Timber Purlins Guide: Sizing, Spacing & Installation

Timber purlins are horizontal structural members that span between roof trusses or rafters to support roofing materials and distribute loads. Correct purlin sizing and spacing is essential for roof stability—undersized purlins lead to sagging roofs and structural failure, while oversizing wastes money and adds unnecessary weight.

This guide explains what purlins do, how to size them correctly for your roof span and loading, when to use C16 versus C24 timber, and proper installation techniques for long-lasting roof performance.

What Are Timber Purlins?

Purlins are secondary structural members in roof construction that run perpendicular to rafters or parallel to the ridge, providing intermediate support and preventing rafter sag over longer spans.

Types of Purlin Configuration

Timber Purlins vs Steel Purlins

Both timber and steel purlins are used in UK construction:

When to choose timber purlins: Residential extensions, traditional timber-frame buildings, agricultural sheds, smaller commercial buildings (up to ~6m purlin spans), retrofit/renovation work, and situations where timber aesthetics are desired or steel fabrication lead times are prohibitive.

Common Timber Purlin Sizes in the UK

Regularised timber is the preferred choice for purlins due to consistent dimensions that make setting out and fixing easier.

Important: These span ranges are indicative only and depend on loading, spacing, roof pitch, and timber grade. Always verify purlin sizing against structural calculations or purlin span tables.

View our regularised timber purlin range →

Factors That Determine Purlin Size and Spacing

Selecting the correct purlin specification requires considering multiple variables:

1. Roof Loading

Dead loads (permanent):

• Roof covering weight (tiles, slate, metal sheeting, felt)
• Insulation and sarking boards
• Purlin and rafter self-weight
• Services (electrical, plumbing if relevant)

Imposed loads (temporary):

• Snow loading (varies by location and altitude—higher in Scotland and northern England)
• Wind loading (uplift and lateral forces)
• Maintenance access (person walking on roof)

Typical UK roof loadings:

2. Purlin Span

Purlin span is the horizontal distance between supports (trusses, walls, or posts). Longer spans require deeper timber sections or closer purlin spacing to prevent excessive deflection.

3. Purlin Spacing (Centres)

Spacing between purlins affects both purlin and rafter design:

• Closer spacing (1.2-1.5m centres) allows smaller purlin sections but requires more material and labour
• Wider spacing (1.8-2.4m centres) needs larger purlin sections and increases rafter span requirements
• Typical residential: 1.5-1.8m centres for economy and standard rafter sizing

4. Roof Pitch

Steeper roofs shed snow more effectively, reducing imposed loads. Pitch also affects the force components on purlins—steeper roofs create greater perpendicular forces on purlins.

5. Timber Strength Grade

C24 timber can span approximately 15-20% further than C16 for the same section size, or achieve the same span in a smaller section.

Example comparison (7×3 purlin, 1.5m centres, typical residential loading):

• C16 7×3: Maximum span ~4.2m
• C24 7×3: Maximum span ~4.8m

(Illustrative only—always check span tables for your specific loading)

Do Purlins Need to Be C24 or Is C16 Sufficient?

The choice between C16 and C24 for purlins depends on your specific span and loading requirements.

When C16 Is Adequate

• Shorter spans (under 3.5m for 7×3, under 4.5m for 9×3)
• Light roof coverings (metal sheeting, lightweight tiles)
• Agricultural buildings with basic loading requirements
• Standard residential roofs where span tables confirm C16 is sufficient
• Budget constraints and C16 meets structural requirements

When C24 Is Required or Beneficial

• Longer spans where C16 in the available section is insufficient
• Heavy roof coverings (thick clay tiles, natural slate)
• High snow load areas (Scotland, Pennines, elevated sites)
• Deflection control critical (sensitive finishes, flat or low-pitch roofs)
• Smaller section preference (C24 7×3 instead of C16 9×3 for same span—saves weight and headroom)
• Building Control requirements (some officers default to C24 for purlins)

Cost-benefit consideration: C24 typically costs 10-20% more than C16. If C24 allows you to drop down one timber size (e.g., use 7×3 instead of 8×3), the total material cost may be similar while reducing weight and easing handling.

Read our complete C16 vs C24 comparison guide →

How to Calculate Purlin Spacing

Purlin spacing affects both purlin performance and rafter span capability. Here’s a practical approach:

Step 1: Determine Maximum Rafter Span

Check rafter span tables for your rafter size, spacing, and loading. This gives you the maximum unsupported rafter length, which equals your maximum purlin spacing.

Step 2: Divide Roof Height into Equal Bays

Divide the vertical rise from eaves to ridge by your maximum purlin spacing to determine how many purlin rows you need.

Example:

• Roof rise (eaves to ridge): 3.6m vertical
• Rafter span tables allow 1.8m maximum unsupported span for your rafter size
• Required purlins: 3.6m ÷ 1.8m = 2 rows of mid-span purlins (plus ridge and wall plate)
• Actual spacing: 3.6m ÷ 2 = 1.8m centres

Step 3: Verify Purlin Capacity

Check purlin span tables to ensure your chosen purlin size can span the required distance at your calculated spacing under the design loading.

Step 4: Adjust If Necessary

If purlins are inadequate:

• Increase purlin section size (e.g., 7×3 to 9×3)
• Upgrade timber grade (C16 to C24)
• Reduce purlin spacing (add more purlin rows)
• Reduce purlin span (add intermediate posts or trusses)

Important: For anything beyond simple garden structures, purlin design should be verified by a structural engineer or checked against published span tables from reputable sources (TRADA, timber suppliers, Building Control guidance).

Installing Timber Purlins: Best Practice

Support and Bearing

• Minimum bearing length: 75mm on masonry walls, 50mm on timber trusses/rafters
• Use joist hangers or purlin cleats where bearing length is restricted
• Pack level: Use slate or hardwood packers (never soft timber off-cuts that compress)
• Lateral restraint: Purlins must be prevented from rolling—use herringbone strutting or direct fixing to rafters

Connections

• Rafters to purlins: Skew-nail or use truss clips/purlin hangers
• Purlin splices: Joints should occur over supports, with both ends adequately bearing. Use steel straps or bolted timber splice plates for continuity
• Fixing spacing: Typically every rafter (400-600mm centres depending on rafter spacing)

Alignment and Setting Out

• Use a string line: Set a tight string line at the correct level along the full purlin run to ensure straightness
• Check for twist: Sight along the purlin to ensure it’s not twisted—regularised timber’s consistent section helps prevent this
• Level check: Use a long spirit level or laser level to verify purlin tops are level (or at consistent fall for drainage)

Treatment and Protection

• External exposure: Use UC3 treated timber for purlins in unclad buildings or where moisture exposure is likely
• Enclosed roofs: Untreated C16/C24 is acceptable in well-ventilated, enclosed roof spaces
• Cut-end treatment: If using treated timber, apply end-grain preservative to all cuts and notches

Common Purlin Installation Mistakes

Mistake 1: Inadequate Bearing

The problem: Purlin ends have insufficient bearing on supports, causing crushing or splitting.
The solution: Ensure minimum 75mm bearing on masonry, 50mm on timber. Use joist hangers if bearing is restricted.

Mistake 2: No Lateral Restraint

The problem: Purlins can roll sideways under load, causing roof distortion or collapse.
The solution: Fix purlins securely to rafters or install herringbone strutting between purlins.

Mistake 3: Splicing Between Supports

The problem: Joining purlins mid-span creates a weak point with inadequate load transfer.
The solution: Always splice purlins directly over supports (trusses, walls, posts) with both ends properly bearing.

Mistake 4: Using Undersized Sections

The problem: Choosing purlin size by guesswork rather than calculation leads to sagging roofs and structural failure.
The solution: Use span tables or consult a structural engineer. When in doubt, go one size larger.

Mistake 5: Poor Alignment

The problem: Wavy or twisted purlins create uneven roof surfaces, making tiling or sheeting difficult.
The solution: Use regularised timber for consistent sections, set string lines for alignment, and fix methodically from one end.

FAQ: Timber Purlins

What size timber do I need for roof purlins?

Common UK residential purlin sizes are 7×3 (75×175mm) for spans up to 5m and 9×3 (75×225mm) for spans up to 6m, but the correct size depends on span, spacing, loading, and timber grade. Always check span tables or consult a structural engineer for your specific application.

Can I use C16 timber for purlins or do I need C24?

C16 is adequate for many purlin applications, particularly shorter spans (under 4m) with light to medium loadings. C24 is required for longer spans, heavy roof coverings, or where deflection control is critical. Check span tables for your specific combination of span, spacing, and loading to determine which grade is sufficient.

How far apart should roof purlins be spaced?

Typical purlin spacing is 1.5-1.8m centres in UK residential construction, but the correct spacing depends on rafter span capability. Purlins must be close enough that rafters don’t exceed their maximum unsupported span. Agricultural buildings often use wider spacing (2.0-2.4m) with heavier purlin sections.

Do purlins run parallel or perpendicular to rafters?

Purlins run perpendicular to (across) the rafters, providing intermediate support points that reduce the unsupported rafter span. They typically run parallel to the ridge and eaves, spanning between gable walls, trusses, or supporting posts.

Can I splice purlins between supports?

No. Purlin splices should only occur directly over supports (walls, trusses, posts) where both ends of the splice have adequate bearing. Splicing mid-span creates a structural weakness and should be avoided. If unavoidable, splices must be engineered with bolted splice plates.

Should I use regularised or sawn timber for purlins?

Regularised timber is strongly preferred for purlins. The consistent dimensions make setting out easier, alignment more accurate, and connection detailing more reliable. Sawn timber’s dimensional variation makes achieving straight, level purlin runs significantly more difficult.

Do I need treated timber for purlins?

In enclosed, well-ventilated roof spaces, untreated C16/C24 timber is acceptable. For agricultural buildings, unclad structures, or anywhere purlins may be exposed to moisture or poor ventilation, specify UC3 treated timber. Coastal locations should always use treated timber due to salt-laden air accelerating decay.

Related Guides

• Regularised Timber Sizes: UK Guide to Nominal vs Finished Dimensions – Understanding actual purlin dimensions for accurate ordering
• C16 vs C24 Timber: Which Strength Grade Do You Need? – Choosing the right grade for purlin applications
• Tanalised & Treated Carcassing Timber: Complete Protection Guide – When to specify treated timber for purlins
• Regularised Timber Category Page – Browse our purlin timber range

Need help specifying purlins for your roof project? Contact Ryedale Timber on 07403 011246. We supply 7×3, 9×3, and other purlin sizes in C16 and C24 grades across North Yorkshire and East Yorkshire with expert technical advice.