Purlin spacing Kenya guide: compare roofing purlin distances for different sheet profiles and spans, and learn how sheet choice affects roof structure.
Purlin spacing in Kenya refers to the distance between the horizontal roof supports that carry roofing sheets and transfer loads to the main roof structure. The correct roofing purlin distance depends on the sheet profile, material thickness, roof span, structural design, wind exposure and the manufacturer's installation requirements.
Purlin centres should therefore not be selected from a universal table without checking the complete roofing system. An IT5 sheet, box profile sheet, corrugated sheet or tile-profile product can have different support requirements, even when used on similar buildings.
Important: The spacing figures used in any reference table should be treated as general guidance, not structural design values. Final purlin centres should follow the roofing manufacturer's published span tables and the requirements of the project's structural engineer.
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Purlin Spacing in Kenya: A Reference Guide by Profile and Span
The spacing between purlins in Kenya refers to the distance between the horizontal roof supports that carry roofing sheets and transfer loads to the main roof structure. The correct roofing purlin distance depends on the sheet profile, material thickness, roof span, structural design, wind exposure and the manufacturer's installation requirements.
Purlin centres should therefore not be selected from a universal table without checking the complete roofing system. An IT5 sheet, box profile sheet, corrugated sheet or tile-profile product can have different support requirements, even when used on similar buildings.
Important: The spacing figures used in any reference table should be treated as general guidance, not structural design values. Final purlin centres should follow the roofing manufacturer's published span tables and the requirements of the project's structural engineer.
What Is Purlin Spacing?
The spacing between purlins is the distance between adjacent purlins supporting the roofing sheets, normally measured along the roof slope. It determines how much unsupported span the sheet has between structural supports.
Purlins can be made from:
- Timber
- Steel angle sections
- C-sections
- Z-sections
- Other engineered structural members
Their job is to support the roofing sheets and transfer roof loads into the main structure.
The purlins themselves must also be adequately designed for their span and loading.
Why Purlin Spacing Matters
Correct purlin spacing helps prevent excessive sheet deflection, movement and damage while providing the support required by the selected roofing profile.
If purlins are spaced too far apart, the roofing sheet may have excessive unsupported areas.
Potential problems include:
- Sheet deflection
- Denting
- Fastener stress
- Distortion
- Water ponding in unsuitable roof geometries
- Poor long-term performance
If purlins are placed unnecessarily close together, however, the project may require more structural material and labour than necessary.
The objective is to use a spacing that is appropriate for the roofing system.
What Determines Roofing Purlin Distance?
Roofing purlin distance is determined by several factors rather than by roof size alone.
Important considerations include:
- Roofing profile
- Steel thickness
- Sheet length
- Roof pitch
- Purlin material
- Purlin span
- Wind exposure
- Roof loading
- Building location
- Manufacturer's span tables
This is why two roofs with the same dimensions can require different purlin arrangements.
Purlin Spacing and Roofing Profile
Different roofing profiles have different geometries and stiffness characteristics, so the selected profile can change the recommended support spacing.
For example, a deeper profiled sheet may have different spanning characteristics from a shallow corrugated sheet.
Common profiles in Kenya include:
- Corrugated roofing
- Box profile
- IT5
- Tile-profile sheets
- Stone-coated roofing systems
Always check the specific manufacturer's technical information for the product being installed.
General Purlin Spacing Reference
A general reference table can help explain how roofing systems differ, but it should not replace the manufacturer's span table or structural design.
| Roofing TypeGeneral ConsiderationFinal Spacing | ||
| Corrugated sheets | Often require relatively close support depending on thickness | Manufacturer/design |
| Box profile | Profile geometry may permit different support spacing | Manufacturer/design |
| IT5 | Commonly used for longer-span applications | Manufacturer/design |
| Tile-profile sheets | Support requirements depend on the product system | Manufacturer/design |
| Stone-coated tiles | Battens and structural support are product-specific | Manufacturer/design |
The key point is that there is no universal purlin centres for every sheet called "mabati."
IT5 Purlin Span and Support
IT5 purlin span and support requirements depend on the exact IT5 product, steel thickness, profile dimensions, loading and manufacturer specifications.
IT5 and other long-span profiles are often selected where reducing the number of end laps or supporting a large roof area is desirable.
However, "long span" does not mean the sheet can simply be installed over any distance.
Check:
- Sheet thickness
- Profile
- The spacing between purlins
- Roof pitch
- Wind loading
- Fastener arrangement
The manufacturer should provide the applicable span information.
Does Gauge Affect Purlin Spacing?
Yes, sheet thickness can affect the roofing system's spanning capability, but gauge should never be used alone to determine purlin centres.
Generally, a thicker sheet can have greater stiffness than a thinner sheet of the same profile.
However, the actual performance also depends on:
- Profile geometry
- Steel grade
- Loading
- Support conditions
- Fastening
- Manufacturer's design data
For example, switching from a thicker gauge to a thinner gauge without reviewing support spacing can change the performance of the roof.
Does Roof Pitch Affect Purlin Spacing?
Roof pitch affects the loads and drainage characteristics of the roofing system, so it should be considered when establishing the structural arrangement.
A steeper roof and a shallow roof do not behave identically.
Pitch also affects:
- Rainwater drainage
- Wind interaction
- Roof access
- Sheet overlap requirements
- Overall roof geometry
The spacing between purlins should therefore be determined as part of the complete roof design.
Purlin Spacing for Box Profile Sheets
Box profile roofing sheets have formed ribs that contribute to the sheet's stiffness, but the correct purlin spacing still depends on the specific profile and thickness.
Before installation, confirm:
- Profile depth
- Sheet thickness
- Recommended span
- Purlin centres
- Fastener locations
Do not assume that all box profile sheets can use the same spacing.
A supplier's product specification should identify the recommended support arrangement.
Purlin Spacing for Corrugated Sheets
Corrugated sheets have a different geometry from box-profile products, so their recommended support spacing may differ according to sheet thickness and design.
Corrugated roofing is widely used for:
- Homes
- Sheds
- Agricultural buildings
- Workshops
- Commercial structures
For each application, check the specific product specification.
A thinner corrugated sheet may require closer support than a thicker sheet.
Purlin Spacing for IT5 Sheets
IT5 sheets are commonly associated with long-span roofing, but their maximum support distance must still be confirmed against the specific manufacturer's span and loading information.
Before choosing the spacing, establish:
- Sheet gauge or thickness
- Profile dimensions
- Roof span
- Wind exposure
- Purlin section
- Fixing system
Longer sheets can reduce end laps, but they do not eliminate the need for correctly positioned structural supports.
Purlin Spacing for Stone-Coated Roofing
Stone-coated roofing systems use a different installation method from conventional long sheets, so their battens and support spacing should follow the specific tile manufacturer's system.
The installer may need to consider:
- Battens
- Counter-battens
- Trusses
- Roof pitch
- Tile dimensions
- Fixing positions
Do not apply the spacing used for corrugated or box-profile sheets to stone-coated tiles without checking the manufacturer's instructions.
Purlin Spacing and Roof Span
Roof span describes the distance the main roof structure must cover, while purlin spacing describes the distance between the intermediate supports carrying the roofing sheets.
These are not the same measurement.
A building can have a large overall roof span while using multiple purlins to divide the roofing sheet into shorter supported sections.
The structural engineer must design both:
- Main structural members
- Purlins
The roofing manufacturer then provides information about the sheet's allowable support arrangement.
Purlin Span vs Purlin Spacing
Purlin span and the spacing between purlins are different concepts: spacing refers to the distance between purlins, while purlin span refers to the distance a purlin itself travels between its supports.
For example:
Purlin centres:
Distance between adjacent purlins.
Purlin span:
Distance between the structural members supporting the purlin.
Both affect structural performance.
A purlin can be correctly spaced for the roofing sheet but still be structurally inadequate if its own span or section is unsuitable.
Wind Loading and Purlins
Wind can create uplift forces on roofing sheets and their supporting structure, so purlin centres and fixing arrangements should account for the project's wind exposure.
Particular attention may be needed around:
- Roof edges
- Corners
- Eaves
- Ridges
- Open-sided buildings
The fasteners connecting the sheet to the purlin are also part of the wind-resistance system.
Simply adding thicker sheets does not automatically solve wind uplift problems.
Purlin Spacing for Coastal Buildings
Buildings near the Kenyan coast may require additional attention to corrosion protection because humid, salt-laden air can affect steel roofing and structural components.
Consider:
- Protected steel sections
- Appropriate roofing coating
- Compatible fasteners
- Drainage
- Maintenance access
A purlin system that performs structurally can still experience premature deterioration if corrosion protection is neglected.
Purlin Spacing in High-Wind Areas
High-wind sites require the roof structure and fixing system to be designed for the expected wind actions rather than relying on a standard spacing rule.
Review:
- Building height
- Roof shape
- Roof pitch
- Exposure
- Edge zones
- Fastener density
- Purlin connections
For exposed buildings, the structural engineer should confirm the design.
Why Purlin Alignment Matters
Purlins should be positioned and aligned correctly so roofing sheets can be fixed consistently and supported at the intended locations.
Poor alignment can cause:
- Uneven fixing
- Sheet distortion
- Difficult installation
- Incorrect screw positioning
- Visible roof irregularities
Before installing the sheets, check that the purlins form a consistent plane.
Purlin Spacing and Roofing Screws
The position of roofing screws is related to the supporting purlin arrangement because fasteners must connect the sheet securely to the structure.
The installer should ensure that:
- Screws reach the supporting member
- Correct fasteners are used
- Washers are properly seated
- Fixings are not over-tightened
- Fixing density follows the product requirements
Using the correct purlin centres but installing inadequate fasteners can still compromise the roof.
What Happens When Purlins Are Too Far Apart?
Purlins placed too far apart can leave roofing sheets with excessive unsupported spans, potentially increasing deflection, movement and damage.
Possible consequences include:
- Increased sheet flexing
- Fastener movement
- Localised deformation
- Greater risk of damage during maintenance
- Poor appearance
The exact consequences depend on the sheet and loading.
What Happens When Purlins Are Too Close?
Purlins that are closer together than necessary can increase structural material, labour and installation costs without providing a meaningful benefit for every roofing system.
Additional purlins can mean:
- More steel or timber
- More connections
- More labour
- Higher transport costs
- Longer installation time
The objective is not simply to maximise the number of purlins.
The objective is to provide the required support efficiently.
How to Plan Purlin Spacing Before Buying Sheets
The spacing between purlins should be established before roofing materials are ordered because changing the support arrangement after materials arrive can create unnecessary cost and installation problems.
Before ordering:
- Confirm the roof dimensions.
- Select the roofing profile.
- Confirm sheet thickness.
- Review the manufacturer's span information.
- Establish purlin centres.
- Design the purlin sections.
- Confirm fixing requirements.
- Order the roofing materials.
This creates a more coordinated roofing system.
Should a Contractor Decide the Purlin Spacing?
A roofing contractor can install purlins according to the project design, but structural adequacy should not be based solely on an informal rule or past experience.
For simple projects, the roofing manufacturer's installation information can provide important guidance.
For larger or structurally complex buildings, involve a qualified structural professional.
This is particularly important for:
- Warehouses
- Factories
- Large commercial buildings
- Schools
- Public buildings
- Multi-storey structures
- High-wind locations
Do Not Copy Another Roof
A purlin spacing arrangement that worked on one building may not be suitable for another because roof dimensions, sheet profile, loading and structural conditions can differ.
Two houses may use the same roofing sheets but have:
- Different roof pitches
- Different purlin spans
- Different wind exposure
- Different sheet thickness
- Different structural systems
Copying an existing roof without checking these factors can produce an unsuitable design.
Common Purlin Spacing Mistakes
Many roofing problems begin with support arrangements that were chosen without considering the complete roofing specification.
Common mistakes include:
Using a Universal Spacing
Assuming every roofing sheet uses the same purlin distance.
Ignoring Sheet Thickness
Changing gauge without reviewing the support arrangement.
Ignoring Profile
Using the spacing from one profile for another.
Overlooking Wind
Failing to account for exposed locations.
Poor Purlin Alignment
Installing supports unevenly.
Incorrect Fasteners
Using screws that do not suit the sheet and supporting member.
Adding or Removing Purlins Without Design Review
Changing the structure based only on installation convenience.
A Practical Purlin Planning Checklist
Before installing roofing sheets, confirm that the purlin arrangement matches the selected roofing product and the structural design.
Check:
- Roof dimensions
- Roof pitch
- Roofing profile
- Sheet thickness
- Manufacturer's span table
- The spacing between purlins
- Purlin section
- Purlin span
- Wind exposure
- Fastener specification
- Purlin alignment
- Structural connections
For larger buildings, have the structural design reviewed by the appropriate professional.
Why Choosing the Sheet First Matters
The roofing sheet should be selected early enough in the design process because its profile, thickness and spanning characteristics can influence the supporting structure.
Buying sheets first and designing the purlins afterwards can create avoidable complications.
Instead, coordinate:
Roof design → Sheet specification → Purlin design → Fixings → Installation
This approach makes it easier to ensure that all components work together.
Why Choose Ali Glaziers Roofing Solutions?
Ali Glaziers Roofing Solutions supplies roofing sheets and accessories for residential, commercial, industrial and agricultural projects in Kenya.
Our team can help you compare roofing options based on:
- Roofing profile
- Sheet thickness
- Sheet length
- Coating
- Application
- Project requirements
Browse available roofing products through the Roofing Shop.
Learn more about our company and services through Roofing About Us.
For roofing material specifications and project pricing, use Request a Quote.
Final Thoughts
Purlin centres in Kenya should be determined from the roofing manufacturer's span information and the structural requirements of the building, not from a single universal spacing rule.
The selected sheet profile, thickness, roof pitch, purlin section, purlin span and wind exposure all influence the final arrangement.
Before construction, confirm:
- The roofing profile
- Actual sheet thickness
- Manufacturer's recommended support spacing
- Purlin section
- Purlin span
- Roof loading
- Wind exposure
- Fixing requirements
For larger or complex buildings, have the complete roof structure checked by a qualified structural professional.
Getting the purlin arrangement right before ordering and installing the roofing sheets can help reduce material waste, installation problems and premature roof damage.
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