How to Calculate Roof Trusses
Roof trusses are engineered structural assemblies that support the roof and eliminate the need for load-bearing interior walls. They are ordered by span, pitch, spacing, and type. Each truss is stamped by an engineer for a specific load combination and cannot be modified in the field.
Truss Count Formula
Trusses=Building LengthftSpacingft+1
Truss Types
- Fink (W-truss) - most common residential, up to 33 ft span
- King post - simplest with single center vertical, up to 30 ft
- Queen post - two verticals, up to 36 ft
- Howe - heavy loads, up to 40 ft
- Scissor - angled bottom chord creates vaulted ceiling
- Attic truss - open room space above for storage or living
Spacing Guidelines
Standard truss spacing is 24 inches on center, sufficient for most residential loads with 7/16 or 15/32 inch OSB sheathing. Use 16 inch spacing for heavy snow loads (over 40 psf), cathedral ceilings, or thinner sheathing applications.
Never Modify Trusses in the Field
Cutting, drilling, or notching a truss chord or web member can reduce capacity by 50 percent or more. If you need chases for mechanicals, specify them to the truss manufacturer in advance, or use conventional rafters where field modifications are acceptable.Hurricane Ties Required
The IRC requires hurricane ties (Simpson H1, H2.5) between each truss and the top plate in every region. Coastal and tornado zones may require heavier ties (H10, HTT4) at each bearing end.Trusses vs. Stick Framing
| Factor | Trusses | Stick Framing |
|---|---|---|
| Install time | 1 to 2 days | 1 to 3 weeks |
| Labor cost | Lower | Higher |
| Material cost | Higher | Lower |
| Attic usability | Limited (attic truss excepted) | Full height possible |
| Interior walls | No load-bearing needed | May require load-bearing |
| Lead time | 2 to 6 weeks manufacturing | None - buy lumber on demand |
Truss Bracing and Handling
Trusses are fragile when flat or unsupported. They only achieve full strength once properly installed, braced, and sheathed. Critical installation rules:
- Do not lift trusses flat - always by the top chord with a spreader bar
- Install temporary lateral bracing per the TPI/BCSI standard before releasing the crane
- Install permanent bracing (diagonal and horizontal) per the truss engineer's package
- Sheath the roof before storing any weight on the trusses
- Never stack material on a single truss - loads must transfer to multiple trusses
Order Extra Trusses
Order 5 to 10% extra trusses to cover damage during transport and installation. A cracked truss cannot be repaired in the field - it must be replaced. Having extras on site saves days of lost time if a truss arrives damaged.