Also in: Disaster Response and Recovery
Existing Roof Snow and Ice Weight Against Design Load
After a heavy snowfall the question is not the design load but the load on the roof now, measured with a ruler.
Example
You enter
- Layer 1 (bottom) depth (in) 18
- Layer 1 type wet
- Layer 2 depth (in) 2
- Layer 2 type ice
- Layer 3 (top) depth (in) 0
- Layer 3 type dry
- Entered unit weight (psf per ft of depth) 15
- Ground snow load for the ASCE 7 density (psf) 0
- Entered weight outside 3-21 (measured core) no
- Roof design snow load (psf) 30
You get
- Layer1 (psf) 31.5
- Layer2 (psf) 9.5
- Total load on the roof 41.0 psf
- Design ratio 1.367
Checked against published worked examples from: FEMA.
Details, formula, and sources
ScopeAfter a heavy snowfall the question is not the design load but the load on the roof now, measured with a ruler, and whether it is time to clear the roof or leave the building. A foot of snow can weigh 3 pounds a square foot or 21, and the difference is water; ice at about 57 psf per foot is almost as heavy as water itself. FEMA P-957 gives the range, and the calculator takes up to three layers separately because a roof after a long storm is a stack: old settled snow, an ice layer from a freeze-thaw, and fresh snow on top. The ASCE 7 density relation is available as an option, and an entered unit weight is held to the FEMA 3 to 21 range unless a measured core is used. A roof carrying 18 in of wet, heavy snow over 2 in of ice holds 31.5 + 9.5 = 41.0 psf; on a roof designed for 30 psf that is a ratio of 1.37, 37% over its design snow load -- time to get the load off by a safe method, and to leave if any warning sign appears. The same 18 in as light, dry snow over the same 2 in of ice is 14.0 psf, under half the design load, which is why the ruler alone does not answer the question. The comparison is with the roof's design snow load from the drawings, never the ground snow load on a map. Sagging, new cracks, creaking, or sticking doors override the arithmetic. It does not compute design, drift, sliding, or rain-on-snow loads or evaluate the structure, and clearing is hazardous in itself. FEMA P-957, the building's design documents, and a structural engineer govern.
Formula or tableload = sum of layer depth (ft) x unit weight (psf per ft): dry snow 3, settled 12.5 (mid of 10-15), wet 21, ice 57, ASCE 7 density 0.13 pg + 14 <= 30, or an entered 3-21 value (override for a measured core); ratio = load / roof design snow load.
Source and editionFEMA P-957, Snow Load Safety Guide (2013), Section 2.2 unit weights and warning signs; the ASCE 7 snow density relation cited by section as an optional basis. The building's design documents and a structural engineer govern.
Free accessFEMA P-957 is a free federal publication; ASCE 7 is cited by section only.
What governsEstimate. AHJ and licensed structural engineer govern.
API field nameslayer1_depth_in, layer1_type, layer2_depth_in, layer2_type, layer3_depth_in, layer3_type, entered_weight_pcf, ground_snow_psf, override_range, design_snow_psf, layer1_psf, layer2_psf, total_psf, design_ratio
- Snow unit weight about 3 psf per ft light and dry to 21 psf per ft wet and heavyFEMA P-957 Section 2.2
- Ice unit weight about 57 psf per ft, a little under 5 psf per inchFEMA P-957 Section 2.2
- Settled snow 12.5 psf per ft, the middle of the commonly cited 10-15 rangetile default