Snow Load Defined
Snow load is the downward force per unit area exerted by snow accumulation on a structure, expressed in pounds per square foot (PSF). It's a structural engineering input — not a roofing material specification — that drives framing member sizing, beam design, and connection requirements.
Two key snow load values:
- Ground snow load (Pg): snow load that would accumulate on flat ground in the area. Statistical 50-year recurrence interval. ASCE 7-22 Figure 7.2-1 maps Pg across the US.
- Design roof snow load (Ps): roof snow load after applying exposure factor (Ce), thermal factor (Ct), importance factor (I), and slope factor (Cs). Always less than Pg due to roof slope shedding + thermal melt + wind exposure.
Massachusetts Snow Load Regional Map
Per ASCE 7-22 Table 7.2-1 + MA-specific addenda:
| Region | Ground Snow Load (Pg) | Typical Design Roof (Ps) |
|---|---|---|
| Cape Cod (immediate coast) | 40 PSF | 28-32 PSF |
| South Coast (New Bedford, Fall River) | 40-45 PSF | 30-35 PSF |
| Greater Boston (Boston, Cambridge) | 50 PSF | 35-42 PSF |
| MetroWest (Worcester area) | 50 PSF | 35-42 PSF |
| Berkshires (Pittsfield, Lenox) | 60 PSF | 42-50 PSF |
| Pioneer Valley hill towns | 55-65 PSF | 40-50 PSF |
Local building department confirms exact Pg for your specific location on permit application.
Roof Snow Load Calculation
Per ASCE 7-22 Section 7.3:
Ps = 0.7 × Ce × Ct × I × Pg
Where:
- Pg = ground snow load (50 PSF most MA)
- Ce = exposure factor (0.9 sheltered, 1.0 partial, 1.1 fully exposed). Most MA homes: 1.0.
- Ct = thermal factor (1.0 heated buildings, 1.1 unheated, 1.2 cold roofs). Most MA homes: 1.0.
- I = importance factor (1.0 residential, 1.1 essential facilities like hospitals, schools).
For typical heated MA single-family: Ps = 0.7 × 1.0 × 1.0 × 1.0 × 50 = 35 PSF design roof snow load.
Slope Adjustment
Slope factor Cs reduces snow load on steeper roofs (snow sheds). Cs = 1.0 for slopes 0-30°, decreasing linearly to 0 at 70°. A 9:12 (37°) roof: Cs ≈ 0.83, reducing design snow to 29 PSF.
Structural Impact: What 50 PSF Means in Practice
50 PSF snow load drives material specifications:
Roof Framing
2x10 rafters at 16" o.c. span ~14'-2" in 50 PSF MA. Vs 16'-8" span in 30 PSF Texas. To get 16-ft spans in MA: 2x12 at 12" o.c. or LVL engineered joists.
Beams
Triple 2x10 ledger beam supports 16-ft span deck in 50 PSF; would support 19-ft in 30 PSF.
Footings
Snow load increases column loads + footing loads. Larger footings + deeper foundations needed in MA vs warmer-climate equivalents.
Heavy Material Roofs
Slate (9-12 PSF dead load) + 50 PSF snow = 60+ PSF total. Pre-1980 rafter systems often need sistering or strongback addition before slate install.
Frequently Asked Questions
Why is Massachusetts snow load so high?
Does the 50 PSF apply to my roof's actual snow load?
Do metal roofs reduce snow load?
Can I clear snow off my roof during a heavy storm?
How does snow load affect deck design?
Does MA snow load vary by elevation?
References & Sources
