Are you thinking about using a king post truss for the next roof of your next project? Or want to design a king post roof truss? We have got you.
The king post roof truss is the simplest type of roof truss used in timber wood. The main feature of this roof truss is that it has only one central vertical post.
However, in this blog post, we have discussed everything about King post roof trusses, how to design them, and their benefits. So, let’s check it out.
King Post Truss Definition
A king post truss is a fundamental triangular framework in timber wood structural framing, renowned for its simplicity and efficiency.
A king post truss features a central vertical member—the king post—that connects the apex of the truss to its base. This design efficiently distributes loads, making it ideal for spans ranging from 5 to 8 meters.
This type of roof truss is especially suitable for residential and small commercial buildings, such as barns, sheds, and small churches.
Components of a King Post Truss
The main components of a king post roof truss are as follows:
King Post: The central vertical post featuring framing in King post roof trusses. It connects the apex to the tie beam, providing support and stability.
Tie Beam: The horizontal member at the base of the truss, connecting the ends of the rafters and resisting outward thrust.
Principal Rafters: Inclined beams extending from the tie beam to the apex, supporting the roof covering.
Struts: Diagonal members extending from the king post to the rafters are called struts. They help in load distribution.
Ridge Beam: A ridge beam, or ridge board, is the horizontal beam at the apex where the rafters meet. It mainly supports the upper ends of the rafters.
Purlins: These are common in traditional king post trusses. It is basically a horizontal member resting on rafters, supporting the roof covering.
How To Calculate A King Post?
Before you calculate the size of the king roof truss structure, make sure you know what size wood to use for roof trusses and get the right measurements, such as:
Determine the Truss Span: Measure the distance between the two supporting walls.
Each rafter length (l) =
√(L/2)2+H2
Select Roof Pitch: Commonly between 30° and 45°, depending on aesthetic and climatic considerations. Equation for calculating roof pitch:
θ=tan−1(H/ (L/2))
Here,
L = Span
H = Height
Assess Loads: Calculate dead loads (self-weight) and live loads (snow, wind) as per local building codes.
Generally, it is determined by multiplying the roof snow load with truss spacing.
Total vertical load on truss = 2 x Total Load on Each Rafter
Let’s consider,
Roof snow load = 1 kN/m²
Truss spacing = 2 m
Then:
Line load on each rafter = 1 kN/m² × 2m = 2 kN/m
Converting to point loads:
- At joints (1 m spacing): The point loads applied on two struts (purlin in cleats) =
2 kN/m x 1 m = 2 kN
- Mid-span (2 m spacing): The point loads applied on two struts (purlin in cleats) =
2 kN/m x 2m = 4 kN
This may vary due to the live load, dead load and wind load values.
Analyze Forces: Now, it’s time for calculating the forces on each beam. Use 3 static equilibrium equations to ensure all members are adequately sized to handle tension and compression forces.
Vertical support reactions, forces on both corners of the tie beam (Aᵥ, Bᵥ):
Aᵥ=Bᵥ=∑P/2
Where:
Sum ∑P = total vertical load on truss
Horizontal force, ∑H = 0
Therefore,
Fн = 0
And
∑V= Aᵥ+ Bᵥ
Rafter force (Fᵣ) from vertical component:
Fr = (P/2)/sin(θ)
= P/2 sin(θ)
King post tension (Fₖ):
Fₖ = 2. Fr sin(θ)
Substituting the value of Fr from earlier, we get
Fₖ= 2. P/2 sin(θ). sin(θ)
∴ Fₖ=P
Note: Here, we have shown the calculation for rafter forces. For total force calculation, you must consider the internal forces as well.
King Post Truss Span
King post trusses are suitable for spans ranging from 5 to 8 meters (16 to 26 feet). In regions with minimal snow loads, spans up to 11 meters (36 feet) may be achievable with appropriate design considerations .
Example Calculation:
For a truss with a span (L) of 24 feet and a rise (H) of 6 feet:
Half-span (L/2) = 12 feet
Therefore, Roof Pitch:
θ = tan−1(6 / 12)
=tan−1 (0.5)
=26.57°
Rafter length,
l= √(L/2)2+H2
= √(12)2+62
= √180
≈ 13.42 ft
Average King Post Truss Dimensions
On average, king post truss dimensions are as follows:
Height: Approximately 2.5 to 4 metres (8 to 13 feet), depending on roof pitch.
Generally, Member Sizes: (Example reference)
Tie Beam: Typically 150 x 300 mm (6×12 inches).
Principal Rafters: Around 150×250 mm (6×10 inches).
King Post: Approximately 150×150 mm (6×6 inches).
Timber Thickness: Commonly 1.5, 3, or 4.5 inches (3.8, 7.6, or 11.4 cm)
Dimensions
Pitch Ratio: Typically between 1:1 and 1:6
How Many Posts Does A King Post Truss Have?
A standard king post truss features a single central vertical post—the king post. This differentiates it from a queen post truss, which has two vertical posts.
What Is The Angle Of A King Post Truss?
The angle of the rafters (pitch) typically ranges between 30° and 45°, depending on design requirements and aesthetic preferences. These angles ensure efficient load transfer and aesthetic appeal.
What Are The Forces In The King Post Truss?
Tie Beam: Under tension, resisting the outward thrust of the rafters.
Principal Rafters: Subjected to compression, transferring roof loads to the supports.
King Post: Under tension, pulling the apex down to prevent sagging.
Struts: In compression, supporting the rafters and transferring loads to the tie beam.
Proper analysis ensures each member is appropriately sized to handle these forces, maintaining structural integrity.
King Post Truss Analysis
Analyzing a king post truss involves:
Load Assessment: Determine all applied loads, including dead, live, wind, and snow loads.
Support Reactions: Calculate reactions at the supports using equilibrium equations.
Member Forces: Apply methods like the method of joints or sections to find internal forces in each member.
Deflection Check: Ensure deflections are within permissible limits to prevent structural issues.
Software tools or structural engineering expertise may be required for complex analyses.
How To Build A King Post Truss
Materials
Timber: Strong and durable, like Douglas Fir, Pine, or Oak.
Fasteners: Galvanized bolts, screws, or traditional wood joinery such as mortise and tenon.
Construction Steps
- Plan the Design: Start by measuring your intended span, rise, and pitch. This informs the size and angle of each timber component.
- Cut the Timber: Use exact measurements to cut rafters, tie beams, king posts, and struts. Clean, accurate cuts ensure strong joints.
- Assemble on the Ground: It’s safer and easier to fit components together while still on the ground. Use clamps and layout guides to check alignment.
- Fasten the Joints: Join the members using your chosen method—mechanical fasteners or traditional timber joints. Make sure all connections are secure and flush.
- Raise and Install: Carefully lift the truss into place on the structure, checking level and plumb. Use temporary supports until fully secured.
You can also learn how to order wood roof trusses for a better and smarter build.
Advantages And Disadvantages Of Using King Post Truss
Advantages
- Simple to Build: Straightforward design and construction.
- Cost-Effective: Requires less material and labor compared to more complex trusses.
- Aesthetic: Exposed timber trusses add visual interest to interiors.
Disadvantages
- Limited Span: Not suitable for spans exceeding approximately 26 feet without additional support.
- Design Constraints: Less flexibility in architectural design compared to other truss types.
Applications of King Post Trusses
King Post truss is mostly used in residential buildings and other small to medium-sized building designs. But for some other roof structural builds, king post truss is used, such as
Residential Buildings: Common in homes, especially in vaulted ceilings.
Agricultural Structures: Used in barns and sheds.
Historical Buildings: Found in churches and other heritage structures.
Bridges: Employed in small-span pedestrian bridges.
What Is The Difference Between King Post Truss And Queen Post Truss?
| Feature | King Post Truss | Queen Post Truss |
| Number of Posts | One central post | Two vertical posts |
| Suitable Span | Up to 8 meters | 8 to 12 meters |
| Structural Efficiency | Simpler, less material | Supports wider spans |
| Common Use Cases | Small to medium roof spans, bridges | Larger roof structures |
| Visual Design | Clean and minimal | More complex and symmetrical compared to a king post truss. |
Final Words
The king post truss remains a viable and attractive option for various construction projects. Its enduring popularity stems from its balance of simplicity, strength, and aesthetic appeal.
However, in this comprehensive guide, we have discussed everything you need to know about king roof trusses, how to design them, king post truss analysis, and more. So, hopefully, after reading this, you can have enough ideas before you think of building.
FAQs
What Is A King Post Truss?
A king post truss is a simple triangular roof truss with a central vertical post, known as the king post. Along with this, it features two angled rafters, a tie beam, and two struts. It’s commonly used in timber wood roof framing for short to medium spans.
What Is The Maximum Span Of A King Post Truss?
The maximum span of a king post truss is around 26 feet or 8 meters. Beyond this, structural performance declines unless reinforced.
What Is The Ratio Of King Post Trusses?
The ratio of a king post truss, called the pitch ratio, generally ranges from 1:1 to 1:6, depending on design needs. A 1:4 ratio is commonly used in residential roof trusses.
What Are the Components of a King Post Truss?
The main components of a king post truss are:
- Tie beam (horizontal base)
- Rafters (angled sides)
- King post (central vertical member)
- Struts (diagonal braces)
- Ridge beam (optional at the apex)
What Are The Disadvantages Of King Post Trusses?
Three major disadvantages of king post trusses are:
- Limited span capacity
- Less design flexibility compared to complex trusses
- Not ideal for large or heavily loaded roofs