulation and ventilation are crucial factors in maintaining a roof's longevity. Due to the space created by the scissor truss design, it often allows for better insulation and ventilation in the roof.

This design feature reduces the problem of dampness and moisture build-up, which can lead to costly repairs and premature roof replacement.