9.3 Roof Framing, Trusses, Snow Load & Wind Load Design
Key Takeaways
- Conventional roof framing utilizes sloping rafters supported by either a non-structural ridge board (when rafter ties tie opposite walls together) or a structural ridge beam (when rafter ties are absent or ceilings are vaulted).
- Collar ties must be installed in the upper third of the attic space to resist wind uplift and rafter separation at the ridge, while rafter ties (or ceiling joists) are installed in the lower third to prevent outward horizontal thrust on exterior walls.
- Manufactured wood roof trusses are pre-engineered structural units that must never be altered, field-cut, drilled, or notched without written authorization and sealed repair drawings from a licensed professional engineer.
- Minnesota Roof Snow Load design requires accounting for Ground Snow Load (Pg), ranging from 50 to 60 psf in Southern/Central Minnesota and up to 70 psf in Northern Minnesota, from which Roof Snow Load (Pf) is calculated.
- Wind exposure categories (B, C, D) and uplift forces require continuous load path tie-downs, including engineered hurricane ties connecting rafters/trusses directly to wall top plates.
Roof Framing, Trusses, Snow Load & Wind Load Design
Roof framing systems form the top structural enclosure of residential buildings, shedding water and snow while transmitting gravity and wind loads down to supporting walls and foundations. In Minnesota, roof framing must endure severe winter snow accumulation, extreme temperature differentials, and high wind gusts. Building code rules under IRC Chapter 8 and Minnesota Rules Chapter 1303 dictate structural roof design. This section covers stick-framed rafters, manufactured trusses, snow load calculations, and wind uplift resistance.
1. Conventional Rafter Framing vs. Manufactured Wood Roof Trusses
Residential roofs are constructed using either site-assembled conventional rafter framing (stick framing) or factory-manufactured wood roof trusses.
System Comparison
- Conventional Rafter Framing (Stick Framing):
- Built on site using dimensional lumber (2x6, 2x8, 2x10, or 2x12 rafters spaced 16 or 24 inches o.c.).
- Best Suited For: Custom roof shapes, dormers, complex hips and valleys, and usable attic living spaces or cathedral vaulted ceilings.
- Structural Requirements: Opposing rafters meet at a central ridge member. Vertical hip and valley rafters carry heavy tributary loads and must be sized larger than common rafters (e.g., 2x10 valley rafter for 2x8 common rafters).
- Manufactured Wood Roof Trusses:
- Pre-fabricated off site using dimensional lumber connected by metal connector plates (gang-nail plates) pressed into the wood.
- Best Suited For: Clear-span residential construction, rapid jobsite erection, and uniform structural quality.
- Structural Principle: Employs triangular web configurations (e.g., Fink, Howe, or Scissors trusses) to convert bending stresses into pure axial tension and compression forces, allowing long clear spans without interior load-bearing walls.
2. Ridge Boards vs. Structural Ridge Beams
A critical structural distinction tested on state licensing exams is the difference between a non-structural ridge board and a load-bearing structural ridge beam.
Non-Structural Ridge Board
- A 1-inch nominal (1x) or 2-inch nominal (2x) board placed between opposing rafter pairs at the roof peak.
- Function: Serves strictly as an alignment board and nailer for opposing rafters. It carries no vertical gravity load.
- Condition: Can only be used if opposing rafters are tied together at their bases by continuous ceiling joists or lower-third rafter ties to prevent the rafters from pushing exterior walls outward.
- Depth Requirement: The ridge board must equal or exceed the depth of the rafter cut end (e.g., a 2x10 ridge board for 2x8 rafters cut at a slope).
Structural Ridge Beam
- A heavy solid sawn timber, engineered LVL, or glulam beam designed to carry full vertical roof loads.
- Function: Carries half the weight of all attached rafters and transfers vertical loads to end posts, foundation columns, or bearing walls.
- Mandatory Application: Must be used whenever rafter ties or ceiling joists are omitted (such as in cathedral or vaulted ceiling designs). Because there are no lower ties to resist outward wall thrust, the structural ridge beam holds the top of the rafters up, preventing outwards wall spreading.
┌───────────────────────────────────────────────────────────────────────────┐
│ RIDGE BOARD VS. STRUCTURAL RIDGE BEAM │
├───────────────────────────────────────────────────────────────────────────┤
│ │
│ NON-STRUCTURAL RIDGE BOARD STRUCTURAL RIDGE BEAM │
│ (Requires Rafter Ties / Ceiling Joists) (Vaulted / Cathedral Ceilings) │
│ │
│ /\ /\ │
│ / \ ◄── Ridge Board / \ ◄── Heavy LVL │
│ / \ (No vertical load) / \ Ridge Beam │
│ / \ / \ (Carries load)│
│ / \ / \ │
│ /══════════\ ◄── Rafter Tie / \ │
│ / \ (Lower 1/3) / \ │
│ /==============\ ◄── Ceiling Joist / \ │
│ ┌──┐ ┌──┐ ┌──┐ ┌──┐ │
│ │ │ Wall │ │ Outward thrust │ │ Wall │ │ No outward │
│ └──┘ └──┘ resisted by ties └──┘ └──┘ thrust │
└───────────────────────────────────────────────────────────────────────────┘
3. Collar Ties vs. Rafter Ties: Placement & Mechanical Purpose
Collar ties and rafter ties are horizontal 1x4 or 2x4 framing members installed in stick-framed roofs. Though often confused, they serve completely different structural functions.
| Feature | Collar Ties (IRC R802.4.6) | Rafter Ties (IRC R802.5.2) |
|---|---|---|
| Vertical Location | Installed in the upper third of the attic space (near the ridge board). | Installed in the lower third of the attic space (at or near the top wall plate). |
| Primary Function | Resists wind uplift forces that attempt to separate rafters from the ridge board. | Resists outward horizontal thrust exerted by sloped rafters on exterior bearing walls. |
| Minimum Size | Minimum 1x4 or 2x4 lumber. | Minimum 2x4 or 2x6 lumber (or ceiling joists attached to rafter feet). |
| Maximum Spacing | Installed at intervals not exceeding 4 feet on center. | Installed on every rafter pair (16 in. or 24 in. o.c.) to form a complete triangle. |
Exam Tip: Remember location and purpose! Collar ties = Upper 1/3 (Wind Uplift). Rafter ties = Lower 1/3 (Outward Wall Thrust).
4. Engineered Roof Truss Handling, Installation & Bracing
Manufactured wood roof trusses are precision-engineered components designed to carry exact design loads. Improper handling or field modification can cause catastrophic failure.
Critical Engineering Rules
- Truss Engineering Drawings: A complete package of manufacturer truss design drawings sealed by a licensed Professional Engineer (PE) must be delivered to the jobsite and submitted to the building official prior to installation.
- NO Field Alterations: Truss members (chords, webs, or metal connector plates) must never be cut, notched, drilled, or altered on site without a written engineered repair detail sealed by a licensed professional engineer or the truss manufacturer.
- Temporary & Permanent Bracing (BCSI Guidelines):
- Trusses must be installed according to Building Component Safety Information (BCSI B1) standards.
- Temporary Lateral Bracing: Ground bracing and continuous lateral web bracing installed during erection to prevent truss dominoing/buckling before roof sheathing is applied.
- Permanent Lateral Bracing: Continuous 2x4 lateral webbing and diagonal bracing installed inside the truss attic cavity as specified on the engineering drawings to stabilize slender compression web members.
5. Minnesota Roof Snow Load Requirements
Minnesota's severe winter climate requires robust structural roof load design. Roof snow load specifications are defined in the Minnesota State Building Code (Minnesota Rules Chapter 1303).
Ground Snow Load (Pg)
Ground snow load (Pg) is the baseline weight of snow accumulated on flat ground determined by historical weather data:
- Southern & Central Minnesota (including Minneapolis-St. Paul, Rochester, Mankato): Ground Snow Load (Pg) = 50 to 60 psf (50 psf for Twin Cities metro).
- Northern Minnesota (including Duluth, Brainerd, Bemidji, International Falls): Ground Snow Load (Pg) = 60 to 70 psf.
Roof Snow Load Calculation (Pf)
Design Roof Snow Load (Pf) acting on a flat or low-slope roof is calculated using the statutory formula:
Pf = 0.7 x Ce x Ct x I x Pg
Where:
- 0.7: Basic snow exposure factor.
- Ce: Exposure factor (accounting for wind exposure around the building structure).
- Ct: Thermal factor (accounting for heated vs unheated building interiors).
- I: Importance factor (1.0 for standard residential structures; higher for essential facilities).
- Pg: Statutory Ground Snow Load in pounds per square foot (psf).
Drift Snow Loads & Unbalanced Loads
Roof assemblies must also account for drift snow accumulation against rooftop step-downs, parapets, and chimneys, as well as unbalanced snow loads on sloped roofs caused by wind drifting snow from the windward slope to the leeward slope.
6. Wind Exposure Categories & Uplift Connector Tie-Downs
In addition to gravity snow loads, roofs must withstand powerful aerodynamic wind uplift forces that attempt to peel the roof assembly off the walls.
Wind Exposure Categories (IRC R301.2.1.4)
- Exposure B: Urban and suburban areas, wooded areas, or terrain with closely spaced obstructions. (Standard for most residential subdivisions).
- Exposure C: Open terrain with scattered obstructions, flat open country, grasslands, and agricultural fields. (Higher wind velocity pressures).
- Exposure D: Flat, unobstructed areas directly exposed to wind blowing over open water bodies (e.g., Lake Superior shoreline).
Continuous Load Path & Hurricane Ties
To prevent wind uplift structural failure, residential buildings must maintain a continuous load path transferring uplift forces from the roof sheath down to the concrete foundation.
- Hurricane Ties (Framing Anchors): Engineered metal connectors (e.g., Simpson H2.5A or Hurricane ties) must be nailed directly connecting each rafter or truss bottom chord to the double top wall plate.
- Toenailing Limitations: Standard toenailing (three 8d or 16d nails) is insufficient in high-wind or wide-overhang roof designs; approved structural uplift connectors must be installed to resist calculated net uplift forces.
What is the primary difference in vertical location and structural function between collar ties and rafter ties in conventional stick-framed roof construction?
In Southern and Central Minnesota (such as the Minneapolis-St. Paul metropolitan area), what is the statutory Ground Snow Load (Pg) specified in Minnesota Rules Chapter 1303 for structural roof design calculations?
What action is legally required under the Minnesota State Building Code before any web or chord member of an engineered wood roof truss can be field-cut or altered on a construction jobsite?