how far apart should flat roof joists be

How Far Apart Should Flat Roof Joists Be: A Comprehensive Guide

Flat roofs have gained popularity in modern architecture due to their sleek design, space-saving potential, and adaptability. However, beneath the surface of these contemporary roofs lies a critical component: the joists. These horizontal structural elements bear the weight of the roof and any additional loads, making the spacing of flat roof joists a crucial factor in ensuring structural integrity. In this comprehensive guide, we will explore flat roof joists, their role, and how to determine the ideal spacing between them.

Whether you’re embarking on a new construction project or renovating an existing flat roof, understanding joist spacing is essential. This guide will provide an in-depth look at the factors influencing joist spacing, calculations, compliance with building codes, and common practices, ensuring that your flat roof remains sturdy and long-lasting.

Part 1: Understanding Flat Roof Joists

1.1 What Are Flat Roof Joists?

Flat roof joists are the unsung heroes that provide structural support for the roof. They distribute the weight of the roof and any additional loads evenly, preventing structural issues such as sagging and damage. Joists can be constructed from various materials, including wood, steel, or engineered trusses. The choice of material depends on the design and load requirements of the flat roof.

Understanding the role of joists and their material options is the first step in determining the appropriate spacing between them. Each material has its own unique properties, influencing how it should be spaced to ensure the roof’s stability.

1.2 The Role of Joist Spacing

Joist spacing plays a pivotal role in maintaining a flat roof’s structural integrity. Proper spacing ensures that loads are evenly distributed, preventing structural damage and maintaining the roof’s load-bearing capacity. Conversely, incorrect spacing can lead to structural problems, jeopardizing the roof’s longevity.

Flat roof joists are akin to the backbone of the structure. They provide stability and guarantee the roof can withstand external forces, including snow loads, equipment installations, or regular foot traffic. Accurate spacing between these joists is crucial during construction to uphold the roof’s integrity.

Part 2: Factors Affecting Joist Spacing

2.1 Roof Load Requirements

One of the foremost factors affecting joist spacing is the expected load on the roof. It includes the weight of the roof itself, known as the “dead load,” as well as additional weights such as snow, equipment, and foot traffic, referred to as the “live load.”

The type and magnitude of loads should be carefully evaluated during the design phase to ascertain the suitable joist spacing. In areas susceptible to heavy snowfall, for instance, joist spacing should be adjusted to accommodate the additional weight during snowy seasons.

2.2 Material and Type of Joists

The material and type of joists used have a considerable influence on determining permissible joist spacing. Different materials exhibit distinct load-bearing capacities, directly impacting the overall strength and stability of the flat roof.

Wood Joists: Wooden joists are a popular choice for residential flat roofs. Their spacing varies based on factors such as wood species, grade, and size. Consultation with a structural engineer is advised to determine specific requirements.

Steel Joists: Steel joists are often employed in commercial and industrial applications due to their high load-bearing capacity. However, spacing for steel joists differs from that of wooden counterparts.

Engineered Trusses: Engineered trusses are pre-manufactured components designed for specific load requirements. The spacing of these trusses is established during the design phase, ensuring compliance with load-bearing standards.

2.3 Local Building Codes and Regulations

Adhering to local building codes and regulations is a critical consideration when deciding on joist spacing. These codes are in place to guarantee the safety and structural integrity of buildings and typically encompass specific guidelines for minimum and maximum joist spacing, tailored to various roof types, including flat roofs.

Neglecting building codes can lead to expensive fines, legal complications, and possible structural hazards. Therefore, consulting local authorities, obtaining necessary permits, and ensuring code compliance are essential aspects of any flat roof project.

Part 3: Calculating Joist Spacing

3.1 Load Calculations

Calculating the anticipated load on your flat roof is a fundamental step in determining appropriate joist spacing. Accurate load calculations consider both dead and live loads. Dead loads comprise the weight of the roof, while live loads encompass external forces like snow, equipment, and foot traffic.

Structural engineers can perform load calculations to ensure that the roof’s structure, including joists, can withstand the expected weight. These calculations form the basis for determining the spacing between joists, ensuring they can effectively distribute the load without compromising the roof’s structural integrity.

3.2 Maximum Span Tables

Joist manufacturers supply maximum span tables that aid in determining the safe distance between joists for specific load requirements. These tables vary according to the material, size, and type of joists. Span tables take into account factors such as material load-bearing capacity and expected roof loads.

Span tables are invaluable resources that ensure joist spacing aligns with safety and structural guidelines. They are readily available from manufacturers and structural engineering references.

Part 4: Common Joist Spacing Practices

4.1 Typical Joist Spacing for Flat Roofs

While the specific joist spacing may vary based on factors such as load requirements, material, and design, some general practices can be considered:

Residential Flat Roofs: For residential flat roofs, joist spacing typically falls within the range of 16 to 24 inches. In wooden joist constructions, 16 inches is the most common spacing. Nevertheless, spacing should always be determined by load calculations and material specifications.

Commercial and Industrial Roofs: Joist spacing for commercial and industrial flat roofs can vary significantly, with steel or engineered trusses often in use. The spacing depends on the unique load requirements and the specific building design.

4.2 Adjusting Spacing for Unique Situations

In unique situations, standard joist spacing may need adjustment. Consider the following scenarios:

Heavy Snowfall Regions: In regions experiencing heavy snowfall, it is advisable to reduce joist spacing to ensure the roof can accommodate the added weight of accumulated snow.

Rooftop Installations: When your flat roof is expected to support heavy equipment or installations like rooftop gardens, it is advisable to consult a structural engineer to determine the appropriate joist spacing.

Structural Integrity: If signs of sagging or structural issues become apparent, it is imperative to engage a professional for an assessment and recommended adjustments.

In cases with specific requirements or challenges, seeking the expertise of a structural engineer or architect is paramount. These professionals provide tailored solutions to ensure the flat roof meets specific needs, safeguarding its stability.


Determining the appropriate spacing between flat roof joists is a critical aspect of any flat roof construction or renovation project. Proper joist spacing is vital for ensuring structural integrity and load-bearing capacity. By considering factors such as expected loads, material choices, and local building codes, you can make informed decisions about joist spacing that will lead to a safe and reliable flat roof.

Whether you’re designing a residential flat roof or planning a substantial commercial project, it is highly recommended to consult with structural engineers or architects. Their expertise will ensure that the joist spacing aligns with safety standards, offering a robust, durable, and dependable flat roof that protects your investment and provides peace of mind for years to come. With this comprehensive guide, you now have the knowledge needed to make informed decisions regarding the spacing of your flat roof joists.

Flat roof joists should typically be spaced 400mm apart; however, this may differ depending on the size and design of each individual joist.

A home’s roof space requirements depend on variables like load factors, wood species and grade; to make sure you build it to code, check prescriptive tables or consult a structural engineer before beginning construction. 

Some building codes allow rafters to be spaced 19.2″ on centre, using 5 structural members over 8-feet instead of 6 at 16″ or 4 at 24″ on-centre.

Another consideration for flat roof joist spacing is the potential for snow and rain loads. These factors determine the amount of trusses needed to support the rafter and purlin spans. But, as always, our flat roofing specialists can help if you need professional help.

Then there are other requirements such as slope, eave and rake overhangs, roof tie-downs, collar ties, ridge connection, rafters supporting the ceiling, a ceiling, ventilation, and a roof finish.

Many buildings are built with steel roofing, which requires a rafter spacing that is set by the metal panel manufacturer. The rafter spacing is set to support the panels directly, so you will need to plan accordingly for lead time, if any.

For wood shingles or metal, the rafter spacing is typically set by the roofing company. The rafter and purlin spanning distances are also based on the roof load, which is a measure of how much weight a rafter can bear or support.

It is important to note that a rafter and roof joist spacing will have many factors in common. They include rafter size, rafter span, thickness, wood species, grade, and load variables.

Flat roof joists are structural elements that make up the roof structure. They should be strong enough to support the roof covering while providing maintenance access. Structural timber decking is usually plywood or OSB that is of roofing grade and BBA certified. 

It must be laid with staggered joists, providing expansion gaps of three to ten millimetres at the abutment with a rigid upstand. Learn more here.


Rafter spacing, or the distance between rafters, is an integral element of flat roof design. It helps determine each rafter’s maximum span as well as determines how many are required based on building size and other considerations.

Typically, roof joists are spaced at 12-, 16-, or 24-inch increments on centre; however, this distance may change depending on the material being used to construct it.

Concrete joists tend to be spaced more closely than timber ones due to their greater strength compared to wood ones.

As part of building a roof, it’s also vital that its joists remain level. This can be accomplished using hard packing methods such as tile or slate bedded in mortar on each of the joists.


Flat roof joist spacing should be determined on an individual basis based on loads that will be applied to it and allowed deflection of its joists.

Design of the roof structure must also be considered. For instance, if a roof features trusses it may be more cost effective to support ceiling joists on tie beams located within its purlins rather than placing ceiling joists directly beneath them.

With steel trusses, it is often more cost-effective and secure for the rafters to rest upon purlins at their joints – giving greater stability and fireproof roof construction up to 60 feet wide spans.

Steel purlins are typically constructed of cold-formed sheet metal that has been thin enough for screws to penetrate. 

This type of purlin tends to be stronger than its hot-formed counterpart and easier to work with as it doesn’t bend under stress; making it the preferred choice for most flat roof applications.


Flat roof joists are structural members that support both roof sheathing and building walls, depending on requirements such as span length, load capacity and insulation needs. Their size and spacing depend on these criteria.

Typically, joists should be spaced 400mm to 600mm apart; however, there may be times when installing wider gaps may be necessary.

Rafters must be properly sized and spaced based on several considerations, including size, span, wood species grade thickness deflection wind loads as well as attachment of ceiling.

Roof joist sizes typically used are 150mm wide by 50mm deep; 175mm wide and 200mm deep are also popular options, which should be measured against either the BS EN 1995 or TRADA span tables and installed using joist hangers for optimal results.


Flat roof joists should be spaced at least 400 mm apart to prevent bending. This spacing may need to be altered according to your roof’s span and weight requirements.

Joists can also be supported by beams, which are structural members used to connect opposing walls and carry loads across a room. 

There are numerous kinds of beams used for this purpose such as steel, concrete or manufactured wood varieties.

Beams and joists not only support the weight of floors and ceilings, but are also integral parts of maintaining structural integrity. Attaching to sidewalls prevents spreading or leaning in.

What distance between joists should be set depends on their design and type of timber construction. You can use AF&PA’s Span Tables for Joists and Rafters to help determine this distance.

Read our other article: What Are The Types of Flat Roofing Systems?

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