The following guide explains the most widely used roof truss designs for residential and light commercial buildings, helping owners, builders, and designers choose the best option for span, load, cost, and interior layout. Roof truss types vary by geometry, structural behavior, and application, and understanding their differences improves design decisions and construction efficiency.
| Truss Type | Best Use | Key Benefit |
|---|---|---|
| King Post | Short Spans, Attics | Simplicity |
| Queen Post | Medium Spans | Open Space |
| Pratt | Large Spans | Efficient Load Path |
| Howe | Heavy Roof Loads | Compression/ Tension Balance |
| Fink | Residential | Material Efficient |
| Scissors | Vaulted Ceilings | Interior Height |
| Mono | Additions, Sheds | Simple Slope |
| Mansard | Architectural Styles | Extra Attic Space |
| Pagoda | Decorative/Small Structures | Visual Appeal |
| Bowstring | Wide Clear Spans | Long-Span Arched Roof |
Overview Of Roof Trusses
Roof trusses are engineered assemblies of members that create a rigid framework to support roof loads. They replace traditional rafters and purlins in many projects and are manufactured from wood, steel, or engineered members. Choosing a truss type affects span capability, attic space, roof pitch, and installation cost.
King Post Truss
The King Post truss is the simplest triangular truss consisting of two rafters, a horizontal tie beam, and a central vertical post. It suits short spans up to about 20 feet and is common in small residences and historic restorations. Advantages include low material cost and straightforward fabrication; limitations are limited clear span and modest attic height.
Queen Post Truss
The Queen Post truss uses two vertical posts and a central straining beam, enabling longer spans than the King Post—commonly up to 30–45 feet. It provides a more open attic space and better distribution of compression and tension forces. It is appropriate for medium-span roofs where a simple aesthetic or exposed timber interior is desired.
Fink Truss
The Fink truss, identifiable by its W-shaped web pattern, is widely used in residential construction for spans up to 60 feet. It is highly material-efficient, offers good load distribution, and is economical to produce in engineered wood. Limitations include limited attic headroom unless modified; it is typically chosen for cost-sensitive projects.
Pratt Truss
The Pratt truss has diagonal web members that slope toward the center under tension and verticals in compression, making it efficient for longer spans. It is common in bridges and long-span roofs where repetitive, prefabricated members expedite construction. The geometry provides predictable load paths and straightforward fabrication with timber or steel.
Howe Truss
The Howe truss is the inverse of the Pratt, with diagonal members in compression and verticals in tension, which benefits heavy roof loads and timber construction. It performs well under heavy snow loads and is often chosen where compressive strength of wood members is a key factor. It can be heavier and less material-efficient than Pratt for the same span when steel is used.
Scissors Truss
Scissors trusses feature bottom chords that cross and slope upward, forming a vaulted ceiling beneath the roof plane. They allow for open, cathedral-like interiors without requiring ridge beams, offering aesthetic and volume advantages. They require careful design for lateral stability and are slightly more expensive than standard flat-bottom trusses due to additional geometry complexity.
Mono Truss
Mono trusses have a single sloping plane and are ideal for lean-tos, additions, porches, and modern minimal roofs. They provide a simple, economical solution for one-way drainage and solar orientation adjustments. Mono trusses are limited by their unidirectional load span and are typically used for shorter spans or shed roofs.
Mansard Truss
Mansard trusses create a double-pitched profile with a steep lower slope and a shallow upper slope, producing additional usable attic space. They are common in architectural styles seeking maximum attic volume and historically inspired facades. Complexity and flashing requirements increase cost, but they deliver high storage or living space without adding another full story.
Pagoda And Decorative Trusses
Pagoda and other decorative trusses serve mainly aesthetic and small-structure functions, often used on porches, pavilions, or entryways. They prioritize form and detail—curved members, exposed joinery, or ornamental profiles—while still providing structural support for limited spans. Engineers often adapt standard truss forms for these applications to ensure safety and durability.
Bowstring Truss
Bowstring trusses have an arched top chord and straight bottom chord, offering long clear spans for gymnasiums, hangars, and warehouses. The shape distributes loads efficiently across wide spans and creates a distinctive vaulted interior without intermediate supports. Fabrication can be more complex and costlier, but they excel where unobstructed interior space is essential.
Selecting The Right Truss Type
Selection depends on span, roof pitch, live loads (snow, wind), desired interior space, aesthetic priorities, and budget. Engineered trusses are typically specified by span and load criteria; builders should consult structural engineers for non-standard or long-span cases. Local building codes and climate-related loads significantly influence truss design choices.
Materials And Manufacturing
Trusses are commonly built from dimensional lumber with metal connector plates or from steel sections for longer spans. Engineered wood products—like laminated veneer lumber (LVL) and truss plates—provide consistent strength and reduce on-site fabrication time. Steel trusses may be preferred for very long spans or where fire resistance and slender profiles are needed.
Installation And Handling Considerations
Proper handling during delivery and erection is critical; trusses should be lifted by designed points to avoid distortion. Bracing is essential during installation to maintain geometry and prevent collapse under construction loads or wind. Contractors must follow manufacturer instructions and local code requirements for bracing, fastening, and anchorage.
Cost Factors And Comparative Estimates
Cost varies by truss type, span, material, and finish. Simple King Post or Mono trusses are among the least expensive per unit, while Bowstring and Mansard trusses require more material and labor. Engineered mass production reduces per-truss cost for common designs like Fink, but custom shapes increase price. Consider life-cycle costs including insulation, ventilation, and maintenance when comparing alternatives.
Maintenance And Longevity
Truss longevity depends on material durability, moisture control, and maintenance. Wood trusses require proper ventilation, termite protection, and periodic inspection of connectors and fasteners. Steel trusses should be checked for corrosion and protective coatings. Addressing minor issues early prevents costly repairs and preserves structural performance.
Common Design And Code Considerations
Key design factors include snow and wind loads, seismic considerations, roof pitch, and eave overhangs. Trusses must meet local building codes and be engineered for load combinations relevant to the project location. Architects and engineers should provide accurate plans and coordinate truss shop drawings before fabrication.
FAQs About Roof Trusses
How Long Can A Truss Span Without Support? Span capability varies by truss type and material; wooden trusses commonly span up to 60 feet with standard designs, while steel or engineered solutions can exceed that significantly.
Can Trusses Be Modified After Fabrication? Cutting or altering truss members in the field is generally unsafe and prohibited; any modification requires engineer approval and often replacement or reinforcement.
Do Trusses Affect Attic Ventilation? Yes; truss design influences attic volume and airflow. Proper ventilation planning—ridge vents, soffit vents, and baffles—is necessary to control moisture and roof longevity.
Resources For Further Research
Industry manuals, local building codes, truss manufacturer catalogs, and structural engineering references provide detailed span tables and load charts. Consulting licensed structural engineers and local truss fabricators ensures compliance, optimized design, and accurate cost estimates.