Quick Answer
Purlins run horizontally across the top of trusses to support roofing materials, while girts run horizontally across the posts to support siding. Standard spacing is 24 inches on-center for both, though heavy snow loads may require tighter purlin spacing. These components are the "skeleton" that transfers loads to the posts and trusses.
What Are Purlins and Girts?
In post-frame (pole barn) construction, purlins and girts are horizontal framing members that create the attachment surface for roofing and siding:
- Purlins - Horizontal members attached to the top chord of trusses, supporting roofing
- Girts - Horizontal members attached to posts, supporting siding
- Both - Typically 2x6 lumber, spaced 24 inches apart
According to the National Frame Building Association (NFBA), purlins and girts are critical for:
- Load distribution - Transfer roofing and siding loads to structural frame
- Attachment points - Provide nailing surface for metal panels
- Building rigidity - Help tie the structure together
- Insulation cavity - Create space for insulation materials
Purlin Guide (Roof Framing)
Standard Purlin Spacing
For metal roofing, standard purlin spacing is:
| Roofing Type | Purlin Spacing | Notes |
|---|---|---|
| 29 gauge metal | 24 inches o.c. | Standard residential application |
| 26 gauge metal | 24-32 inches o.c. | Heavier gauge allows wider spacing |
| Shingles (solid decking) | 12-16 inches o.c. | Requires plywood/OSB sheathing |
| Heavy snow areas | 16-24 inches o.c. | Tighter spacing for load capacity |
Purlin Installation Methods
Method 1: On-Edge (Stronger)
The purlin is installed vertically on its edge, resting on the truss top chord.
Advantages:
- Greater strength and load capacity
- Less deflection under snow load
- Provides 5.5-inch cavity for insulation
Installation: Screw through the wide face of the purlin into the truss top chord using two #10x2½ wood screws per connection.
Method 2: Flat (Simpler)
The purlin is installed flat on top of the truss top chord.
Advantages:
- Easier and faster installation
- Less precision required
- Common in agricultural buildings
Disadvantages:
- Reduced strength (2x6 flat is weaker than on-edge)
- Less insulation cavity (1.5 inches)
- More prone to sagging over time
Purlin Overhang Handling
At the eaves, purlins should be flush with or slightly extending past the truss end. For overhangs:
- Standard overhang (6-12 inches) - Extend purlins and add fly rafters
- Large overhangs (24+ inches) - Use outriggers or ladder-framing
Ridge Purlins
A continuous ridge purlin (also called a ridge board) at the peak:
- Provides attachment for roofing panels at the ridge
- Connects trusses together at the peak
- Sometimes required by code in high-wind areas
Girt Guide (Wall Framing)
Standard Girt Spacing
For metal siding, standard girt spacing is:
| Siding Type | Girt Spacing | Notes |
|---|---|---|
| 29 gauge metal | 24 inches o.c. | Standard application |
| 26 gauge metal | 24-32 inches o.c. | Heavier gauge allows wider spacing |
| Plywood/OSB sheathing | 16-24 inches o.c. | Depends on panel thickness |
| Board and batten | 24 inches o.c. | Standard for vertical siding |
Girt Placement Methods
Method 1: Attached to Outside of Posts
Girts are fastened to the outside face of the posts.
Advantages:
- Maximum interior space
- Easier installation
- Standard method for most pole barns
Disadvantages:
- Posts are visible from inside
- Less wall insulation depth (5.5 inches max)
Method 2: Between Posts (Commercial/Residential)
Girts are installed between posts, flush with the post face.
Advantages:
- Smooth interior wall surface
- Better for finished interiors
- Looks cleaner for barndominiums
Disadvantages:
- More complex installation
- Requires notching or blocking
- Reduces interior width slightly
Method 3: Commercial Girts (Bookshelf Girts)
Girts are attached flat between posts, creating a shelf for insulation.
Advantages:
- Creates deep insulation cavity (6+ inches)
- Provides shelf for blown-in insulation
- Smooth interior when finished
Best For: Climate-controlled buildings requiring heavy insulation
Door and Window Rough Openings
When framing openings:
- Header - Double 2x6 or engineered lumber across opening top
- King studs - Full-height posts on each side
- Jack studs - Support the header
- Trimming - Additional girts may be needed around openings for siding attachment
Material Specifications
Lumber Grade and Treatment
The APA - The Engineered Wood Association provides guidelines for purlin and girt lumber:
| Application | Minimum Grade | Treatment |
|---|---|---|
| Roof purlins | #2 SPF or better | No treatment required (under roof) |
| Wall girts | #2 SPF or better | Pressure-treated if exposed to weather |
| Bottom girts | #2 or better | Pressure-treated (ground contact) |
Fastener Requirements
Per the International Building Code:
- Purlins to trusses - Two #10x2½ wood screws per connection
- Girts to posts - Two #10x2½ wood screws or 16d nails
- Splicing - When splicing purlins/girts, use a 2-foot splice member
- Corrosion-resistant - Use hot-dipped galvanized or stainless fasteners
Structural Considerations
Load Path
Purlins and girts are part of the continuous load path that transfers forces to the ground:
- Snow/wind load hits roofing/siding
- Purlins/girts distribute load to trusses/posts
- Trusses/posts transfer load to foundation/ground
Break this chain at any point, and the building can fail.
Deflection Limits
Purlins and girts must not deflect (bend) more than L/180 under total load (L = span length). This means:
- 24-inch spacing = 0.13-inch maximum deflection
- 32-inch spacing = 0.18-inch maximum deflection
Excessive deflection causes oil canning (waviness) in metal roofing and siding.
Bracing and Diagonals
Large buildings may require diagonal bracing in the purlin/girt bays:
- Wind bracing - Diagonal members in end walls
- Diaphragm action - Metal skin can act as bracing when properly attached
Installation Best Practices
- Start at the bottom - Install bottom girts first to establish baseline
- Work upward - Install girts progressively up the wall
- Layout with story pole - Create a measuring stick for consistent spacing
- Check for plumb - Verify girts are level as you install
- Pre-drill metal - When installing metal panels, pre-drill to prevent distortion
- Stagger splices - When splicing lumber, stagger splices between adjacent bays
- Seal penetrations - Use flashing or sealant where roof/wall penetrations occur
Common Mistakes
Mistake 1: Inconsistent Spacing
Varying spacing causes metal panels to wave and creates weak points. Use a consistent measurement method and verify spacing before fastening.
Mistake 2: Wrong Lumber Orientation
Installing 2x6s flat when on-edge is specified reduces strength by over 60%. Follow engineering specifications.
Mistake 3: Ignoring Treatment Requirements
Bottom girts in contact with concrete or earth must be pressure-treated. Untreated lumber will rot within 5-10 years.
Mistake 4: Inadequate Fastening
One screw per connection is insufficient. Use two fasteners at each purlin/girt intersection for proper load transfer.
Mistake 5: Not Accounting for Future Finishes
If you plan to finish the interior later, consider using commercial girt methods or adding furring strips. Retrofitting insulation over exposed girts is challenging.
Common Questions
Q: What's the difference between purlins and girts?
A: Purlins are horizontal roof members that support roofing. Girts are horizontal wall members that support siding. Both are typically 2x6 lumber on 24-inch centers.
Q: Can I use 2x4s instead of 2x6s?
A: For small sheds with light loads, 2x4s may be acceptable. However, 2x6s provide significantly more strength and only cost marginally more. The 5.5-inch depth also provides better insulation cavity.
Q: How do I attach purlins to scissor trusses?
A: Use vertical blockers (squash blocks) between truss webs to create a flat surface for purlins. Don't rely on screws into the angled top chord alone.
Q: Should purlins run parallel or perpendicular to the ridge?
A: Purlins run perpendicular to the ridge (parallel to the truss span). They connect across the top chords of multiple trusses.
Q: Do I need girts if I'm using board and batten siding?
A: Yes, girts provide the structural backing for board and batten. The boards attach to the girts, and battens cover the gaps between boards.
Q: How do I handle purlins at a roof transition (broken roof)?
A: Install valley flashing and frame the transition with additional purlins and blocking. Complex roof transitions should be detailed in your engineered plans.
Sources & References
- National Frame Building Association (NFBA), "Post-Frame Building Design Manual"
- APA - The Engineered Wood Association, "Pole Barn Construction Guidelines"
- International Code Council, "2021 International Building Code"
- Steel Manufacturers Institute, "Metal Roof and Siding Installation Guide"
- Wood Works, "Post-Frame Construction Fastener Requirements"