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The required timber size varies based on factors such as the load it must bear (including snow, people, and the roof), the spacing of the joists, and the type of wood selected. For a 6-meter-span flat roof, the suitable timber size would be 75 x 245mm joists spaced at 600mm centres, assuming a standard loading.
Factors Influencing Timber Size for a 6m Span Flat Roof
The timber size depends on multiple factors, including:
Span
A 6-meter span is a relatively long distance for a flat roof, and it requires stronger joists to support the load.
Joist Spacing
Joists are typically spaced between 400mm and 600mm centres. 600mm spacing is a common choice, but it’s crucial to consider the loads the roof will bear.
Timber Size
The 75 x 245mm joists offer a good balance between strength and material usage.
Load
The specific load the roof will support will influence the final joist size and spacing. A 75 x 225mm joist is generally suitable for a roof that will help a permanent load (excluding joist weight) of up to 50kg/m2 and an imposed load of up to 150kg/m2.
Material Grade
C24 timber is known for its strength and suitability for structural applications, but other grades like C16 can be used by making necessary calculations.
Building Regulations
To provide specific guidance on timber sizes and spans, local building regulations and relevant codes of practice are adhered to.
- BS 5268-7.2 provides guidance on the design of flat roof joists, including permissible clear spans for different timber sizes and loads.
- BS EN 1995-1 (Eurocode 5) defines timber structure design, including flat roofs, and provides general building rules.
- BS 6399-3 provides information on imposed loads, including snow loads and wind loads, which need to be considered when designing the roof.
Other Factors
Other factors, such as insulation thickness, ventilation requirements, and any specific design constraints, will also influence the final joist size.
How to Calculate Timber Size for a 6m Span Flat Roof?
To calculate the size of timber for a 6-meter span flat roof, you can follow these steps:
Given
Span length of joist = 6m
Roof pitch for a flat roof = 10 degrees (Max.)
Calculate
Width =?
Depth =?
Solution
Several factors, such as construction needs and the spacing of joists, influence timber dimensions, which are crucial in calculating the appropriate size.
- Spacing Between Joists
Min = 400mm
Max = 600mm
- Imposed Load
The imposed load includes additional factors like snow load, wind activity, and any live loads from the use of the roof.
L (Imposed load) = snow load + wind load + live structure load
- Calculate Dead Load
To calculate dead load, sum the total dead load from all structural components.
Dead load = roof load + walls load + beams load + column
- Building Code Area
Ensure that all calculations adhere to local building codes and standards, such as BS 5268-7.2 and BS 5268-3, which govern structural design.
Based On Standard Requirements:
Width of Joists = 75mm
Depth of Joists = 245mm
Result
Timber size for a 6m span is 75 x 245 mm.
This calculation ensures that the timber used will provide sufficient support for the expected loads while complying with relevant regulations. For particular projects, it is advisable to consult a structural engineer or refer to span tables to consider unique conditions or requirements.
Timber Size for 4m or 5m Span Flat Roof
For a 4m Span Flat Roof
When constructing a flat roof with a span of 4 metres, it is essential to choose the right size for the joists to ensure suitable support and stability. Typically, joists sized around 47x220mm or 75x195mm are ideal for this span. However, the spacing between the joists and the live load will impact the final sizing decision. Appropriate calculations and considerations are necessary to maintain the structural integrity of your project.
For a 5m Span Flat Roof
For a flat roof with a 5-metre span, you typically require 75x220mm timber joists spaced at 600mm on centre. However, the exact size and spacing can vary depending on the load being supported, the type of timber (C16 or C24), and other factors. You need to ensure that you select the right materials and dimensions to create a strong and resilient flat roof that can withstand different weather conditions and demanding uses.