Live load is everything on a structure that is not its own permanent weight — the people using it, goods stored inside, grain piled in a granary, snow on the roof, wind against the walls — and unlike dead load it changes constantly with use. A residential room or classroom only needs to carry about 40 pounds per square foot, while a granary with grain piled in it needs 100 to 200 and heavy-goods storage needs 125 to 250, so using a residential figure for a granary floor would be a dangerous underestimate. Dynamic loads — the impact of a loaded cart crossing a bridge, or machinery vibration on a floor — can run several times higher than the equivalent static load and have to be accounted for on top of the static number, not folded into it.
Part of Structural Engineering
Estimating the variable loads from people, goods, animals, snow, and wind that a structure must carry in addition to its own weight.
Live Loads Change With Use, Snow, Wind, and Impact
Dead load — the weight of the structure itself — is constant and predictable. Live load is everything else: the people who use the building, the goods stored inside, the grain piled in a granary, the snow on the roof, the wind pressing against the walls. Live loads change over time and vary with use. They can be much larger than the dead load for some building types, or relatively small for others.
Getting live loads wrong is dangerous in both directions. Underestimating live load leads to structural failure when the building is put to its intended use. Overestimating live load wastes materials in the foundation and structure unnecessarily. For a rebuilding community operating with limited materials, this balance matters.
Live loads also include dynamic effects — loads that change rapidly, like the impact when a heavy cart rolls over a bridge, or the vibration of machinery on a floor. Dynamic loads can be several times larger than the equivalent static load and must be accounted for separately.
Occupancy Live Loads
These are the loads from people and their activities. Use these values for floors, stairs, and platforms.
Floor live loads by occupancy type:
| Occupancy | Live load (lb per square foot) |
|---|---|
| Residential rooms | 40 |
| Residential corridors | 40 |
| Assembly hall, church | 100 |
| Classrooms | 40 |
| Storage (general light) | 50–75 |
| Storage (heavy goods) | 125–250 |
| Market, trading floor | 75–100 |
| Granary (grain piled) | 100–200 |
| Workshop, light industry | 50–100 |
| Workshop, heavy equipment | 100–200 |
| Roof access (no use) | 20 |
| Roof with congregation | 50–100 |
How these values are derived, and why the arithmetic is only the floor of the answer: a full assembly hall at maximum density holds roughly 1 person per 4 square feet, and a 150 lb person on 4 square feet is 37.5 lb/sq ft. Do not design an assembly floor to that number. The table’s 100 lb/sq ft is the figure to use, and the gap between them is not padding:
- A crowd is not a static array of standing bodies. People move, and in a hall they often move in step — dancing, clapping, rushing one exit — which amplifies the load well beyond standing weight.
- The figure must also carry the furnishings, the stored goods along the walls, and the local concentration where everyone crowds to one side of the room.
Static body weight per unit area tells you a floor cannot possibly need less than about 40 lb/sq ft. It does not tell you what it needs.
Storage live loads: These can be enormous. A floor loaded with stacked grain (density ~50 lb/cu ft) to 3 feet depth exerts 150 lb/sq ft live load. A floor storing iron products or stone blocks can easily reach 500+ lb/sq ft. Always calculate the actual weight of planned storage loads rather than assuming a standard value will cover it.
Snow Loads
Snow accumulates on roofs and must be carried by the structure. Snow load depends on location, roof slope, and wind patterns.
Fresh snow density: 5–15 lb per cubic foot (light, dry snow is lighter; wet snow is heavier) Old compacted snow: 15–25 lb per cubic foot Wet spring snow: 25–40 lb per cubic foot
Typical design snow loads (on horizontal roof projection):
| Climate | Snow load (lb per sq ft) |
|---|---|
| No snow zone | 0 |
| Light snow zone | 10–20 |
| Moderate snow zone | 20–40 |
| Heavy snow zone | 40–70 |
| Very heavy (mountain) | 70–150+ |
Effect of roof slope: Snow slides off steep roofs. For roof slopes over 30°, reduce calculated snow load by 50%. For slopes over 60°, snow load is negligible (snow slides off before it accumulates significantly). Flat or nearly flat roofs retain full snow load — and can accumulate drifts even deeper than the baseline load if surrounded by higher obstructions.
Drift loading: Snow drifts against vertical obstructions (higher roof sections, walls above the roof, equipment housing). A roof adjacent to a taller wall can accumulate drifts 2–3× the normal design snow load immediately adjacent to the wall. Design these areas for extra load.
Calculating roof snow load:
A slope does not collect more snow. It spreads the same snow over more surface, so the total load on the building does not change with pitch. The table above is already given per square foot of horizontal plan area, so:
Total snow on the roof = table value × plan area, whatever the pitch.
A 30 ft × 24 ft building is 720 sq ft in plan. In a moderate zone at 30 lb/sq ft that is 720 × 30 = 21,600 lb of snow, flat or pitched.
If you need the load per square foot of the sloping roof surface — for sheathing, or for battens running up the slope — the sloping area is larger than the plan area by 1 ÷ cos(slope), so the load per square foot of surface is smaller by cos(slope):
- 30° roof: 30 × cos 30° = 30 × 0.866 = 26 lb/sq ft of roof surface
- 45° roof: 30 × 0.707 = 21 lb/sq ft of roof surface
For sizing rafters, do neither conversion. Use the plan-area load with the horizontal span. That pairing is already consistent and it is where mistakes stop happening.
An earlier version of this page divided by cos² instead
It gave 30 ÷ cos²(30°) = 30 ÷ 0.75 = 40 lb/sq ft, which is larger than the flat-roof figure — the wrong direction, since a pitched roof carries no extra snow. It also contradicted the slide-off rule in the paragraph above, which reduces the load by half at this same 30°: 15 lb/sq ft against 40, a factor of 2.7 apart on one page. If a conversion makes a load go up with slope, the conversion is upside down.
Wind Loads
Wind exerts pressure on surfaces facing into the wind (windward) and suction (negative pressure) on surfaces facing away from the wind (leeward and side walls, roof leeward slope).
Basic wind pressure: Pressure = 0.003 × V² (lb per sq ft), where V = wind speed in mph
- 50 mph wind: 0.003 × 2500 = 7.5 lb/sq ft
- 80 mph wind: 0.003 × 6400 = 19 lb/sq ft
- 100 mph wind: 0.003 × 10000 = 30 lb/sq ft
Modify for exposure: Open flat terrain amplifies wind speed; forests and buildings shelter downwind structures. For an exposed site, use the full calculated pressure. For a sheltered site, use 70–80% of calculated pressure.
Roof uplift: Wind creates suction over a roof surface. For typical building geometry, this uplift force is approximately equal to the windward wall pressure (positive). It effectively reduces the dead load holding the roof down. If the roof dead load is less than the wind uplift, the roof will be lifted off — anchor roofing and framing accordingly.
Overturning: Wind on the windward wall of a building creates an overturning moment — the building wants to rotate about its downwind foundation edge. Ensure foundations are heavy enough or anchored to resist this overturning.
Dynamic and Impact Loads
Impact loads: A load dropped or moving rapidly exerts a larger effective force than its static weight. For design purposes, apply these factors to the static load to get the effective design load:
| Impact source | Multiply static load by |
|---|---|
| Light vibrating machinery | 1.25 |
| Moderate vibrating machinery | 1.5 |
| Heavy machinery (hammers, presses) | 2.0–3.0 |
| People jumping or running | 1.5–2.0 |
| Cart or wagon on smooth floor | 1.2 |
| Cart or wagon on rough surface | 1.5–2.0 |
A working example: A blacksmith hammer mounted on a floor weighs 800 lb. With a dynamic factor of 2.5 (heavy hammer): design load = 800 × 2.5 = 2,000 lb concentrated load. The floor must be designed for this, not merely the 800 lb static weight.
Resonance danger: If machinery vibrates at a frequency near the natural frequency of the floor or structure, resonance can amplify vibrations to dangerous levels — small vibration forces produce large structural oscillations. Test: walk across the floor and feel for excessive springiness. If the floor bounces noticeably with each step, it may be near resonance. Stiffen the floor by reducing span or adding intermediate supports.
Load Combinations
Structures carry multiple loads simultaneously. Use load combinations to find the worst case:
Normal case: Dead load + full live load Snow case: Dead load + partial live load + full snow load Wind case: Dead load + partial live load + full wind load
Do not add maximum values of all loads together — maximum snow and maximum occupancy load are very unlikely to occur simultaneously (the building is unlikely to be fully occupied during the worst snowstorm, though it can happen). The code approach: use 100% of the dominant variable load and 75% of secondary variable loads.
For a building: 100% dead + 100% live + 75% snow is a reasonable worst-case combination for floor design. For roof design: 100% dead + 100% snow + 75% wind.