Roof Truss Systems — Design Architect Knowledge Base
Truss Anatomy
╱ Top Chord (compression under gravity)
╱
────╱────────────────╲──── ← Ridge / Peak
╱ Web Members ╲
╱ (tension or ╲
╱ compression) ╲
╱ ╲
╱──────────────────────────╲
Bottom Chord (tension under gravity)
↑ ↑
Bearing point Bearing point
(reaction) (reaction)
Components
| Part | Function | Material |
|---|---|---|
| Top chord | Carries roof sheathing, transfers load to panel points | 2×4 or 2×6 SPF |
| Bottom chord | Tension tie, carries ceiling load | 2×4 or 2×6 SPF |
| Web members | Diagonal + vertical bracing between chords | 2×4 SPF |
| Panel points | Nodes where members intersect | Metal connector plates (MPC) |
| Heel | Junction of top and bottom chord at bearing | Critical for height/insulation |
| Peak | Apex where top chords meet | MPC gusset |
| Bearing point | Where truss sits on wall/beam | Min 1.5" bearing required |
Common Residential Truss Types
Fink (W-Pattern) — Most Common
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- Span: Up to 40–50'
- Best for: Standard residential roofs, no attic use
- Depth: Shallow (economic use of material)
- Why it's #1: Best strength-to-weight ratio, cheapest to fabricate
- ML Systems use: Primary truss type spanning between steel beams
Howe
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- Span: Up to 60'
- Best for: Heavy snow loads, clay tile roofing
- Verticals in compression, diagonals in tension (opposite of Pratt)
- ML Systems use: When snow load > 35 PSF or heavy roofing material
Scissors
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- Span: Up to 30–35'
- Best for: Cathedral/vaulted ceilings
- Bottom chord slopes up (typically 1/2 of top chord pitch)
- Caution: Higher horizontal thrust at bearings — needs robust connection
- ML Systems use: Living room feature ceiling over steel beam
Attic Truss
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ROOM
- Span: Up to 30' (limited by room inside)
- Best for: Habitable attic space in Cycle 2
- Live load increases to 30 PSF for habitable space
- ML Systems use: Cycle 2 expansion — room within truss
Mono (Single Slope)
╱
╱─────
╱ | |
╱───|──|──
- Span: Up to 30'
- Best for: Shed roofs, additions, clerestory windows
- Single bearing point at high end, roller at low end
- ML Systems use: Lean-to additions, covered entries
Hip Truss Set
- Not a single truss — a SET of progressively shorter trusses
- Standard truss at center, stepping trusses reduce toward corners
- Hip jack trusses transfer load to hip girder truss
- More complex framing, but better wind resistance (lower profile at corners)
Truss Spacing
| Spacing | Tributary Width | Common Use |
|---|---|---|
| 24" o.c. | 2.0 ft | Standard residential (default) |
| 16" o.c. | 1.33 ft | Heavy loads, long spans, tile roofs |
| 48" o.c. | 4.0 ft | Engineered trusses, heavy top/bottom chords |
ML Systems default: 24" o.c. wood trusses spanning between steel beams
Span Capacity (Fink Truss, 24" o.c.)
| Truss Depth | Dead Load 10 PSF | Dead + Snow 30 PSF | Dead + Snow 40 PSF |
|---|---|---|---|
| 12" | 28' | 24' | 22' |
| 18" | 36' | 32' | 28' |
| 24" | 42' | 38' | 34' |
| 30" | 48' | 44' | 40' |
Values approximate — actual capacity depends on lumber grade, MPC rating, and specific geometry. Always per engineer's design.
Critical for ML Systems: With 20' bay spacing, trusses span 20' between steel beams — well within capacity for any standard truss depth. This is the clear-span advantage of the hybrid system.
Truss-to-Steel Beam Connections
Bearing on Top of Steel Beam
╱ Truss ╲
╱─────────╲
│ bearing │
═══════════════ ← Steel beam top flange
W12×26
- Truss sits directly on beam top flange
- Wood nailer plate (2× PT) lag-bolted to beam flange
- Truss toe-nailed or hurricane-tied to nailer
- Advantage: Simple, allows full truss depth above beam
- Disadvantage: Adds truss depth to overall building height
Hung from Steel Beam (Joist Hanger)
═══════════════ ← Steel beam
┌─ hanger ─┐
│ ╱ ╲ │
│ ╱truss╲ │
- Welded or bolted joist hanger on beam web
- Simpson HUS or custom steel hanger
- Advantage: Flush ceiling — truss bottom chord aligns with beam bottom flange
- Disadvantage: Requires field-bolted hanger (DfD compatible) or shop-welded (not DfD)
ML Systems Preferred Detail
Top-bearing with bolted nailer plate:
Hurricane Ties — Uplift Resistance
Simpson Connectors for ML Systems
| Connector | Capacity (Uplift) | Use |
|---|---|---|
| H2.5A | 475 lbs | Light wind zones, truss to plate |
| H10 | 1,100 lbs | Standard — RI wind zones |
| H10A | 1,130 lbs | Skewed installation option |
| LSTA | 1,175 lbs | Strap tie over truss, both sides |
| HDU | 3,000+ lbs | Holdown — column to foundation |
RI Wind Uplift Requirement
Uplift per connector = (Wind uplift PSF × tributary area) − (0.6 × Dead load)
Example: 20 PSF uplift × (2' × 10') = 400 lbs gross
0.6 × (12 PSF × 2' × 10') = 144 lbs counteracting
Net uplift = 400 − 144 = 256 lbs → H2.5A adequate
Coastal RI: Higher wind → H10 or LSTA required
Continuous Load Path (Roof to Foundation)
Roof sheathing → Ring-shank nails → Truss top chord
↓
Hurricane tie (H10) → Truss to nailer plate
↓
Nailer plate → Through-bolts → Steel beam flange
↓
Beam end-plate → A325 bolts → Column flange (moment connection)
↓
Column base plate → Anchor bolts → Grade beam
↓
Grade beam → Rebar → Spread footing → Soil
Every link in this chain must be designed for the same uplift force. A single weak link = failure.
Truss Uplift (Moisture-Related)
What It Is
Bottom chord of truss bows UPWARD in winter, pulling ceiling drywall away from partition walls. Not structural failure — a serviceability/cosmetic issue.
Cause
- Top chord: cold, high moisture content (exposed to attic air)
- Bottom chord: warm, low moisture content (insulated, conditioned space below)
- Wood shrinks when it dries → bottom chord shrinks → truss cambers up
Magnitude
- Typical: 1/4" to 1/2" uplift at mid-span
- Severe: 3/4" to 1" (long spans, high insulation, dry climate)
Mitigation Details
ML Systems Advantage
Precast hollow-core floors + steel beams at intermediate levels ELIMINATE truss uplift at floors. Uplift only relevant at roof trusses over top-floor partitions.
Truss Design for Multi-Cycle
Cycle 1 (Current)
- Standard Fink trusses at 24" o.c., spanning 20' between steel beams
- Bearing on bolted nailer plates
- Hurricane ties per RI wind requirements
Cycle 2 (Future +1 Level)
- Existing roof trusses REMOVED by crane (2–4 hour operation)
- New precast floor placed on existing steel beams at former roof level
- New steel columns erected on stub plates (embedded in Cycle 1)
- New trusses installed at new (higher) roof level
- Key: Cycle 1 trusses are DISPOSABLE — DfD by design
Design Implications
- Don't over-specify Cycle 1 trusses — they're temporary (10–15 year life)
- DO over-specify connections (nailer plates, hurricane ties) — they define the DfD procedure
- Bottom chord insulation detail matters less because the entire roof assembly gets replaced
Truss Engineering Notes
When Trusses Must Be Engineered (Not Prescriptive)
- Span > 26' (IRC prescriptive tables max out)
- Snow load > 30 PSF ground snow
- Non-standard configurations (piggyback, cantilever, offset bearing)
- Habitable attic trusses (always engineered)
- ML Systems: ALL trusses engineered — steel hybrid system = non-prescriptive per IBC
Truss Bracing Requirements
- Permanent lateral bracing: Continuous 2×4 nailed to top chord at 45° intervals
- Bottom chord bracing: Every 10' max for bottom chords > 20' long
- Web member bracing: Per truss engineer's design drawings
- T-bracing: Perpendicular to compression web members > 4' long
Truss Camber
- Long-span trusses (> 30') should be manufactured with upward camber
- Typical camber: L/300 to L/360 (prevents visible sag under dead load)
- Example: 30' span → 30×12/360 = 1" upward camber at mid-span
Fire Rating
- Unprotected wood trusses: 0-hour fire rating
- With 5/8" Type X GWB ceiling: 1-hour assembly (UL Design U300 series)
- ML Systems advantage: Steel beams carry gravity loads even if wood trusses burn — steel buys evacuation time